Analysis of Trihalomethanes in Drinking Water Distribution Lines and Assessment of Their Carcinogenic Risk Potentials

Abstract

This study examined the spatial and seasonal variations of trihalomethanes (THMs) and estimated the health risks associated with THM exposure in drinking water through various pathways. Water samples were collected from 14 distribution districts connected to the Ulutan Distribution System (UDS) and the Süleyman Bey Distribution System (SDS), which supply drinking water to Zonguldak Province, Türkiye. THMs were measured using the USEPA 551 method. The median total trihalomethanes (TTHMs) ranged from 41 μg/L to 71 μg/L, which is below the Turkish drinking water standard of 100 μg/L. Chloroform (TCM) was the most common trihalomethane in all distribution networks in UDS and SDS. On the other hand, pre-ozonation oxidation after chlorination in SDS disinfection caused the contribution of brominated THMs (62%) to THM formation to be higher than that of TCM (38%). The study on cancer risk reveals that ingestion (96%) poses the greatest risk of the investigated pathways, followed by dermal contact (3.95%), while inhalation has been found to have a negligible effect. The highest and lowest median TTHMs occurred during winter and summer. The findings of the study show that the distribution areas of Kozlu, Ömerli, Topçalı, and Uzunçayır, for both genders, exhibit an unacceptable cancer risk level according to the criteria established by the USEPA (>10⁻⁴). Bromodichloromethane (BDCM) and chlorodibromomethane (DBCM) are the main contributors to cancer risk for males and females in UDS and SDS. The hazard index (HI) data indicated that the HI value remained below one for both UDS and SDS. Sensitivity analysis of THMs demonstrated that exposure frequency (EF) was the primary parameter contributing to the maximum potential impact on the total cancer risk exposure frequency (EF), followed by body weight (BW) and exposure duration (ED). Further, the results provide valuable information for health departments and water management authorities, enabling the formulation of more specific and efficient policies to minimise THM levels in drinking water distribution networks.

1. Introduction

In recent years, there has been a growing recognition of the importance of access to clean and reliable water resources on a global scale for promoting human health [1,2]. It is a fact that surface water sources, primarily used for drinking, contain organic matter, inorganic chemicals, and microorganisms such as bacteria and viruses [3]. Notably, conventional treatment methods, including coagulation–flocculation, filtration, pre-oxidation techniques, and disinfection, are widely employed in drinking water treatment systems worldwide [4,5].

Disinfection is the final and most important step in water treatment processes. It plays a key role in protecting human health by eliminating pathogenic microorganisms that can transmit infectious diseases through drinking water distribution systems [6,7]. When chlorine reacts with water containing natural organic matter, it forms disinfection by-products (DBPs), which can negatively impact human health. Additionally, toxicology research has shown that prolonged exposure to high levels of DBPs may increase the risk of developing cancerous growths [8]. To reduce the formation of DBPs such as THMs, researchers have explored alternative disinfection methods to replace chlorination [9]. Ozone (O₃), with its strong disinfectant properties and likelihood of generating fewer DBPs, is considered a preferred option in water treatment [10]. However, ozone is naturally unstable in water and cannot sustain long-lasting, effective disinfection. To control toxic DBPs like THM, which are linked to cancer, various chemical pre-oxidation disinfection methods—such as O₃, O₃/Cl₂, and O₃/NH₂Cl₂—are used in treatment plants and distribution systems instead of chlorination [11]. In pre-ozonation applications, selecting the correct ozone dose for disinfection is essential. Using high doses of ozone early in treatment can lead to increased formation of DBPs like THM [12]. Therefore, adjusting ozone doses and treatment procedures to effectively manage organic parameters such as TOC and UV₂₅₄ proves more effective. Additionally, pre-ozonation produces more THM but reduces HAA formation during subsequent chlorination [13].

Among the DBPs that are especially important in this context, trihalomethanes (THMs) are the most notable in chlorinated water. These compounds are carcinogenic, and many countries regulate their levels in drinking water [14,15]. THMs appear in chlorinated water in four forms: chloroform (TCM, CHCl₃), bromodichloromethane (BDCM, CHCl₂Br), bromoform (TBM, CHBr₃), and chlorodibromomethane (DBCM, CHClBr₂), with the total amount of these compounds called total trihalomethanes (TTHMs) [16,17]. While most studies on DBPs in surface water samples show TCM as the dominant compound, there are cases where bromide THM compounds like DBCM and TBM become the main THMs when bromide levels are high, especially during sample chlorination, as seen in seawater [18]. During water disinfection containing bromide, chlorine (HOCl/OCl) or O₃ oxidizes bromide to hypobromous acid (HOBr) through a series of reactions. This process produces brominated THMs, including BDCM, DBCM, and TBM, as well as HAA, alongside TCM [19]. Also, when water with high bromide levels is disinfected with large doses of ozone rather than chlorine, brominated THMs, mainly TBM, are found at high concentrations. At the same time, the ratio of brominated DBPs increases significantly after the pre-ozonation step [20].

Significant restrictions on the pollutant levels of these compounds have been imposed by internationally recognized agencies such as the United States Environmental Protection Agency (USEPA) and the European Community (EC) environmental authorities. The maximum TTHM contaminant levels are set at 80 and 100 µg/L according to USEPA [21] and EC [22], respectively. This value is also established at 100 µg/L in Türkiye by the Ministry of Health (TMH) [23], responsible for drinking water standards. On the other hand, THM formation is influenced by factors such as NOM surrogate parameters, water quality, and seasonal variations. Chowdhury et al. [24] conducted an experimental study to investigate the factors influencing the formation of THMs. The researchers found that UV absorbance at 254 nm (UV₂₅₄), dissolved organic carbon (DOC), pH, disinfectant dose, and bromide levels play vital roles in THM formation. As posited by Ye et al. [25], these factors have been identified as the most significant in determining the presence of disinfection by-products. This assertion was further corroborated by Mahato and Gupta [26].

Spatial and seasonal alterations are directly related to changes in THM concentrations within water distribution systems [27]. It has been suggested that these variations may impact THM formation even more than water temperature and quality, particularly in long network lines. Modifications to water supply infrastructure are associated with the management of THMs in drinking water, as well as long-term human health implications. Consequently, epidemiologists must consider the spatial and seasonal variations in human exposure to DBPs in drinking water [28].

Humans are exposed to DBP compounds, including THMs, through various means. First, ingestion occurs via eating or drinking. Second, dermal contact happens during activities such as cooking, showering, and bathing. Lastly, inhalation of contaminated air represents another potential pathway for DBP exposure [29]. Therefore, it is crucial to consider THM concentrations in drinking water and to conduct multi-pathway exposure analyses and health risk assessments to illustrate their potential effects on human health. This assessment estimates the carcinogenic and non-carcinogenic effects of THM exposure through diverse routes [30]. Several studies have identified a risk profile associated with THM exposure. Lee et al. [31] thoroughly analysed cancer risk in Hong Kong, covering all exposure pathways. Gängler et al. [32] reported that household cleaning activities can increase TCM concentrations in indoor air, potentially heightening THM exposure. Kumari et al. [33] examined both cancer and non-cancer risks of THMs in drinking water from five water treatment plants in eastern India. The study utilized multi-pathway exposure routes for both males and females, with results indicating that residents in this region faced an increased cancer risk through oral ingestion of water compared to the other two exposure routes. The objective of this study is to investigate the seasonal and spatial variations of THMs within water distribution systems and the associated risk assessment regarding chronic exposure to THMs and their impact on human health.

In the present study, a total of 180 water samples were collected from the water distribution systems in four major districts of Zonguldak, a significant city in Türkiye with a population of 250,000. These samples were gathered from 20 September 2022 to 21 August 2023. We analysed THM concentrations for spatial and seasonal variations. This study is among the first to examine THM data in drinking water, considering spatial and seasonal variations in the Western Black Sea region, particularly in Zonguldak province. Additionally, a comparative analysis was conducted to evaluate the relative importance of each THM species and each exposure pathway regarding cancer and non-cancer risks for both males and females across all water distribution systems. No other study has been performed in this region. Therefore, this research provides a novel contribution to the literature by updating the THM values in the drinking water network in Zonguldak province, including the Western Black Sea region, and conducting health risk assessments of the adverse effects on the local population through multi-pathway exposure analyses.

2. Materials and Methods

2.1. Study Area and Water Sampling

Zonguldak Province (41.743°–40.83° N, 31.192°–32.477° E), situated in the Western Black Sea Region, borders the Black Sea to the west and north. Encompassing an area of 3309 km², it accounts for 0.6% of Türkiye’s territory. Starting at the Black Sea coast, the province is bordered by the Black Sea to the north, Bartın to the northeast, Karabük to the east, Bolu to the south, and Düzce to the west. The region’s total population was recorded as 59,492 inhabitants in 2023. Zonguldak experiences a temperate Black Sea climate with precipitation occurring throughout the year. The province receives an average annual rainfall of 1238 mm. Ulutan Dam (UD) and Kızılcapınar Dam (KD) are two key water sources providing drinking and utility water to Zonguldak Province. Notably, there are no natural lakes within the provincial borders. Raw water from UD and KD is channelled to the drinking water distribution systems of Ulutan (UDS) and Süleymenbey (SDS), respectively, thus fulfilling the drinking water needs of most of the province.

Table 1. Characteristics of Zonguldak drinking water utilities.

Water Source	Distribution System	Pre-Treatment Stage	Treatment Process	Average Population	Sampling District
Ulutan Dam (UD)	Ulutan Distribution System (UDS)	Chlorination	Cascade Aeration, PAC Coagulation, Filtration, Disinfection	70,000	1. Kozlu 2. Kilimli 3. Acılık 4. Kavaklık 5. İncivez 6. Karaelmas 7. Dilaver
Kızılcapınar Dam (KD)	Kızılcapınar Distribution System (KDS)	Ozonation	Aeration, Ozonation, Alum Coagulation, AC adsorption, Filtration, Disinfection	90,000	1. Ömerli 2. Armutçuk 3. Topçalı 4. Kışla 5. Kocaali 6. Göktepe 7. Uzunçayır

provides a comprehensive overview of the locations of water sources, treatment and disinfection systems, and sampling lines. Each distribution system employs a distinct set of treatment and disinfection strategies. As shown in Table 1, in this study, water samples were collected from a total of 14 sampling points, with sampling conducted at seven different locations from each distribution system. Further, the location map of the study area and the satellite view of selected water sampling sites are presented in Figure 1.

The study, conducted over one year from September 2022 to August 2023, involved the collection of 180 water samples. Of these, 140 were obtained from tap water sources, while 40 were sourced from raw water. The sampling points included 14 sites within the UDS and SDS regions. The samples were meticulously collected in 500 mL polyethylene bottles following the Sampling and Storage Regulations [34]. The bottles were cleaned, rinsed with tap and ultrapure water, and then rinsed with acetone. The samples were filled into the bottles to minimize air bubbles, and hydrochloric acid (HCl) was added to prevent biodegradation. The samples underwent a drying process in an oven at 150 °C for two hours. Sodium sulfite (Na₂SO₃) was also added to eliminate residual chlorine and prevent the formation of THMs during sampling, analysis, transportation, and storage. In cases where microbial growth occurred before analysis, the water samples were transported in a sealed container with an ice pack and stored in a refrigerator. The schematic flow diagram of the experimental studies conducted in this research is presented in Figure 2.

2.2. Chemicals

Water samples were collected from all designated sampling sites within the UDS and SDS, as well as the UD and KD, and filtered through a 0.45 μm membrane filter before analysis. For the chlorination experiments, the sodium hypochlorite (NaOCl) used in this study was laboratory-grade, with a concentration range of 5.65–6%. This compound was obtained from Fisher Scientific (Pittsburgh, PA, USA). The DPD-free chlorine reagent powder test pillows were supplied by Hach (Loveland, CO, USA). Potassium hydrogen phthalate (C₈H₅KO₄) was purchased from Sigma-Aldrich (St. Louis, MO, USA). This substance was used in two distinct ways: first, for TOC measurements and calibration in experimental studies; second, as a calibration solution in UV absorbance measurements.

Potassium dihydrogen phosphate (KH₂PO₄) (99.8%) was utilized to adjust pH levels in UV absorption and free chlorine measurements, while sodium sulfite (Na₂SO₃) served as a quenching agent. The substances were supplied by Merck (Merck, Darmstadt, Germany). TTHM quantification in a 200 µg/L mixed THM standard was performed using the EPA 551.1 method [35]. The Agilent brand (VOC-mix) with catalogue number 0500001 was acquired from American Council of Independent Laboratories (Washington, DC, USA). The concentrations in this study were 200 µg/L for TCM (CHCl₃), BDCM (CHCl₂Br), DBCM (CHClBr₂), and TBM (CHBr₃). Additionally, pentane, utilized in the THM extraction process, was sourced from Fisher Scientific (Pittsburgh, PA, USA).

2.3. Analytical Procedure

To prevent interference, all water samples used in the experimental study were filtered through Whatman 0.7 GF/F filters before analysis. Total Organic Carbon (TOC) analyses utilized the combustion method as outlined in Standard Method 3510-B [36], employing a Shimadzu TOC-5000 analyser equipped with an auto-sampler. UV₂₅₄ absorbance measurements were taken following Standard Method 5910B, using a Shimadzu 1608 UV-vis spectrophotometer (Tokyo, Japan) at a wavelength of 254 nm with a 1 cm quartz cell. In this study, Specific UV Absorbance (SUVA) is calculated using the following Equation (1):

SUVA(L/mg·m) = 100 × [UV₂₅₄(cm⁻¹)/TOC(mg/L)]

(1)

The raw water samples were chlorinated according to Standard Method 5710 B [36]. The samples underwent a treatment process involving an NaOCl solution with a concentration range of 1–20 mg/L, corresponding to the equivalent dosage of chlorine found in a 100 mL vial. The chlorination process used a stock solution of NaOCl at 5 mg/mL. The vials were securely sealed to prevent air bubbles. Subsequently, the samples were maintained at ambient temperature to ascertain the generation of THM. For free chlorine analysis, the DPD solution was added to the chlorinated water samples after incorporating the phosphate buffer into the mixture.

THM concentrations were analysed using the liquid–liquid extraction method with pentane as the solvent. The instrument utilized for THM measurements was an HP 6890 (Agilent Technologies Inc., Santa Carla, CA, USA) Series II Gas Chromatograph equipped with a micro-electron capture detector (µECD). A capillary column, designed after a J&W Science DB-1 (Santa Clara, CA, USA), with a length of thirty meters, a diameter of 0.32 mm, and a thickness of 1.0 μm, was employed [37].

2.4. Statistical Analysis

In this study, Jamova 2.6.2 was used for the following purposes: (1) To determine the statistical significance, sensitivity, accuracy, and robustness of THM concentrations in different distribution networks during periods of local and seasonal fluctuations. (2) To calculate Spearman’s rho coefficients for the correlation between THMs and organic compound parameters.

2.5. Cancer Risk Assessment

A cancer risk assessment is a tool that estimates the likelihood of an individual developing cancer based on the contaminants they have been exposed to over their lifetime. The process involves three sequential steps: toxicity and exposure assessment, risk characterization, and risk management. The potential for human cancer development due to THMs is evidenced by three distinct exposure pathways: oral ingestion, dermal contact, and inhalation [38,39]. Cancer risks were calculated based on THM concentration, considering chronic daily intake (CDI) and the corresponding slope factor (SF). The statistical distributions and parameter values are shown in

Table 2. The statistical distributions and values of exposure parameters.

Input Parameters	Symbol	Unit	Value	Reference
THM concentration	Cw,i	mg/L	Tables S2 and S3	This study
Ingestion rate	IR	L/day	2	[40]
Exposure frequency	EF	days/year	365	[40]
Exposure duration	ED	years	Female: 81 Male: 75.6	[41]
Body Weight	BW	kg	Female: 68.4 Male: 77.4	[41]
Average Time	AT	years	Female: 29,565 Male: 27,594	[41]
Skin surface area	SA	m2	Female: 1.77 Male: 1.89	[42]
Exposure time	ET	hours	Female:0.13 Male: 0.12	[42]
Dermal permeability constant	PC	cm/h	TCM: 0.16 BDCM: 0.18 DBCM: 0.20 TBM: 0.21	[42]
THMs in air	Ca,i	mg/m3	Model	[42]
Inhalation rate	IRa	m3/h	Female: 0.66 Male: 0.84	[43]
Overall mass transfer coefficient	KOLA	L/min	TCM: 7.4 BDCM: 5.9 DBCM: 4.6 TBM: 3.7	[44]
Henry’s constant	H	Unitless	TCM: 0.35 BDCM: 0.186 DBCM: 0.102 TBM: 0.058	[44]
Flow rate in liquid	QL	L/min	5	[45]
Air flow rate	QG	L/min	50	[45]
Air volume in the shower	Vs	m3	6	[45]
Exposure time in air	ET	h/day	0.2	[46]
Reference dose	RfD	mg/kg/day	TCM: 0.01 BDCM: 0.02 DBCM: 0.02 TBM: 0.02	[46]

.

An estimation of the CDI for each exposure route (oral, dermal, and inhalation for THMs) was obtained through the application of Equations (2)–(4), as follows:

CDI_ingestion = (C_w × IR × EF × ED × CF)/(BW × AT)

(2)

CDI_dermal = (C_w,i × SA × PC × ET × EF × ED)/(BW × AT)

(3)

CDI_inhalation = (C_a,i × IR_a ×ET × EF × ED)/(BW × AT)

(4)

where CDI_ingestion, CDI_dermal, and CD_Iinhalation are the chronic daily intake for the ith THM through ingestion, dermal, and inhalation, respectively (mg/kg/day), C_w is the concentration of the ith THMs in distribution networks (mg/L), IR is the ingestion rate of water (L/day), EF is the exposure frequency (days/year), ED is the exposure duration (years), BW is body weight (kg), AT is the average lifetime (days), CF is the mass conversion factor from μg to mg (0.001), SA is the skin surface area exposed to water (m²), PC is the specific dermal permeability for the ith THM (cm/h), ET is the exposure time (hours/day), C_air is the concentration of THMs in shower air for the ith THM (mg/m³), and IRa is the inhalation rate (m³/h).

The C_a,i was evaluated by the following Equations (5)–(9);

C_air = Ys(t) + Ys(i)/2

(5)

Ys (t) = [1 − exp(−bxt)] × (a/b)

(6)

a = {Q_L × C_w [1 − exp(−N)]}/V_s

(7)

b = {(Q_L/H)[1 − exp(−N)] + Q_G}/V_s

(8)

N = K_oLA/Q_L

(9)

In this context, the variables Ys(t), Ys(0), b, t, a, QL, N, Vs, H, QG, and KoLA represent the following quantities: concentrations of THMs in the bathroom at 15 min (mg/L), and the initial concentration of THMs at 0 min (0 mg/L).

The following parameters were used in the calculation: the minimum water flow rate during the shower (5 L/min), a dimensionless coefficient (calculated from Equation (7)), and the volume of the bathroom (5 m³). The Henry’s law coefficients (TCM: 0.12, TBM: 0.0219, BDCM: 0.0656, DBCM: 0.0321 unitless), the air flow rate (50 L/min), and the mass transfer coefficients (TCM: 7.4, TBM: 3.7, BDCM: 5.9, DBCM: 4.6 L/min) were also used [47].

2.6. Cancer Risk Characterization

The lifetime cancer risk is calculated by summing the exposure pathways for each type of THM and multiplying by the slope factors (SF_ingestion,i, SF_dermal,i, and SF_inhalation,i) as shown in Equation (10) below.

CR_i = Σ CDI_ingestion,i × SF_ingestioni + Σ CDI_dermal,i × SF_dermal,i + Σ CDI_inhalation,i × SF_inhalation,i

(10)

where CR_i is the lifetime cancer risk for the ith THM, and SF_ingestion,i, SF_dermal,i, and SF_inhalation,i are slope factors for the ith THM from the exposure routes via ingestion, dermal, and inhalation, respectively (mg/kg/day).

The SF, identified as the maximum estimate of the lifetime probability of developing cancer due to exposure to a specific chemical in the environment, was used to calculate the probability that an individual would suffer from cancer due to exposure to that chemical [48]. The SF values for the ith THM (i.e., TCM, BDCM, DBCM, and TBM) were established at 0.0061, 0.062, 0.084, and 0.0079 mg/kg/day, respectively. The SF values for dermal exposure, SF_dermal,i, were found to be equivalent to those for inhalation exposure, SFinhalation,i. As for the values of SF_inha, i, these were assigned as 0.081, 0.13, 0.094, and 0.0039 mg/kg/day for TCM, BDCM, DBCM, and TBM, respectively [49].

2.7. Non-Cancer Risk

The hazard index (HI) of THMs is an essential tool for evaluating the non-cancer risk associated with exposure pathways, including ingestion and dermal absorption. This risk is calculated using the following Equations (11) and (12):

HI_inge,i = CDI_inge,i/RfD_i

(11)

HI_derm,i = CDI _inge,i/RfD_i

(12)

where RfDi is the reference dose for the ith THM (mg/kg/day), as shown in Table 1, and HI_inge, i and HI_derm, i are the hazard indices for the ith THM through ingestion and dermal contact, respectively.

Given that the CDI for the inhalation exposure route was significantly smaller than the CDI for ingestion and dermal exposure, the HI associated with inhalation contact was deemed negligible. In studies where HI ≥ 1, there is a high level of concern regarding potential toxicity.

2.8. Sensitivity Radar Chart

A radar chart was used to conduct a sensitivity analysis. The chart is known as a spider chart or a star chart. Data were processed in Microsoft Excel. This allowed calculation of both cancer risk and the value of the potential for carcinogenesis from each route.

3. Results and Discussion

3.1. Characterization of Source Waters and THMs Concentrations

Table 3. Physical and organic characteristics of raw waters.

Parameters	Unit	Ulutan Dam (UD)			Kızılcapınar Dam (KD)
		Median	SD	Range	Median	SD	Range
PH		7.56	±0.39	6.87–8.2	7.96	±0.33	7.42–8.44
Turbidity	NTU	4.41	±1.05	2.69–6.32	3.58	±1.01	2.25–5.48
Br-	µg/L	112	±40.4	75.1–207	189	±48.2	95.1–240
TOC	Mg/L	4.3	±0.93	3.02–6.30	4.18	±0.81	2.82–6.03
UV254	cm−1	0.13	±0.029	0.07–0.16	0.12	±0.029	0.07–0.17
SUVA	L/mg.m	2.75	±0.199	2.32–2.99	2.91	±0.246	2.47–3.19
TCM	µg/L	170.88	±37.91	93.08–207.61	200.88	±43.02	112.52–229.05
BDCM	µg/L	27.55	±7.18	14.2–42.14	41.15	±14.94	23.14–73.36
DBCM	µg/L	17.56	±6.79	7.4–29.03	31.79	±13.31	13.1–49.5
TBM	µg/L	7.19	±2.87	1.92–11.38	6.13	±4.96	3.59–17.2
TTHM	µg/L	223.19	±54.13	116.6–289.32	283.88	±74.04	152.35–368.58

presents the median water quality parameters that illustrate the physical and organic characteristics of UD and KD waters used in UDS and SDS, which supply drinking water to Zonguldak province. The recorded median values of physical parameters in UD and KD were as follows: pH, 7.56–7.96; turbidity, 3.58–4.41 NTU; bromide, 112–189 µg/L. The organic content of NOM in UD and KD, along with its potential for THM formation, is characterized by TOC, UV₂₅₄, and the SUVA (UV₂₅₄/TOC) parameter values, recognized as THM precursor components [50]. The median UV₂₅₄ and TOC values in UD water samples (4.43 mg/L—0.13 cm⁻¹) were slightly higher than those in KD (4.18 mg/L—0.122 cm⁻¹), respectively. Meanwhile, the median SUVA values in KD and UD waters were 2.91 L/mg·m and 2.75 L/mg·m.

SUVA is a reliable predictor of the carbon aromaticity content of the NOM and the formation of DBPs in water [51]. As demonstrated by Ozdemir et al. [52] and Reckhow et al. [53], there is a direct correlation between the formation of THM and the increased activated aromatic content of the NOM. Similarly, this study found that TTHM values were higher in KD (283.88 µg/L) than in UD (223.19 µg/L), depending on SUVA values. The most dominant THM species in UD and KD were TCM, accounting for 77% and 71% of TTHM, respectively. The measured mean levels of TCM in UD and KD varied between 200 µg/L and 171 µg/L. TBM concentrations in UD (7.19 µg/L) and KD (6.13 µg/L) represent the least contributing THM species, accounting for approximately 3% of the TTHM concentration.

The observed sequence of concentrations of brominated species in both UD and KD is BDCM > DBCM > TBM. The findings of this study align with the existing literature on the subject [54]. Higher brominated THM levels (i.e., BDCM and DBCM) were analysed in a water sample collected from KD. This is attributed to the raw water used in the KD sample, which may account for the elevated bromide content observed in the results compared to those obtained from UD, as reported by Uyak et al. [55].

3.2. Correlation Analysis

The present study investigated the correlation between NOM surrogate parameters, such as TOC, UV₂₅₄, and SUVA, and THM species in both UD and KD water samples. This investigation employed Spearman’s coefficient and partial correlation analysis. The findings revealed a moderate correlation between TOC and TCM (rho = 0.56, p < 0.001), BDCM (rho = 0.44, p < 0.001), DBCM (rho = 0.46, p = 0.001), and TBM (rho = 0.5, p = 0.001) (Table S1). Similarly, a moderate correlation was noted between TOC and TTHM (rho = 0.51, p < 0.001). These results indicate that TOC plays a crucial role in the formation of THM during the chlorination of UD and KD reservoirs. In summary, THM formation is observed to increase in parallel with rising TOC levels in both water sources. Past studies demonstrated a positive correlation between TOC and THM formation [56]. In a similar observation, a study by Tsitsifli et al. [57] reported moderate correlations between THMs and TOC in the water distribution system of Kayseri city (Türkiye), with a correlation coefficient of 0.48 [58].

UV₂₅₄, an important organic parameter with an aromatic structure, correlates well with TTHM (rho = 0.61, p < 0.001) and TCM (rho = 0.61, p < 0.001). Conversely, there is a moderate correlation between brominated THMs and UV₂₅₄, with a correlation coefficient ranging from 0.55 to 0.57. Supporting these findings, Kelly-Coto et al. [59] reported positive correlations of TTHMs with TOC and UV₂₅₄. Furthermore, among the NOM surrogate parameters, SUVA displays the strongest correlation with TTHM and THM species, with a Spearman’s coefficient exceeding 0.90 (rho > 0.90, p < 0.001). The correlation between TTHM and SUVA is rho = 0.99, followed by TCM (rho = 0.98), then DBCM (rho = 0.97), BDCM (rho = 0.94), and finally TBM (rho = 0.91).

This study utilized Spearman’s correlation analysis to ascertain the relationship between SUVA and THMs. The results indicated that SUVA is the primary precursor of THM formation, and an increase in SUVA level strongly correlates with a rise in THM concentration. This finding supports results reported in previous studies [60]. Meanwhile, Spearman’s correlation analysis revealed highly significant relationships between THM types and TCM. For instance, the following correlations were observed: TCM–BDCM (rho = 0.93), TCM–DBCM (rho = 0.97), TCM–TTHM (rho = 0.99), BDCM–TTHM (rho = 0.96), BDCM–DBCM (rho = 0.97), DBCM–TBM (rho = 0.88), DBCM–TTHM (rho = 0.99), and TBM–TTHM (rho = 0.90). Spearman rank correlation values indicate a strong and positive correlation between TTHM and SUVA, as well as THM species. Additionally, a moderate correlation is observed between TOC, UV₂₅₄ parameters, and THMs in UD and KD water samples.

To evaluate the statistical significance of the TTHM values measured in water samples from all distribution points in UDS and SDS, a one-way ANOVA analysis was performed. As presented in

Table 4. The Homogeneity of Variances Test (Levene’s) in UDS.

UDS	F	df1	df2	p
TTHMs	3.13	6	140	0.007

and

Table 5. The Homogeneity of Variances Test (Levene’s) in SDS.

	F	df1	df2	p
TTHMs	5.90	6	140	<0.001

, firstly, a test of homogeneity of variances was conducted for a total of 280 TTHM values at all distribution points in UDS and SDS. It was observed that there was no homogeneity between distribution regions in UDS and SDS (F = 3.13, p = 0.007; F = 5.90, p ˂ 0.001).

In the subsequent normality test conducted for both distribution systems, it was observed that normality was achieved because the skewness value was between −1 and 1 in all distribution regions (Tables S4 and S5). As shown in

Table 6. The analysis of One-Way ANOVA (Welch’s) in UDS.

	F	df1	df2	p
TTHMs	51.4	6	61.9	<0.001

and

Table 7. The analysis of One-Way ANOVA (Welch’s) in SDS.

	F	df1	df2	p
TTHMs	179	6	61.7	<0.001

, the results of Welch’s one-way ANOVA analysis indicated a statistically significant variation in TTHM values among all distribution regions within the UDS (F = 51.4; p < 0.007) and SDS (F = 179; p < 0.001). The statistical data for TTHM values in the distribution regions of UDS and SDS are presented in Tables S2 and S3. Since normality was achieved in the distribution regions of UDS and SDS but homogeneity was not, the Games–Howell Post-Hoc Test was applied to all distribution regions in the one-way ANOVA analysis to examine the significance of TTHM values (Tables S6 and S7). As a result, a notable difference in TTHM values was found between the distribution regions of UDS and SDS when compared to each other. For example, in UDS, there was a significant discrepancy in TTHM values between the Kozlu-Kilimli (p < 0.001) and Kilimli-İncivez (p < 0.001) regions. As shown in Table S6, there is a significant difference in the mean values across these regions in UDS. In contrast, no statistically significant difference was found in TTHM values within the Acılık-Karaelmas regions (p > 0.05). Similar comparisons are observed between other distribution regions in SDS (Table S7). Overall, the statistical significance, sensitivity, and robustness of the TTHM concentration values analysed in water samples collected from all distribution areas in UDS and SDS are revealed by one-way ANOVA analysis. The present findings are consistent with the extant literature [61,62], which also demonstrates the formation of THMs.

3.3. Levels of THMs in Drinking Water Distribution Systems

Figure 3 displays the statistical distribution of TTHM concentrations in water samples collected from 14 sampling points across two water distribution systems. This box plot depicts the median, standard deviation, and minimum–maximum ranges for each TTHM value. The analysed median TTHM concentrations in both UDS and SDS at each sampling point have been determined to range from 29.1–91.4 µg/L. The maximum median concentrations of THMs are lower than the MCL of 100 μg/L regulated in TMH and EC. The result obtained in this study is consistent with those of analogous studies [63]. Almoiqli et al. [64] analysed the average THM concentration (70.39 µg/L) in water samples taken from drinking water networks in Riyadh, Saudi Arabia, below the MCL value specified by the EC. For UDS, TTHM analyses were carried out on a total of 140 water samples, which were taken from seven sampling points along the route from Kozlu to Dilaver (Figure 3a). The median TTHM concentrations ranged from 29.11 to 91.4 µ/L. The lowest median TTHM concentration was 29.1 µg/L in water samples from the Kilimli distribution network, and the highest median TTHM concentration of 72.3 µg/L was observed in water samples from the Kozlu distribution network, followed by Kavaklık (66.3 µg/L), Acılık (63.7 µg/L), Karalemas (59.2 µg/L), İncivez (56.3 µg/L), and then Dilaver (49.1 µg/L) networks (Table S2). This finding can be attributed to the fact that drinking water is pumped over long distances from the Kilimli sampling district to the Kozlu sampling point due to the constant residual chlorine in the distribution lines. This observation is consistent with the findings reported by Nikolaou et al. [65].

In the SDS analyses conducted on a total of 140 water samples collected from the Ömerli distribution area to the Uzunçayır distribution area, the median TTHM concentration (82.71 µg/L) was highest in Ömerli, while the lowest was recorded at the Kocaali distribution points (58.11 µg/L) (Figure 3b). In other words, these lower values are strongly correlated with the high quality of the water supply source and the effective operation of the SDS, which results in lower TOC levels and improved control during chlorination [66]. Furthermore, this finding arises because water for consumption is transported over long distances from the SDS to the Ömerli area. In contrast, since the Kocaali distribution area is closer to the SDS, TTHM values are lower. In the Göktepe, Armutçuk, Kışla, Uzunçayır, and Topçalı distribution areas, the median TTHM concentrations ranged between 61.2 mg/L and 78 mg/L, respectively (Table S3). A comparative analysis of TTHM concentrations in UDS and SDS revealed higher occurrences in SDS than in UDS distribution systems. The discrepancy in median TTHM concentrations between the two systems is associated with various factors, including differences in treatment processes at water treatment plants, water residence times, hydraulic conditions, chlorination methods, network line characteristics, and water quality. These factors greatly influence TTHM formation in distribution systems [67]. As demonstrated by multiple studies, the concentration and composition of THMs vary significantly between water systems depending on the water source and the characteristics of each system [68,69]. Similar findings were reported by Mishaqa et al. [70] in their study on THM analyses in Egyptian drinking water.

3.4. Spatial and Seasonal Speciation of THMs in Drinking Water Distribution Systems

Figure 4 displays a visual representation of the spatial distributions of chlorinated and brominated THM species across both distribution systems, along with each one’s contribution to the TTHM. In all distribution areas in UDS and SDS, TCM is the dominant THM component, with the highest concentrations of 42.5 mg/L and 36.7 mg/L, except in the Kocaali sampling area. As shown in Figure 4a, in the UDS, median TCM values with the highest concentrations ranged from 42.5 µg/L to 22.7 µg/L, from Kozlu to Dilaver sampling areas, followed by BDCM (19.7 µg/L), DBCM (13.4 µg/L), and TBM (5.24 µg/L). In SDS, TCM levels follow the order of Ömerli (36.7 µg/L) > Topçalı (32.32 µg/L) > Uzunçayır (30.5 µg/L) > Kışla (29.3 µg/L) > Armutçuk (25.2 µg/L) > Göktepe (24.4 µg/L) > Kocaali (20 µg/L), as shown in Figure 4b.

The analysis revealed a higher proportion of brominated THMs (BDCM and DBCM) in SDS than in UDS, while chlorinated THM species were detected in UDS. Moreover, higher levels of BDCM and DBCM were observed in Topçalı (28.1 µg/L, 18.1 µg/L) and Uzunçayır (26 µg/L, 18.7 µg/L) in SDS compared to other distribution lines (Table S3). The outcomes of the present study may be explained by the observation that distribution networks for SDS exhibit a higher bromide content in comparison with those of UDS, a phenomenon that can be attributed to the treatment applications. This finding is in alignment with the conclusions reported in the study conducted by Zhang et al. (2024) [71]. On the other hand, Uyak et al. [72] conducted a study on determining THM concentrations in Istanbul’s drinking water distribution systems in Türkiye. They observed an increase in TCM, BDCM, and TTHM values following the application of pre-ozonation and chlorination processes for the oxidation and disinfection of organic matter, and this result is also supported by the present study.

While the median DBCM levels among the brominated THMs peaked at 18.4 µg/L at the Ömerli distribution point in SDS, in Uzunçayır, Topçalı, and other distribution regions, median DBCM concentrations were recorded between 18.7 µg/L and 12 µg/L. However, the highest DBCM level in the UDS was detected in water samples from the Karaelmas (13.4 µg/L) distribution point. TBM is the least significant contributor to the TTHMs in both distribution systems. Median TBM values at certain distribution points were roughly two to three times higher in SDS compared to UDS. For instance, median TBM levels in Ömerli, Armutçuk, and Kocaali in SDS were recorded at 8.99, 5.3, and 8.23 µg/L, respectively, whereas median TBM levels in Kozlu, Kilimli, and İncivez in UDS were 2.58, 1.76, and 3.17 µg/L, respectively.

As a summary of the findings in this section, Figure 5 illustrates the percentage contribution of different THM types at each distribution point to the median TTHM concentrations after undergoing various treatment processes and disinfection methods in UDS and SDS in Zonguldak Province.

TCM contributed the most to the median TTHMs, accounting for 53% and 38% across all distribution points in UDS and SDS, respectively, followed by BDCM (25–32%), DBCM (16–21%), and TBM (6–9%). Comparing the distribution of THM species in UDS and SDS reveals a notable shift in TTHM contributions. In SDS, there is an increase in the percentage contributions of BDCM, DBCM, and TBM—by 8% and 3%, respectively—compared to UDS. Conversely, TCM’s contribution decreased by 15%.

A review of the existing literature explains the primary factors behind these findings. (1) Disinfection strategies differ: UDS used conventional chlorination, while SDS employed ozone oxidation alongside chlorination. As in UDS, chlorination increases the formation of chlorinated by-products like TCM. In SDS, chlorination and O₃/Cl₂ disinfection have a greater potential to produce brominated THM species such as BDCM and DBCM. This result aligns with previous studies in the literature [73,74]. Liao et al. [75] found that pre-O₃ disinfection in a Chinese distribution network produced more THMs than chlorination alone, demonstrating that pre-O₃ is associated with more THM formation during subsequent chlorination, supporting our findings. (2) Compared to UDS, water with higher bromide levels in SDS, disinfected with ozone or chlorine, forms hypobromous acid (HOBr) during bromide oxidation, which reacts more quickly with organic matter than HOCl formed during chlorine oxidation. Consequently, higher levels of brominated THMs are observed in SDS, and this aligns with previous studies [76]. Moreover, Zhai et al. [77] found that brominated THM species like BDCM and TBM increased with pre-ozonation, similar to our results. Furthermore, a significant finding is that one of the main reasons for higher concentrations of brominated THM species in SDS compared to UDS is the slightly elevated bromide level, attributed to the regional geology surrounding the SDS. The accuracy of these results is supported by several studies [78,79]. (3) The effective use of ozone dosing in SDS is crucial, as changes in THM formation are strongly correlated with pre-O₃ dosage. Yang et al. [80] conducted laboratory-scale studies where water containing natural organic matter was oxidized with ozone doses of 2 and 6 mg/L. The maximum concentrations of THM, THAA, and DHAA were recorded as 1.83, 0.65, and 0.56 μM, respectively, at approximately 2 mg/L ozone. These concentrations were about 1.38, 1.23, and 1.43 times higher than those without ozonation. When oxidation occurred at a 6 mg/L ozone dose, there was a 10% reduction in THM and THAA levels. Therefore, in treatment systems like SDS, proper ozone dosing is vital for managing DBP levels during disinfection with pre-O₃. Conversely, Zhang et al. [81] reported that low ozone doses before chlorination reduced HAA but increased THMs, reinforcing the findings of this study.

Figure 6 provides a quantitative view of the fluctuations in TTHM production, as measured by the median TTHM concentration, across the four seasons studied in the water distribution systems of Zonguldak. On the other hand, the statistical data concerning seasonal variation in TTHM concentrations are shown in Table S8.

In this particular section of the study, THM analyses were exclusively conducted on water samples extracted from UDS and SDS outlets. Also, the procedural framework for the treatment processes applied in UDS and SDS is summarised in Table 1. A median analysis of TTHM concentrations across the two distribution systems reveals that the highest median concentrations were recorded during the winter months. In other words, the median TTHM concentration was 78.1 mg/L in SDS and 58.2 mg/L in UDS during the winter season. Furthermore, during that season, TTHM concentrations reached up to 92.6 µg/L and 76.1 µg/L in SDS and UDS, respectively (Figure 6).

It is concluded that seasonal changes in TTHM are linked to variations in the amount and properties of water sources. The presence of THMs is caused by chlorine reacting with organic matter. Therefore, it is reasonable to infer that higher levels of TOC are likely to increase THM production. Moreover, considering the higher TOC levels seen in KD compared to UD, a greater tendency for THM formation is indicated due to the chlorination process. This conclusion is supported by an analysis of water samples collected from distribution lines. Similar findings were reported by Volk et al. [82]. The median TTHM concentrations (51.3–67.7 µg/L) in the SDS samples were higher during autumn and spring than the TTHM levels (41.2–47.2 µg/L) in the UDS (Table S8). The TTHM ranges were 40.9–62.5 µg/L and 51.1–82.6 µg/L in SDS, and about 29 to 64 µg/L in UDS during fall and spring, respectively. However, TTHM values in both distribution systems were lower than those seen in winter. This suggests a change in the amount and makeup of NOM following precipitation, which may also be due to the higher rainfall intensity in winter compared to spring. Increased rainfall likely causes more humic substances to enter the water from the upper soil layers [83], consistent with other studies on seasonal THM variations [84]. The lowest median TTHM values, 36.3 µg/L in SDS and 37.2 µg/L in UDS, were recorded during winter. This can be explained by the following: lower precipitation during summer reduces humic substance input from land into water, while evaporation rates increase. These two factors contribute significantly to the decrease in THM formation during summer months relative to winter and spring. Conversely, median TTHM concentrations in SDS and UDS during winter increased by 2.16 and 1.56 times, respectively, compared to summer (Table S8). These findings are consistent with other research on seasonal THM variations [85,86].

On the other hand, the statistical significance of changes in TTHM concentrations according to season in all distribution areas was tested using one-way ANOVA analysis. As demonstrated in

Table 8. Homogeneity of Variances Test (Levene’s) in UDS for seasonal variations.

	F	df1	df2	p
TTHM	1.82	3	64	0.153

and

Table 9. Homogeneity of Variances Test (Levene’s) in SDS for seasonal variations.

	F	df1	df2	p
TTHM	0.237	3	64	0.870

, the distribution of TTHM concentrations according to season is homogeneous in UDS (F = 1.82; p = 0.153) and SDS (F = 0.237, p = 0.87).

Furthermore, given that the skewness values in both distribution systems were between −1 and 1, it can be concluded that the normality parameter was also met (Tables S9 and S10). Following the completion of these steps, a significant difference was observed between TTHM values across seasons in the one-way ANOVA analysis table (p < 0.001) (

Table 10. One-Way ANOVA (Fisher’s) in UDS.

	F	df1	df2	p
TTHM	44.2	3	64	<0.001

and

Table 11. One-Way ANOVA (Fisher’s) in SDS.

	F	df1	df2	p
TTHM	9.83	3	64	<0.001

).

The Tukey Post-Hoc analysis in Tables S11 and S12 provides clear evidence of the significance of these differences when seasonal pairs in UDS and SDS are compared. For instance, in UDS, a substantial discrepancy was identified in all pairwise comparisons, as indicated by p < 0.05. A significant difference is observed between autumn–summer, winter–summer, and spring–summer (p < 0.05), while no significant difference is found between other groups (p > 0.05). Significant differences were found between fall–winter and summer–spring in SDS (p < 0.05). As indicated in Tables S13 and S14, other statistical parameters, including the mean between groups, are presented.

The results of the one-way ANOVA analysis show that the TTHM concentration values measured in different seasons using UDS and SDS are statistically significant, accurate, and robust.

3.5. Total Lifetime Cancer Risk Assessment

The adverse effects of THM formations on human health are evaluated using lifetime cancer risk assessment data, calculated based on ingestion, dermal, and inhalation exposure routes, as outlined in Equations (1)–(3). The cancer risk is assigned to four categories: negligible risk (Cancer risk < 10⁻⁶), acceptable low risk (1 × 10⁻⁶ ≤ Cancer risk < 5.1 × 10⁻⁵), acceptable high risk (5.1 × 10⁻⁵ ≤ Cancer risk < 10⁻⁴), and unacceptable risk (Cancer risk ≥ 10⁻⁴) [87].

The present study aims to assess the cancer risk to males and females posed by the presence of THMs in water samples from distribution networks in UDS and SDS in Zonguldak Province. The lifetime cancer risk for males and females, estimated for all potential exposure pathways at all distribution districts in UDS and SDS, is presented in Figure 7.

Concerning the ingestion route, the highest cancer risk in UDS was found to be 9.8 × 10⁻⁵ at the Kozlu distribution point for males and 7.54 × 10⁻⁵ at the Karaelmas distribution point for females, while the lowest cancer risk was recorded at 3.63 × 10⁻⁵ at the Kilimli distribution point for males and 3.62 × 10⁻⁵ at the Kozlu distribution point for females (Figure 7a). The lifetime cancer risk values in UDS and other distribution districts range from 4.64 × 10⁻⁵ to 6.96 × 10⁻⁵ for males and 4.11 × 10⁻⁵ to 6.74 × 10⁻⁵ for females (Figure 7b). A higher cancer risk has been observed in SDS compared to UDS. The highest cancer risk for males and females in SDS is found to be almost identical in Ömerli, Uzunçayır, and Topçalı distribution points. In these distribution points, cancer risks were recorded as 9.82 × 10⁻⁵, 9.51 × 10⁻⁵, and 9.05 × 10⁻⁵ for males and 1.11 × 10⁻⁴, 1.02 × 10⁻⁴, and 9.96 × 10⁻⁵ for females, respectively. The lowest cancer risk was observed in the Göktepe distribution area with 6.37 × 10⁻⁵ (males) and the Armutçuk distribution area with 4.54 × 10⁻⁵ (females). Elsewhere in the literature, Zhu et al. [88] found that the mean lifetime cancer risks associated with the ingestion of TTHMs in water samples from the outlets of four drinking water treatment plants in Jiangsu Province, China, ranged from 5.81 × 10⁻⁶ to 1.67 × 10⁻⁵ and from 5.16 × 10⁻⁶ to 1.48 × 10⁻⁵. In another study by Mishra et al. [89], it was determined that the carcinogenic risk of THMs through ingestion exposure was 3.18 × 10⁻⁴ for males and 2.60 × 10⁻⁴ for females, respectively [90]. These results are consistent with this study. The highest cancer risk through dermal contact was detected as 2.62 × 10⁻⁶ (males) and 3.01 × 10⁻⁶ (females) in the Karaelmas distribution area, while the lowest cancer risk was identified as 1.42 × 10⁻⁶ (males) and 1.63 × 10⁻⁶ (females) in the Kilimli distribution area. Furthermore, the highest cancer risk associated with dermal contact of SDS was found in the Ömerli distribution area for both males (3.82 × 10⁻⁶) and females (4.38 × 10⁻⁶). Low cancer risks were observed in the Kocaali and Göktepe distribution areas for males, at 2.53 × 10⁻⁶ and 2.50 × 10⁻⁶ (Figure 7a), respectively, and in the Göktepe and Armutçuk distribution areas for females, at 2.91 × 10⁻⁶ and 3.01 × 10⁻⁶, respectively (Figure 7b). This study revealed that females had a higher lifetime cancer risk through the dermal route than males at all distribution sites in UDS and SDS, depending on factors such as skin surface area and body weight. The findings outlined above are corroborated by the research conducted by Basu et al. [91]. Concurrently, the results of previous studies on the carcinogenic risks arising from THMs in the dermal contact pathway corroborate the findings of this study [92]. The risk values for the dermal route across all distribution districts in UDS and SDS are within acceptable limits compared to the prescribed limit by USEPA (1 × 10⁻⁶ < Cancer Risk < 5.1 × 10⁻⁶). Consequently, it can be concluded that the risk of exposure to THMs via dermal absorption is at a low level. Notably, the findings of this study indicated that the cancer risk through the dermal route in UDS across all distribution network lines exceeds the acceptable low risk threshold by a factor of 2.17 to 2.49 for both males and females. In SDS, the risk is 3.05 and 3.47 times higher than the acceptable level in males and females, respectively, and the present study has yielded favourable results, which are consistent with findings from related studies [93,94]. Research findings in UDS and SDS have indicated that the lifetime cancer risk in males and females by the dermal exposure route is found to be at an acceptably low level, and that the cancer risk by inhalation exposure route is found to be at a negligible level in all distribution networks.

As is clear from the literature, inhalation exposure occurs when volatile compounds are inhaled by humans during water activities such as showering, bathing, washing, and cooking, which cause them to become airborne. In this study, the lifetime cancer risk via the inhalation route for TTHM was found to be lower than the recommended negligible risk (10⁻⁶) for both sexes at all distribution network points in UDS and SDS (Tables S15 and S16). Of the three exposure routes, inhalation contributes the least to lifetime total cancer risk, followed by dermal exposure and then the ingestion pathway in UDS and SDS across all distribution networks and both sexes. In UDS, the maximum cancer risk via the inhalation route was documented at the Kozlu sampling point, with a value of 3.71 × 10⁻⁷ (males) and 3.30 × 10⁻⁷ (females). Moreover, the minimum cancer risk values were recorded at the Kilimli distribution area for males (2.22 × 10⁻⁷) and at the Karaelmas distribution area for females. On the other hand, in SDS, the highest cancer risk values of 4.91 × 10⁻⁷ (males) and 4.36 × 10⁻⁷ (females) were recorded in the Ömerli distribution region, and the lowest values of 3.21 × 10⁻⁷ (males) and 2.94 × 10⁻⁷ (females) were recorded in the Kocaali and Armutçuk distribution regions. A review of related research reveals that Sadeghi et al. [95] investigated this topic and found that the overall cancer risk from THMs in drinking water in Iran’s Ardabil region was negligible, with inhalation exposure risk far below the recommended limits. These findings align with the current study. In the meantime, results indicate that, in contrast to the ingestion and dermal pathways, the inhalation pathway exhibits a heightened cancer risk for males compared to females within the distribution regions of both UDS and SDS. This outcome can be clarified by the observation that the inhalation rate parameter (IRa) in the CDI calculations for the inhalation exposure route is elevated for males in comparison to females.

In the present study, ingestion was identified as the main pathway of THM exposure, with dermal absorption and inhalation ranking second and third, respectively. Overall, this outcome shows that the variation of THM concentrations is associated with the concentrations of other THM species, SF values, and various additional parameters within CDI estimates. Also, the CDI estimates are specified within Equations (2)–(4) for each of the specified cancer exposure pathways, and this observation was consistent with the findings of earlier studies [96,97]. In other words, the study’s findings underscore the significance of ingestion exposure as a predominant contributor to cancer risk, surpassing other exposure pathways due to its pivotal role in generating TTHM concentrations and diverse THM species within drinking water networks. This assertion is further substantiated by exposure parameters such as the cancer slope factor (SF), ingestion rate (IR), inhalation rate (IRa), and exposure frequency (EF), as elucidated in this study. Besides, Kumari and Gupta [98] reported that oral ingestion was the primary exposure pathway, accounting for up to 97.67% of the total cancer risk. In other words, inhalation contributed 2.32%, while dermal absorption posed an almost negligible risk (0.002%), which matches the current study’s findings. However, Kumari and Gupta [99] found that inhalation was a major route of exposure. This might be due to a shift toward a more sedentary lifestyle among certain age groups, increasing susceptibility to the harmful effects of contaminants. For example, younger individuals are more likely to develop respiratory-related cancers from activities like showering and swimming, whereas older individuals are more prone to ingestion-related cancers because of higher drinking water consumption.

Table 12. Evaluation of the potential cancer risk from TTHMs via multiple exposure routes for males at all sampling locations in the UDS.

Water System	Distribution Districts	Males
		Exposure Pathways			Total Cancer Risk	Health Risk Assessment
		Ingestion	Dermal	Inhalation		Risk Level *	Carcinogenicity **
UDS	Kozlu	9.80 × 10−5	2.26 × 10−6	3.39 × 10−7	1.04 × 10−4	Unacceptable	Carcinogenic
	Kilimli	3.63 × 10−5	1.42 × 10−6	2.22 × 10−7	3.65 × 10−5	Low	Possibly carcinogenic
	Acılık	5.46 × 10−5	2.32 × 10−6	3.39 × 10−7	5.73 × 10−5	High	Probably carcinogenic
	Kavaklık	5.95 × 10−5	2.50 × 10−6	3.70 × 10−7	6.24 × 10−5	High	Probably carcinogenic
	İncivez	5.76 × 10−5	2.22 × 10−6	3.63 × 10−7	6.02 × 10−5	High	Probably carcinogenic
	Karaelmas	6.66 × 10−5	2.62 × 10−6	3.57 × 10−7	6.95 × 10−5	High	Probably carcinogenic
	Dilaver	4.64 × 10−5	1.84 × 10−6	2.85 × 10−7	4.85 × 10−5	Low	Possibly carcinogenic

*: Classification defined by USEPA [100] for total cancer risk assessment; Negligible risk (Cancer risk < 10⁻⁶), Low risk (1 × 10⁻⁶ ≤ cancer risk < 5.1 × 10⁻⁵), High risk (5.1 × 10⁻⁵ ≤ cancer risk < 1 × 10⁻⁴), Unacceptable risk (>1 × 10⁻⁴), **: THM carcinogenicity classification by IRIS [101]. Colours cods of carcinogenicity; Red: carcinogen, Yellow: Possibly carcinogenic, Orange: Probably carcinogenic.

and

Table 13. Evaluation of the potential cancer risk from TTHMs via multiple exposure routes for females at all sampling locations in the UDS.

Water System	Distribution Districts	Females
		Exposure Pathways			Total Cancer Risk	Health Risk Assessment
		Ingestion	Dermal	Inhalation		Risk Level *	Carcinogenicity **
UDS	Kozlu	3.62 × 10−5	2.60 × 10−6	3.30 × 10−7	3.91 × 10−5	Low	Possibly carcinogenic
	Kilimli	4.11 × 10−5	1.63 × 10−6	1.98 × 10−7	4.29 × 10−5	Low	Possibly carcinogenic
	Acılık	6.18 × 10−5	2.66 × 10−6	3.01 × 10−7	6.48 × 10−5	High	Probably carcinogenic
	Kavaklık	6.74 × 10−5	2.87 × 10−6	3.29 × 10−7	7.06 × 10−5	High	Probably carcinogenic
	İncivez	6.51 × 10−5	2.55 × 10−6	3.23 × 10−7	6.80 × 10−5	High	Probably carcinogenic
	Karaelmas	7.54 × 10−5	3.01 × 10−6	1.82 × 10−7	7.86 × 10−5	High	Probably carcinogenic
	Dilaver	5.25 × 10−5	2.11 × 10−6	2.54 × 10−7	5.49 × 10−5	High	Probably carcinogenic

*: Classification defined by USEPA [100]) for total cancer risk assessment; Negligible risk (Cancer risk < 10⁻⁶), Low risk (1 × 10⁻⁶ ≤ cancer risk < 5.1 × 10⁻⁵), High risk (5.1 × 10⁻⁵ ≤ cancer risk < 1 × 10⁻⁴), Unacceptable risk (>1 × 10⁻⁴), **: THM carcinogenicity classification by IRIS [101]). Colours cods of carcinogenicity; Yellow: Possibly carcinogenic, Orange: Probably carcinogenic.

present lifetime cancer assessments for males and females in all sampling areas in UDS, respectively, based on different exposure routes from TTHMs.

Table 14. Evaluation of the potential cancer risk from TTHMs via multiple exposure routes for males at all sampling locations in the SDS.

Water System	Distribution Districts	Males
		Exposure Pathways			Total Cancer Risk	Health Risk Assessment
		Ingestion	Dermal	Inhalation		Risk Level *	Carcinogenicity **
SDS	Ömerli	9.82 × 10−5	3.82 × 10−6	4.91 × 10−7	1.03 × 10−4	Unacceptable	Carcinogenic
	Armutçuk	6.76 × 10−5	2.62 × 10−6	3.31 × 10−7	7.06 × 10−5	High	Probably carcinogenic
	Topçalı	9.05 × 10−5	3.34 × 10−6	4.39 × 10−7	9.43 × 10−5	High	Probably carcinogenic
	Kışla	6.68 × 10−5	2.78 × 10−6	3.55 × 10−7	6.99 × 10−5	High	Probably carcinogenic
	Kocaali	7.32 × 10−5	2.53 × 10−6	3.23 × 10−7	7.61 × 10−5	High	Probably carcinogenic
	Göktepe	6.31 × 10−5	2.50 × 10−6	3.34 × 10−7	6.59 × 10−5	High	Probably carcinogenic
	Uzunçayır	8.80 × 10−5	3.22 × 10−6	4.18 × 10−7	9.16 × 10−5	High	Probably carcinogenic

*: Classification defined by USEPA [100] for total cancer risk assessment; Negligible risk (Cancer risk < 10⁻⁶), Low risk (1 × 10⁻⁶ ≤ cancer risk < 5.1 × 10⁻⁵), High risk (5.1 ×10⁻⁵ ≤ cancer risk < 1 × 10⁻⁴), Unacceptable risk (>1 × 10⁻⁴), **: THM carcinogenicity classification by IRIS [101]. Colour cods of carcinogenicity; Red: carcinogen, Orange: Probably carcinogenic.

and

Table 15. Evaluation of the potential cancer risk from TTHMs via multiple exposure routes for females at all sampling locations in the SDS.

Water System	Distribution Districts	Females
		Exposure Pathways			Total Cancer Risk	Health Risk Assessment
		Ingestion	Dermal	Inhalation		Risk Level *	Carcinogenicity **
SDS	Ömerli	1.11 × 10−4	4.38 × 10−6	4.36 × 10−7	1.16 × 10−4	Unacceptable	Carcinogenic
	Armutçuk	4.54 × 10−5	3.01 × 10−6	2.94 × 10−7	4.87 × 10−5	High	Probably carcinogenic
	Topçalı	1.02 × 10−4	3.83 × 10−6	4.33 × 10−7	1.06 × 10−4	Unacceptable	Completely Carcinogenic
	Kışla	7.56 × 10−5	3.19 × 10−6	3.16 × 10−7	7.91 × 10−5	High	Probably carcinogenic
	Kocaali	8.29 × 10−5	3.31 × 10−6	2.87 × 10−7	8.65 × 10−5	High	Probably carcinogenic
	Göktepe	7.13 × 10−5	2.91 × 10−6	2.97 × 10−7	7.45 × 10−5	High	Probably carcinogenic
	Uzunçayır	9.96 × 10−5	3.69 × 10−6	3.72 × 10−7	1.04 × 10−4	Unacceptable	Completely Carcinogenic

*: Classification defined by USEPA [100] for total cancer risk assessment; Negligible risk (Cancer risk < 10⁻⁶), Low risk (1 × 10⁻⁶ ≤ cancer risk < 5.1 × 10⁻⁵), High risk (5.1 × 10⁻⁵ ≤ cancer risk < 1 × 10⁻⁴), Unacceptable risk (>1 × 10⁻⁴), **: THM carcinogenicity classification by IRIS [101]. Colour cods of carcinogenicity; Red: carcinogen, Orange: Probably carcinogenic.

present similar assessments for SDS.

In the UDS, an unacceptable level of cancer risk was detected in the Kozlu drinking water network for males, while high levels were detected in water samples from four regions in both distribution systems for both females and males. Conversely, in the SDS, cancer risk was determined to be at an unacceptable level in water samples collected from the Ömerli drinking water distribution network for both females and males, and at high levels in all other distribution regions. However, it was only found to be at an unacceptable level in water samples taken from the Topçalı and Uzunçayır drinking water distribution networks for females.

The results indicate that the overall lifetime cancer risk in water samples from distribution lines in most districts of Zonguldak exceeds the risk level established by the USEPA. A study conducted in the distribution areas of Kozlu, Ömerli, Topçalı, and Uzunçayır found that the cancer risk level was deemed unacceptable according to USEPA criteria. The findings of the study by Tokmak et al. [102] align with these results. Notably, a significant portion of the lifetime cancer risk results from ingestion exposure, with BDCM and DBCM being the THM species contributing most to the total cancer risk. Additionally, the presence of interactive effects and diseases, such as synergistic and antagonistic interactions [103], and kidney, cardiac, and liver diseases [104], among the exposed population in these districts could be expected, given the elevated cancer risk values observed. Therefore, this study is crucial for risk management, as it offers a detailed analysis of procedures that can be implemented to effectively reduce risks. Addressing and minimizing potential hazards is vital to safeguarding public health. In summary, the study’s main finding is presented in Figure 8, which shows the percentage contribution of each of the three exposure routes to the total cancer risk across all distribution lines in UDS and SDS, considering both males and females.

The ingestion route is determined to be the predominant potential risk source for cancer in all distribution areas in UDS and SDS in Zonguldak province, accounting for 96% of the total cancer risk. This is followed by dermal absorption (4%), with inhalation contributing very little (<1%).

Accordingly, the study’s findings could inform the efforts of the Zonguldak province water management authority and health departments to regulate THM species levels and protect community health. For instance, in European countries, including France, Germany, and Denmark, water management authorities have initiated booster disinfection applications to reduce THM species concentrations that are typically found exclusively within potable water networks [105]. In the meantime, greater emphasis must be placed on the significance of BDCM and DBCM, as they have been observed to pose a considerable cancer risk, even at low levels, thus, as indicated by the findings of related studies in the academic literature [106], environmental engineers and environmental experts must optimise other operating parameters, particularly disinfectants such as ozone and chlorine, which have been demonstrated to result in elevated levels of THM in drinking water treatment facilities in Zonguldak. Moreover, disinfection methods such as chlorine dioxide and UV radiation could be used instead of chlorination to prevent THM formation, as is practised in many European countries [107]. On the other hand, in light of the elevated risk of exposure identified for all drinking water works within the investigated Zonguldak districts, the implementation of appropriate treatment methods and continuous monitoring through modelling approaches or electronic information systems is imperative, which is consistent with similar studies in the literature [108].

3.6. Contribution of THM Species to Total Cancer Risk

To compare the contribution of THM species to cancer risks, the average lifetime cancer risks for each THM through different exposure routes among males and females across all districts in UDS and SDS are summarized in Figure 9.

For ingestion, brominated THMs (BDCM, DBCM) contribute the most to the overall cancer risk in both men and women across all points in the UDS distribution system. The ranking of THM species contributing to cancer risk via ingestion is as follows: in males, BDCM (43.83%) > DBCM (39.22%) > TCM (15.9%) > TBM (1.26%); in females, DBCM (45.71%) > BDCM (42.16%) > TCM (10.55%) > TBM (1.53%). In SDS, BDCM is the main contributor to ingestion-related cancer risk for both males (49.02%) and females (52.6%), followed by DBCM (44.1–39.51%), TCM (5.56–6.11%), and TBM (1.32–1.98%). These results are explained by two factors: (1) the variation in risk estimates mainly results from SF values, which are higher for brominated THMs compared to chlorinated ones, consistent with other water system studies, supporting the study’s reliability; (2) the risk potential of BDCM and DBCM [109], which are ten times greater than TCM and TBM. Furthermore, this finding indicates that the variation in risk estimates is strongly correlated with SF values, which are higher for brominated THMs than chlorinated ones. The study’s recommended SF values for brominated THMs are higher than those for chlorinated THMs. The results from this study align with findings from other research on water distribution systems, further supporting its credibility [110]. Regarding dermal contact, in both males and females, THM species follow the order DBCM > BDCM > TCM > TBM across all UDS distribution points (Figure 9a,b). In SDS, BDCM is the main contributor to dermal cancer risk at all sites for both sexes, followed by DBCM, TCM, and TBM, in that order. The differences in rankings between men and women in SDS stem from the varying risk contributions of TCM and TBM. The study reveals that in SDS, TBM’s contribution to cancer risk exceeds that of TCM for both sexes, accounting for 19.02% in males and 20.43% in females, while TCM contributes 13.14% in males and 13.03% in females (Figure 9c,d). The higher bromide levels in SDS compared to UDS likely cause this pattern. Additionally, pre-ozonation during disinfection is associated with increasing TBM formation. For inhalation, TCM contributes the most to the total cancer risk in UDS at all sites, with 55.86% in males and 52.18% in females. As shown in Figure 10a,b, BDCM is the second-largest contributor, making up 34.38% in males and 37.14% in females, followed by DBCM and TBM. Due to TCM’s high volatility and low boiling point, inhalation exposure accounts for 52.86% to 55.86% of the total risk. This aligns with results from several studies [111,112]. Compared to UDS, BDCM has the highest percentage contribution at all sampling points for inhalation exposure in SDS, with 44.92% in males and 44.01% in females, mainly because of the lower SF for TCM compared to BDCM and DBCM. This difference can also be attributed to the variations in disinfection methods used in each distribution system and operational parameters such as ozone, chlorine dosage, retention, and reaction time. This pattern is consistent with other research [113,114]. Additionally, TCM accounts for 40.61% in men and 42.02% in women across SDS, while DBCM accounts for 14.27% and 13.77%, respectively. TBM’s contribution to total cancer risk is negligible.

Overall, as illustrated in

Table 16. Mean total cancer risk of THMs through multi-exposure routes and the contribution percentage of each THM species to the exposure route of ingestion, dermal contact, and inhalation in Zonguldak, Wuxi, and Marargheh cities.

City, Country	Water System	Expos. Routes	TCM	%	BDCM	%	DBCM	%	TBM	%	TTHM	%	Reference
Zonguldak, Türkiye	Distribution * networks	Ingestion	5.03 × 10−6	6	2.44 × 10−5	28	5.48 × 10−5	64	1.72 × 10−6	2	8.60 × 10−5	100	This study
		Dermal	1.12 × 10−6	20	1.39 × 10−6	24	2.82 × 10−6	49	4.05 × 10−7	7	5.74 × 10−6	100
		Inhalation	1.09 × 10−7	43	1.16 × 10−7	45	3.04 × 10−8	11	1.30 × 10−9	1	2.57 × 10−7	100
		Cancer risk	6.26 × 10−6	7	2.59 × 10−5	28	5.77 × 10−5	63	2.13 × 10−6	3	9.19 × 10−5	100
Wuxi, China	Distribution networks	Ingestion	2.87 × 10−6	7	2.23 × 10−5	55	1.46 × 10−5	36	1.16 × 10−6	2	4.09 × 10−5	100	[115]
		Dermal	1.01 × 10−7	7	7.01 × 10−7	47	6.40 × 10−7	43	6.09 × 10−8	3	1.50 × 10−6	100
		Inhalation	1.18 × 10−5	15	3.79 × 10−5	48	2.82 × 10−5	36	1.55 × 10−6	1	7.95 × 10−5	100
		Cancer risk	1.48 × 10−5	12	6.09 × 10−5	50	4.34 × 10−5	36	2.77 × 10−6	2	1.22 × 10−4	100
Maragheh, Iran	Distribution networks	Ingestion	3.07 × 10−6	7	2.85 × 10−6	55	1.48 × 10−5	36	3.44 × 10−8	2	2.08 × 10−5	100	[93]
		Dermal	2.62 × 10−7	7	2.32 × 10−7	47	1.66 × 10−6	43	6.68 × 10−9	3	2.13 × 10−6	100
		Inhalation	2.09 × 10−8	15	1.04 × 10−10	48	5.85 × 10−10	36	2.80 × 10−12	1	3.05 × 10−8	100
		Cancer risk	3.35 × 10−6	15	3.08 × 10−6	13	1.65 × 10−5	71	4.11 × 10−8	1	2.29 × 10−5	100

*: Classification of parameters affecting cancer risk in distribution networks in different countries according to colour; Colourless: for the distribution network in Türkiye, Yellow: for the distribution network in China, Green: for the distribution network in Iran.

, this study’s comprehensive findings in Türkiye encompass all distribution areas of the UDS and SDS systems in the city of Zonguldak. The analysis considers the contributions of THM types to the total cancer risk for each exposure route. A study was conducted by Abdolahnejad et al. [93] in Maragheh, Iran, and by Ding et al. [115] in Wuxi, China, to compare the contributions of THM types measured in drinking water distribution lines to cancer risk through different exposure pathways.

As indicated in Table 16, according to the USEPA criteria for cancer risk level assessment, the risk level in Zonguldak drinking water networks is classified as high (9.19 × 10⁻⁵), unacceptable (1.22 × 10⁻⁴) in Wuxi city China, and low (2.29 × 10⁻⁵) in Maragheh City, Iran. Furthermore, it has been determined that the greatest contribution to cancer risk in each city is from brominated THM types, BDCM and DBCM. In other words, DBCM was the most significant contributor to the cancer risk in the drinking water networks of Zonguldak and Maragheh, at 63% and 71%, respectively. Conversely, BDCM was the most significant contributor in Wuxi province, at 50%. This result, as corroborated by the present study, shows that brominated THMs are formed in greater quantities in water with elevated bromide levels and where the disinfection process uses ozone pre-oxidation.

3.7. Non-Cancer Risk Assessment

Figure 10 and Figure 11 present hazard index (HI) values for ingestion and dermal exposure routes across all distribution lines for females and males in UDS and SDS, respectively. This study found that, similar to previous research, HI values for the ingestion route were higher than those for the dermal route [116]. There is an increased likelihood of adverse outcomes when the HI exceeds 1, suggesting potential negative effects on human health [117]. The ingestion and dermal exposure HI values for TTHMs in UDS and SDS remained below the USEPA toxicity threshold (HI ≥ 1), with slightly higher values observed in females compared to males across all distribution networks from both sources. The highest HI value for the ingestion route of TTHMs in UDS was recorded in the Acılık distribution line for females (1.78 × 10⁻¹), while in SDS, the highest ingestion route HI value was found in the Ömerli distribution line for females (1.93 × 10⁻¹) (Figure 10). Conversely, the maximum dermal exposure HI values were recorded at 3.28 × 10⁻³ in Kozlu for UDS and 1.09 × 10⁻² in the Kocaali distribution line for SDS in females, consistent with the findings for ingestion (Figure 10). The HI values for TTHMs via ingestion across six distribution points for both males and females in UDS ranged from 7.97 × 10⁻² to 1.7 × 10⁻¹, while the HI values for TTHMs via the dermal route ranged from 1.60 × 10⁻³ to 3.14 × 10⁻³. In SDS, HI values for TTHMs via ingestion ranged from 9.59 × 10⁻² to 1.71 × 10⁻¹, and HI values for TTHMs via the dermal route ranged from 2.29 × 10⁻³ to 3.87 × 10⁻³. Supporting the findings of this study, Lee et al. [118] established that the average HI values associated with ingestion and dermal contact were between 0.001 and 0.004, while Wang and Dong [119] determined an average HI value of 0.007 in the non-carcinogenic risk assessment for THMs, which is below the acceptable risk level defined by the USEPA.

In both UDS and SDS, TCM was identified as the primary contributor to the total HI risks associated with ingestion and dermal exposure among THM species across all distribution networks, followed by BDCM, DBCM, and TBM. This significant contribution is attributed to the higher concentrations of TCM relative to other THMs in both UDS and SDS, as well as its low reference dose. The highest HI values for TCM regarding ingestion in the UDS were recorded at 1.10 × 10⁻¹ and 1.24 × 10⁻¹ for males and females, respectively, within the Kozlu distribution network, and 9.48 × 10⁻² and 1.07 × 10⁻¹ in the Omerli distribution line in SDS. Furthermore, the highest HI values of TCM for dermal exposure in females from UDS and SDS were recorded at the Kozlu (1.72 × 10⁻³) and Kocaali (9.38 × 10⁻³) distribution points, respectively. Following TCM, the highest HI values for BDCM, related to ingestion and dermal routes, were noted as 4.62 × 10⁻² and 9.54 × 10⁻⁴, respectively, for females in the Omerli distribution network in SDS (Figure 10 and Figure 11). The HI values for DBCM regarding ingestion and dermal routes were 2.69 × 10⁻² and 6.19 × 10⁻⁴, respectively, in the Omerli distribution network in SDS.

As indicated by numerous studies [120,121], this outcome suggests that the non-cancer risk for THMs is associated with the ingestion route of exposure from TCM, as the HI values for dermal contact are negligible compared to the ingestion route. Conversely, TBM was observed to contribute the least among all THM species to the total HI value in both SDS and UDS. Specifically, the average contribution of TCM to the total HI values for ingestion and dermal exposure was 68–65%, followed by BDCM (18%), DBCM (11–13%), and finally TBM (3–7%) for males and females across all distribution networks in UDS. In SDS, the contributions were found to be 56%, 21–24%, and 15–16% for TCM, BDCM, and DBCM, respectively, while TBM accounted for 4–7%. Furthermore, the data obtained in the studies conducted by Viana et al. [122] are consistent with the results of this study.

3.8. Sensitivity Analysis

In this study, a sensitivity analysis using the radar or spider sensitivity tool revealed the carcinogenic risk assessments of THMs in water samples collected from all distribution networks in UDS and SDS for both females and males. This analysis identifies the parameters that most significantly influence the risk, thereby allowing for a focused approach on the most important variables and, subsequently, the potential measures that may be implemented to reduce the risk.

The exposure parameters that significantly affect cancer risk through ingestion and dermal exposure in both females and males in UDS and SDS are shown in Figure 12.

EF was determined to be the primary parameter contributing to the maximum impact on total cancer risk, followed by BW and ED. This study provides evidence suggesting that the risk of developing cancer increases in proportion to the increase in EF, ED, and BW. The impact of the remaining factors was found to be negligible regarding brominated THMs such as BDCM and DBCM. The behaviour exhibited by both males and females during sensitivity analysis demonstrated consistent trends within the two distinct contexts, i.e., UDS and SDS. Nonetheless, slight disparities were noted in the sensitivity values, as presented in Figure 12a–d. The other parameters, exposure time (ET), slope factor (SF), infrared inhalation (IRa), and skin area (SA), were considered non-sensitive variables. Overall, in this study, the sequence of variables used in the sensitivity analysis was as follows: EF > BW > ED > BDCM > DBCM. Furthermore, the results from the sensitivity analysis conducted in this study suggest that the management process must determine which procedures can mitigate risk and thereby protect public health. Conversely, following the detection of cancer risk in the Kozlu, Ömerli Topçalı, and Uzunçayır drinking water distribution networks by USEPA criteria, it has become mandatory to continuously monitor TTHM and THM species concentrations. This is to be achieved by optimising the treatment processes in UDS and SDS, as well as developing new model approaches or systems in the drinking water distribution networks connected to them.

The EF, ED, and BW parameters are key metrics used to calculate exposure routes such as ingestion, dermal contact, and inhalation. These parameters are important for assessing cancer risks related to THMs and play a critical role in evaluating the potential for cancer development, thus significantly contributing to overall cancer risk. Textiera et al. [123] conducted a sensitivity analysis to determine how likely THMs are to pose a cancer risk to humans in samples from indoor swimming pools in 17 cities in northern Portugal. The study found that the EF and ED parameters were major contributors to the total cancer risk. The results from this study suggest that it is essential to develop and enforce adequate regulatory guidelines and other mitigation measures to protect human health in indoor swimming pool environments, without compromising the microbiological quality of the water. In a related study, Mishra et al. [89] examined the cancer risk from THMs in samples from the effluent of drinking water treatment plants in various regions of India. Sensitivity analyses indicated that EF and TCM concentrations were the most significant parameters affecting total cancer risk, with BW and ED also playing important roles. These findings highlight the need for significant changes in the operational procedures and process choices of treatment facilities to reduce cancer risk. This information can help guide public health authorities. The results of the aforementioned studies in the literature on the subject confirm the findings related to sensitivity analyses presented in this paper [98]. In addition, the findings of this study are supported by previous research, including that of Kumari et al. [33] and Anchal et al. [124].

Furthermore, range-based sensitivity analysis for both genders in all sampling regions in UDS and SDS is presented in Figures S1 and S2.

The study’s notable strengths enhance the current literature, especially its large sample size and meticulous individual exposure assessment to THMs.

The present study is unique in that it is the first to investigate local and seasonal THM concentration changes in drinking water lines connected to distribution systems where different treatment and disinfection strategies are implemented in the Western Black Sea region of Türkiye, and their cancer effects on human health according to the criteria and mathematical expressions developed by the USEPA. The statistical analysis of the significance, sensitivity, and robustness of THM measurements in water samples collected from various sampling locations within the studied distribution systems reveals a notable difference from similar studies conducted in Türkiye. The findings of the present study corroborated the efficacy of DBP regulations in curtailing THM concentrations in drinking water, a phenomenon particularly evident in developed countries [125,126]. Moreover, the study elucidated the disparities in drinking water safety between developed and developing countries. In other words, this study also provides evidence that a spatial-based risk analysis can help identify suitable risk management strategies. Conversely, during the investigation of lifetime cancer risk from THMs at sampling sites within each distribution network, exposure parameters for ingestion, dermal, and inhalation routes were precisely and accurately determined for individuals, categorized by gender. These parameters effectively contributed to assessing overall cancer and non-cancer risks. To date, research on this topic remains limited, particularly in Türkiye. In one such study, Sofuoğlu et al. [127] conducted cancer and non-cancer risk assessments on humans due to THM formation in tap water samples taken from different regions of Izmir province. In the present study, the lifetime cancer risk was found to be solely based on ingestion exposure. It was determined that the cancer risk exceeded 1 × 10^−6. Among the THM species, BDCM and DBCM were found to have a higher potential for carcinogenesis than other species. It was concluded that the HI value was below the value (HI < 1) prescribed by the USEPA in all regions. In laboratory-scale studies conducted by Tanatti et al. [128], it was found that the exposure route was the primary contributor to overall cancer risk. Furthermore, it was identified that BDCM exhibited the highest potential to induce cancer risk.

Compared to this study, existing studies have not conducted a detailed statistical analysis of cancer risk based on the distribution of THM and THM species in drinking water networks, and the total cancer risk was also assessed by calculating only one CDI. The human exposure assessment in both studies was based on theoretical data. Moreover, the health risk analyses’ findings do not discuss the impact of these findings on public health in the studied regions, nor do they address the development of treatment methods and measures to raise public awareness. In this study, sex-stratified data were utilised due to the recognised importance of gender-related differences and the CDI estimations and total cancer risk calculations. Additionally, the sensitivity analysis performed enabled estimation of the influence of input variables on risk outcomes; EF, BW, and ED most significantly impacted the majority of risk estimates. Further, the effects of the cancer risk data found in the public health analyses of the water samples taken from the distribution regions examined in Zonguldak province on the inhabitants of those regions, the precautions to be taken, and the technological methods to be applied to improve the quality of drinking water, are discussed. Besides, the necessity to apprise water management authorities of the imperative for technical and social measures and recommendations to protect the health of individuals residing in distribution networks and in regions where a substantial cancer risk from THMs and THM species has been identified has been articulated in detail for the first time in Türkiye with the findings of this study.

4. Conclusions

In this study, THM concentrations were measured in water samples collected from 14 sites linked to two water distribution systems in Zonguldak province. The potential lifetime cancer and non-cancer risks from exposure to THMs in drinking water through ingestion, dermal contact, and inhalation were evaluated. Compared to source water samples, KD water samples showed higher levels of THM formation, which was attributed to a greater presence of UV-absorbing aromatic compounds in the NOM structure. Additionally, when examining the NOM precursors in UD and KD, as determined by Spearman’s rank analysis, the SUVA parameter demonstrated a stronger correlation with TTHM and THM species than UV₂₅₄ and TOC.

In the Zonguldak province, median TTHM concentrations detected in all network lines in UDS and SDS were below 100 mg/L, conforming to standards set by the EC and TMH. The TTHM values from the 14 distribution networks examined in UDS and SDS exhibited differences, caused by various factors. On the other hand, the findings of this study demonstrated that the TTHM concentrations measured in all distribution networks in UDS and SDS were statistically significant and robust, as determined using one-way ANOVA analysis.

In the ozone pre-oxidation process applied as a disinfection strategy in SDS, the contribution of brominated THMs (62%) to TTHM was found to be higher than that of chlorinated THMs (38%), which had a much higher carcinogenic potential. In other words, in this study, it was found that ozone, as a pre-oxidant, and then chlorination increased THM concentrations. Therefore, this study suggested that using ozone independently as a disinfectant instead of chlorine could be an alternative solution.

If this were the case, lower levels of THMs would be generated. Besides, it was shown that median TTHM concentrations increased in all distribution networks in Zonguldak province since the input of humic substances from land to the water environment rose due to high winter rainfall compared to other seasons. The lowest TTHM levels were recorded in summer when rainfall is lowest. In the meantime, the significance of TTHM measurements in UDS and SDS according to seasonal changes was demonstrated by means of one-way ANOVA analysis. It is evident from the aforementioned data that the Tukey test, which was performed within the framework of the one-way ANOVA analysis, also indicated significant disparities (p < 0.05) between seasonal pairs in TTHM measurements.

In this study, we assessed the overall cancer risks linked to THMs in water samples collected from 14 sites connected to two main water sources, UDS and SDS, in Zonguldak province. The findings indicated that the highest risk resulted from ingestion, with dermal and inhalation exposures being less significant. Overall, ingestion was identified as the main source of cancer risk, accounting for an average of 96% of the total risk, while dermal exposure contributed 3.65%, and inhalation (<1%) was almost negligible for both males and females. Moreover, based on the criteria set by the US EPA, cancer risk assessments in the distribution areas of UDS and SDS, especially in the Kozlu, Ömerli, Topçalı, and Uzunçayır drinking water networks, revealed an unacceptable level of cancer risk (>10⁻⁴). Consequently, to protect residents’ health, public health authorities should first review these results and promptly implement risk reduction initiatives to lower the cancer risk. The primary recommendation of the study was to adopt alternative disinfection methods instead of chlorination, such as chlorine dioxide and UV radiation, and chloramination, which are used in European countries like Germany and the Netherlands, to decrease THM levels in distribution systems and networks. Further, only the usage of ozonation and post-ozonation was associated with reducing THM levels, instead of pre-O3 ozonation with chlorine. In general, THM compounds have been found to contribute most to the total cancer risk in men and women across all distribution regions through BDCM and DBCM, except via the respiratory exposure route, followed by TCM and TBM. However, a significant finding of this study was that, although BDCM and DBCM were identified as the THM compounds with the highest levels in all exposure routes within both distribution networks, TCM (52.86%) was the primary THM compound contributing to the total cancer risk in the inhalation exposure pathway across all distribution areas in UDS. Therefore, it was recommended that personnel involved in chlorination disinfection in both UDS and UDS distribution lines consider factors such as chlorine dosage, waiting time, and other key water quality parameters. Additionally, before implementing the chlorination process, safety measures such as wearing masks and specialized work attire should be enforced, given chlorine’s toxicity when inhaled. Brominated THMs (BDCM, DBCM) were observed to contribute significantly to cancer risk for three exposure routes in the SDS, ranging from 24.49% to 52.6% and 13.77% to 44.1%, respectively. Consequently, this study proposed the utilisation of disinfectants such as chlorine dioxide instead of chlorine within the SDS disinfection process, intending to minimise the formation of brominated THMs, which possess a higher carcinogenic potential in comparison to TCM. In addition, the results of this study indicate that water managers and specialists responsible for distribution systems in these regions were advised to consider disinfection with chloramines as an alternative to chlorine. This is because increasing chlorine or ozone doses and longer contact times with organic matter, particularly in long network lines in the SDS and UDS, would increase THM concentrations. Conversely, although the HI value was below the acceptable risk level (HI > 1) as stipulated by the USEPA, all sampled districts in UDS and SDS exhibited unexpected adverse non-cancer health effects. Moreover, the risk of developing cancer was strongly correlated with higher EF, ED, and greater BW. The results of this study demonstrated that the standards for drinking water quality in our country should be revised, emphasizing the importance of public health conditions. On the other hand, the establishment of continuous water monitoring initiatives at the regional level was recommended. Concurrently, implementing regular flushing procedures and monitoring water residence time within distribution networks was proposed as an effective measure for reducing THM concentrations. It was anticipated that this study would bring public health attention to the negative effects of chlorine and provide a foundation for future research on THMs in Türkiye. This study had the potential to make a substantial contribution to the development of effective optimization and control strategies by water management authorities in response to health risks that could arise due to THMs and other DBPs in sustainable, safe drinking water, with a particular emphasis on public health in the present and the future.

In summary, this paper reported that changes in THM levels in water distribution systems, cancer risk assessments, and proposed regulations might provide a framework for future studies to address knowledge gaps in this area.