Assessment of Potentially Toxic Elements in Water from the Protected Natural Area Barranca de Metztitlán, Mexico, and Human Health and Ecological Risk

Abstract

Water is a critical resource for both environmental integrity and human health. This study assessed the concentrations of potentially toxic elements (PTEs)—Pb, Cd and Hg—in surface waters adjacent to six urban settlements within the Barranca de Metztitlán Biosphere Reserve (MBR), Mexico. Ecological and health risks were evaluated for vulnerable groups, including children, adolescents, and the elderly. Cd and Hg water concentrations surpassed the national and international water quality criteria at three locations. Ecological and health risk analysis of Pb was not conducted as Pb concentrations were below LOD (0.02 ppm). Ecological risk analysis revealed a low potential risk for Cd exposure but a high risk for Hg at its highest concentration in the reserve. Health risk assessment revealed that Cd and Hg pose a non-carcinogenic risk, particularly to children under three years old. Hazard quotients (HQ) and cumulative risk indices (HI) exceeded safe thresholds at multiple sites. Infants (0–11 months) were the most susceptible, even at contaminant levels near detection limits. These findings emphasize the importance of routine monitoring and early intervention strategies to mitigate exposure risks, especially in vulnerable populations within the MBR.

1. Introduction

Surface water plays a fundamental role in maintaining ecological processes, agricultural productivity, and human health. However, its quality is increasingly compromised by chemical pollutants from natural and anthropogenic origins. Among these, PTEs, particularly Pb, Cd, and Hg, stand out due to their persistence, potential for bioaccumulation, and their association with adverse health effects even at low exposure levels [1,2]. These PTEs originated from diverse sources, including mining, agriculture, urbanization, and industrial discharges, as well as natural weathering processes, which threaten both environmental stability and human health [3,4].

The adverse effects of PTEs are extensively documented, with chronic exposure linked to multisystemic damage in humans and wildlife. Pb, for example, exerts toxicity across virtually all cell types and has been associated with neurological impairments (including diminished cognitive performance and behavioral alterations), as well as cardiovascular effects such as elevated blood pressure and atherosclerotic changes. Pb exposure also affects renal function, as evidenced by a decreased glomerular filtration rate, and interferes with reproductive hormones [5]. Cd has been implicated in cardiovascular disorders (hypertension and ischemic heart disease), hematological abnormalities (anemia and hypoproteinemia), gastrointestinal irritation (nausea and abdominal pain), musculoskeletal demineralization (reduced bone density and increased fragility), renal dysfunction, metabolic disturbances, neurodevelopmental impairment (altered IQ), and reproductive alterations such as hormonal imbalance and reduced sperm motility [6]. Hg similarly exerts wide-ranging toxic effects, including neurological deficits (impaired cognition, memory, changes, and mood instability), renal injury (reduced glomerular function and tubular damage), cardiovascular disturbances (hypertension and altered cardiac function), immunological modulation (immune stimulation), and reproductive impairment (fertility reduction and decreased sperm quality) [7]. In addition, Cd is classified as a human carcinogen, linked to malignancies of the kidney, lung, breast, prostate, and pancreas, whereas Pb and Hg are regarded as potential carcinogens [1].

The environmental behavior and toxicity of PTEs depend not only on their total concentrations but also on their chemical speciation. In aquatic and terrestrial environments, Pb can occur as free ions (Pb²⁺), oxides, hydroxides, or complexed in oxianionic forms [8]. Anthropogenic activities also contribute synthetic forms, such as lead pyrosilicate (PbSi₂O₅), lead oxide (Pb₃O₄), and lead chromate (PbCrO₄) [9,10]. Cd is predominantly present as Cd²⁺ in natural systems, though it can also occur in elemental (Cd⁰) or monovalent (Cd¹⁺) states under specific redox conditions [11]. Hg exhibits complex behavior depending on the environmental compartment: in the atmosphere, it is commonly found as elemental Hg⁰, while in soils, sediments, and aquatic systems, it appears primarily as Hg²⁺ [12]. These chemical forms differ in their mobility, bioavailability, and toxicokinetic properties, which in turn influence their ecological impact and human health risk.

The Barranca de Metztitlán Biosphere Reserve (MBR), located in Hidalgo, México, is an ecologically and socially relevant region that faces growing pressure from pollution sources, including agrochemical use, road traffic, and discharge of untreated wastewater into its main water bodies (Meztitlán River and Meztitlán Lagoon) [13,14,15]. Surface water in the region not only is essential for irrigation crops destined for human consumption but also provides drinking water for the local population and supports biodiversity, including resident and migratory birds [13]. Reports from previous studies have identified elevated concentrations of Cd, Hg, As, and Mn in soil and surface water in this region, sometimes exceeding Mexican regulatory limits [14,15,16,17,18]. Pulido-Flores et al. [19] found that Hg and Cd concentrations in the Metztitlán River were three to six times higher than the Mexican limits for potable water (Cd: 0.01 ppm; Hg: 0.001 ppm). In the case of Pb, the concentration was close to the maximum permissible limit (Pb: 0.05 ppm). Regulatory standards in Mexico for Pb, Cd, and Hg remain less stringent than those of other countries, such as the United States [20], which may lead to an underestimation of ecological and human health risks.

Despite this evidence, there is a lack of comprehensive risk assessments evaluating the combined impact of PTEs in surface water within MBR. This gap is especially concerning given the widespread use of this water for irrigation and domestic purposes.

The present study aims to quantify the total concentrations of Pb, Cd, and Hg in surface waters of the MBR and to assess the associated ecological and human health risks based on chronic oral exposure scenarios. The findings seek to inform regional environmental management and contribute to a better understanding of the risks posed by.

2. Materials and Methods

2.1. Study Site

The MBR located in the state of Hidalgo, Mexico (Figure 1; 98°23′00″ and 98°57′08″ W, 20°14′15″ and 20°45′26″ N) comprises nine municipalities and is recognized for its ecological and economic importance [13]. Within the reserve, areas of intensive agriculture coexist with urban settlements. The main surface water bodies include the Almolón River, in the northern region, and the Metztitlán River, which flows across the reserve.

2.2. Map of Risk of PTE Contamination

Given the limited information on PTE concentrations in rivers and water bodies within the MBR, a pollution risk map was developed to support the selection of sampling sites through the application of a risk pollution index. Similar indices have been proposed to characterize spatial variation in PTE contamination near potential pollution sources [21].

The map was generated using Geographic Information System (GIS) software (QGIS 3 and ArcMap 10.8) to integrate and analyze spatial variables associated with anthropogenic PTE pollution. The selection of these variables was informed by a literature review on environmental and socioeconomic factors known to influence the distribution of PTEs. Environmental predictors included soil type, pH, slope, elevation, distance to rivers, and distance to the lagoon. Socioeconomic predictors encompassed land use (irrigated and rainfed agriculture), population density, distance to industrial zones and mining areas, proximity to tertiary economic activities, and to road networks [13].

Several studies have reported that specific environmental characteristics, such as clay-rich soils and low pH levels, can enhance the availability and mobility of metals due to their effects on adsorption and solubility processes [22,23]. Similarly, socioeconomic factors such as high population density and intensity of agricultural activities have been linked to increased PTE concentrations [22,24]. Based on this evidence, we assigned each variable an impact value (IV) ranging from 1 to 5, reflecting the strength of its reported association with PTE pollution. For example, population density (commonly correlated with wastewater and surface runoff) was assigned a value of 5, while rainfed agriculture was assigned a lower value of 3, due to its relatively moderate impact. Spatial data for each variable were obtained from national governmental and non-governmental sources [25] and processed as vector layers within the GIS environment. Each variable was reclassified on a five-point ordinal scale (1 = lowest risk, 5 = highest risk) using the natural breaks method. This allowed for the categorization of continuous variables into classes based on their distribution. For instance, pH values between 5.31 and 6.06 were classified as 5, while values between 6.85 and 7.81 were assigned a score of 1.

The final pollution risk index was constructed by multiplying the reclassified value of each variable by its respective IV, generating a weighted spatial layer per variable. These layers were then summed across the entire study area to produce an integrated risk map, highlighting zones with higher cumulative susceptibility to PTE contamination within the MBR.

2.3. Study Population

To assess human health risks, the population was stratified into four age groups: children (0–8 years), adolescents (9–18 years), adults (19–59 years), and the elderly (over 60 years). Population for the corresponding localities was obtained from the National Institute of Statistics and Geography (INEGI, by its Spanish acronym) [25]. The exposure assessment incorporated age-specific physiological parameters, including average body weight and daily water ingestion rates, which were used to estimate chronic oral exposure for each group.

2.4. Determination of Potentially Toxic Elements in Surface Water

Surface water sampling was conducted at six sites within the MBR, following Mexican regulatory guidelines [26]. at six locations within the MBR. Site selection was based on the pollution risk map described in Section 2.2, which identifies locations with the highest potential for PTE contamination. A total of 24 samples were collected during the dry seasons of 2022 and 2023 (August to December), using the direct method described in Mexican regulations [27]. The sampling sites included: Metztitlán River (20°36′9.0138″ N, −98°48′21.1068″ W and 20°39′38.29″ N, 98°50′5.08″ W), a spring south of this river (20°15′1.44″ N, 98°31′57.06″ W), two springs near the Santiago River (20°31′33.20″ N, 98°39′18.33″ W and 20°31′59.09″ N, 98°38′1.80″ W), and the Almolón River (20°43′22.71″ N, 98°54′1.09″ W).

Quality control during sampling and preservation was implemented in accordance with applicable national standards. Samples were collected in acid-washed vials (pre-soaked in 3% HNO₃ and rinsed three times with deionized water) and stored at −18 °C until analysis. Vials were properly labeled on-site [26,27].

Surface water samples underwent microwave-assisted acid digestion prior to quantification, following EPA Method 3015. A volume of 5 ± 0.1 mL of HNO₃ was added to 45 mL of the homogenized water sample in microwave digestion vessels. Then, the vessels were sealed and subjected to controlled digestion conditions (time, pressure, and heating ramp) according to the method’s specifications [28].

Concentrations of Pb and Cd were determined using Graphite Furnace Atomic Absorption Spectroscopy (GFAAS) employing a Spectrometer with a GTA120 Graphite Tube Atomizer (Agilent, Santa Clara, CA, USA). Hg was quantified using the cold vapor generation technique. All samples and blanks were analyzed in triplicate. The analytical sensitivity was assessed by calculating the limits of detection (LOD) and quantification (LOQ). The resulting LODs and LOQs were 0.02 ppm and 0.067 ppm for Pb, 0.0006 ppm and 0.002 ppm for Cd, and 0.0025 ppm and 0.0084 ppm for Hg, respectively. Instrumental calibration was performed prior to each analysis using external calibration curves. Standard concentrations for Cd, Pb, and Hg were 0.000, 0.004, 0.012, 0.016, and 0.020 ppm. The resulting correlation coefficients were as follows: R² = 0.9838 for Cd, R² = 0.9988 for Pb, and R² = 0.9813 for Hg. The selection of GFAAS and cold vapor generation techniques was based on the availability of the equipment and on their capacity to detect concentrations expected to exceed national regulatory limits, as previously reported for the region [29]. Notably, the LODs and LOQs achieved were lower than the Mexican maximum permissible limits for surface water and drinking water for all three PTEs, except for the LOQ of Hg (0.0084 ppm), which slightly exceeds the regulatory limit for drinking water (0.006 ppm) (see

Table 1. Reference concentrations permissible limits for PTEs in Mexican regulations (ppm).

Element	NOM-001-SEMARNAT-2021 1	U.S. National Recommended Quality Criteria	NOM-127-SSA1-2021 2	Toxic Response Index (Tr)
Pb	0.4	0.0025	0.01	5
Cd	0.4	0.0018	0.005	30
Hg	0.02	0.00077	0.006	40

¹ National standard in Mexico that establishes the maximum permissible limits of pollutants in wastewater discharges into national waters and assets; ² National standard in Mexico that indicates permissible limits for water quality in water intended for human use and consumption; Tr, Toxic Response Index.

). The calibration of the equipment was performed prior to the PTE quantification.

Based on the quantified concentration of PTEs in the superficial water, an ecological and health risk assessment was performed. Although this study did not aim to attribute contamination levels to specific sources, potential anthropogenic contributors (e.g., agriculture, wastewater discharge, mining) were discussed in light of previous evidence in the region.

2.5. Risk Assessment

Ecological risk was assessed through the calculation of three indices: the environmental hazard quotient (CP), the potential ecological risk index (ER), and the cumulative ecological risk index (RI). For human health risk assessment, the non-carcinogenic hazard quotient (HQ) and the non-carcinogenic risk index (HI) were calculated.

Given the spatial variability in PTE concentrations across sampling sites in the reserve, the data were not homogeneously distributed. Therefore, to represent contamination scenarios realistically while optimizing resources. Three representative values were selected for each PTE: median, minimum, and maximum concentrations. These values were then used to estimate ecological and human health risks, thereby capturing a range of potential exposure conditions: average (median), worst-case (maximum), and best-case (minimum) scenarios. This strategy reduced the total number of risk assessments from 18 (i.e., individual analysis of Pb, Cd, and Hg at each of the six sampling sites) to 9 (i.e., three contamination levels per PTE), thus enhancing the feasibility of the study. Such an approach is particularly advantageous in studies covering extensive and heterogeneous areas, where site-by-site analysis may be impractical or resource-intensive, as compared to small-scale studies with fewer sampling points [23]. To calculate the median concentrations, survival analysis was applied to account for left-censored data (i.e., concentrations below the detection limit). This method enables the statistically robust estimation of central tendency in datasets that contain non-detects. The detection limits used for left-censoring were 0.02 ppm for Pb, 0.0006 ppm for Cd, and 0.002 ppm for Hg.

2.6. Ecological Risk Assessment

The ecological risk associated with PTE concentrations in surface water was evaluated by calculating the environmental hazard quotient (CP), using Equation (1). This index is defined as the ratio between the measured environmental concentration (Ce) of each PTE and its corresponding reference concentration (Cref) [3].

$$CP={\displaystyle \frac{Ce}{C_{ref}}}$$

(1)

Reference concentrations were obtained from the National Recommended Water Quality Criteria of the United States Environmental Protection Agency [20] (see Table 1), following the recommendation by López et al. [3], who suggested their use to avoid underestimating ecological risks. The use of concentrations from the local reference site (SMA) was discarded, as all measured values were below the detection limits of the analytical equipment. To interpret the ecological risk associated with each PTE, the classification proposed by Sánchez-Olivares et al. [30] was adopted: CP < 1 indicates low risk; CP values between 1 and 1.9 suggest low to moderate risk; values from 2 to 5 indicate moderate to high risk; and CP > 6 reflects high ecological hazard. A CP ≥ 1 is considered to represent potential environmental concern, as it reflects concentrations above ecotoxicological thresholds known to impact aquatic biota, based on the U.S. Water Quality Criteria [21].

2.7. Potential Ecological Risk Index

The potential ecological risk index (Er) was calculated using Equation (2), described by Nozari et al. [31].

$$Er=Tr\times CP$$

(2)

In this equation, Er represents the risk coefficient for each PTE, and Tr denotes the toxic response index specific to each element. According to Hamid and Payandeh [32], Tr values are as follows: Pb = 5, Cd = 30, Hg = 40, and As = 10. Tr values are based on the abundance of each PTE in freshwater, plants, soils, igneous rock, and land animals, the sink factor of each PTE, and the dimension between them. Risk levels associated with Er values were interpreted based on the classification by Hakanson [33]: Er < 40 indicates low risk, 40–80 is considered moderate, 80–160 is considerable, 160–320 is high, and Er > 320 denotes a very high potential ecological risk.

2.8. Cumulative Ecological Risk Index

The cumulative ecological risk index (RI) was calculated by adding the ER of all the analyzed PTEs (Equation (3)) as they are all associated with similar health effects, including impacts on the nervous, circulatory, renal, reproductive, immunological, and hematological systems [33]. Additionally, all four PTEs have been linked to cancer development in animals [10,11,12,34].

$$RI=\sum _{i=1}^{n}Er$$

(3)

In Equation (3), RI is the cumulative ecological risk index, and Er is the potential ecological risk index.

2.9. Health Risk Assessment

The health risk posed by PTEs through oral exposure was assessed, as this is the main pathway for contaminants present in water to enter the body. The evaluation focused on adults, adolescents, children, and the elderly, with the latter two groups being considered vulnerable populations. To calculate health risk, average body weight and water consumption related to each age group were considered (

Table 2. Population groups, average body weight, and water consumption according to age.

Age Group	Age (Years)	Average+ Body Weight (Kg)	Water Consumption (L/day)
Children *	0–11 m	6.90	1
	1–3	12.80
	4–8	24.5 + 5
Adolescents	9–14	39 + 5	1
	15–18	56 + 5
Adults	19–59	73.5 + 10	2
Elderly	>60	65.7 + 10	2

Weight data from Ferreira-Hermosillo et al. 2020 [35], Osuna-Padilla et al. 2015 [36], Uribe-Carvajal et al. 2018 [37], Unikel-Santoncini et al. 2009 [38], and CDC, 2000 [39]. * ATSDR, 2023 [40]; m, months; Kg, kilogram; L, liter; +5 and +10 adjustment to body weight.

).

According to national reports, the prevalence of overweight and obesity in the Mexican population (including children, adolescents, and adults) is among the highest in OECD (Organization for Economic Cooperation and Development) member countries [41]. To account for this, a weight adjustment of 5 kg for children and 10 kg for adults was applied for the analysis.

The group of children is divided into subgroups due to differences in their anatomical and physiological development characteristics that can increase their susceptibility to PTE exposure. Among these, accelerated growth and rapid weight gain, as well as deficiencies in the hepatic and renal systems during the first year of life, are observed [42]; similar cognitive effects related to PTE exposure for children from 1 to 3 years old are also observed, and the greatest susceptibility of cognitive and behavioral development is found in children from 4 to 8 years old [43].

The ages that comprise the adolescent group (10 to 18 years old) are characterized by rapid skeletal growth and changes in the reproductive and endocrine systems. The adult group was not divided into subgroups, as this stage of life is considered more stable, and it is observed that adults absorb fewer PTEs than children. Additionally, regarding lead, in adults, it can be absorbed in bones, so there is a lower risk that lead can reach other tissues [44].

Finally, the Elderly population (>60 years old) is vulnerable to the exposure of PTEs for the following reasons. PTE exposure could lead to an increase in the level of oxidative stress in adults aged 60 years and older [45], and it can aggravate the conditions related to normal aging [46,47].

2.10. Non-Carcinogenic Risk

To calculate the non-carcinogenic risk (HQ), the chronic daily intake (CDI) values for each PTE (as shown in Equation (4)) were first calculated. For this purpose, the ingestion rate and environmental exposure concentration were considered. Subsequently, the HQ for exposure to Pb, Cd, and Hg was calculated by assessing the exposure level through the CDI (Equation (5)).

$$CDI={\displaystyle \frac{CE\times Ti}{BW}}$$

(4)

In Equation (4), CDI represents the chronic intake index (mg/kg/day), CE is the environmental exposure concentration (mg/L), Ti is the ingestion rate (L/day), and BW is the individual’s body weight (kg).

$${HQ}_{oralc}E={\displaystyle \frac{{CDI}_{oral}}{{RfD}_{oral}}}$$

(5)

In Equation (5), HQ_oralcE represents the chronic oral hazard quotient of each PTE (dimensionless), CDI_oral is the chronic intake index, and RfD_oral is the oral reference dose (mg/kg BW-day) (

Table 3. PTE Reference dose for oral exposure and the slope cancer factor.

Element	RfD Oral (mg/kg BW-day)	SCF
Pb	*	0.0085
Cd	0.0005	-
Hg	0.002	-

RfD, oral reference dose; mg, milligrams; kg, kilograms; BW, body weight; SCF, slope cancer factor; * not evaluated by ATSDR or EPA.

).

2.11. Non-Carcinogenic Risk Index

The non-carcinogenic risk index (HI) (Equation (6)) was calculated to account for the simultaneous exposure to the analyzed PTEs associated with possible non-carcinogenic health effects. This index results from the sum of the chronic oral hazard quotients (HQ) for each analyzed element and reflects the risk of developing non-carcinogenic negative effects due to cumulative oral exposure to Pb, Cd, and Hg. Potential health impacts include gastrointestinal, neurological, and renal alterations.

$$HI=\Sigma HQi$$

(6)

In Equation (6), HI is the non-carcinogenic risk index, and ∑HQ is the sum of the chronic oral hazard quotient values of the analyzed PTEs.

To evaluate this, all HI values greater than 1 indicate a potential risk due to chronic exposure to the PTEs [48].

Graphics were generated using Microsoft Excel and OriginPro 8.5, and tables were generated using Microsoft Word.

3. Results and Discussion

In the MBR, increasing urbanization and the expansion of intensive agriculture, and other anthropogenic pressures have created a scenario of potential ecological and human health risks linked to the contamination of surface waters by PTEs [14]. Despite these concerns, a low level of PTE pollution and minimal associated risks were initially expected, given the reserve’s protected status and its geographic distance from the most heavily industrialized and urbanized areas in the state. However, as López et al. [3] point out, there is a lack of pollution research in areas with supposedly low anthropogenic impact, which limits the understanding of environmental risks in such regions. The present study addresses this gap by providing data on PTE concentrations in surface waters and evaluating the corresponding ecological and human health risks. These findings offer a valuable baseline for comparison with more heavily contaminated environments. The results revealed notable spatial variability in PTE concentrations, which translated into differences in both ecological and human health risk levels. This heterogeneity is consistent with patterns observed in other environmental studies and may be explained by variations in land use, hydrological dynamics, and the presence of multiple localized sources of contamination within the reserve [3].

3.1. Map of Risk of PETs Contamination

According to the risk map generated (Figure 2), a substantial portion of the MBR displays a medium to low potential for PTE contamination. Nonetheless, several zones were identified as having medium, medium-high, and high pollution risk levels, primarily corresponding to urbanized areas, regions adjacent to the reserve’s boundary, and zones located near effluent discharges. Specifically, the sections of the Metztitlán River classified as medium to high risk are likely influenced by domestic and urban runoff from the nine municipalities through which the river flows, highlighting the cumulative impact of multiple urban sources. Conversely, the few areas classified as low risk are located in remote regions, relatively isolated from dense population centers and infrastructure, both within and outside the reserve boundaries.

The risk map served as a key tool for selecting sampling locations along the reserve that presented a higher potential for PTE contamination. Based on this spatial analysis, five sites located in areas classified as medium to high risk were selected: Tecruz Cozapa (TC) (20°36′9.0138″ N, −98°48′21.1068″ W), Barrio Nuevo (BN) (20°31′33.20″ N, 98°39′18.33″ W), San Cristóbal (SC) (20°39′38.29″ N, 98°50′5.08″ W), Santa María (SM) (20°31′59.09″ N, 98°38′1.80″ W), and El Vite (EV) (20°15′1.44″ N, 98°31′57.06″ W). Additionally, one site located in a low-risk zone, San Miguel Almolón (SMA) (20°43′22.71″ N, 98°54′1.09″ W), was selected as the reference site (Figure 2).

According to the most recent population census conducted by INEGI [25], the population sizes of the sampled localities are as follows: San Miguel Almolón (496 inhabitants), Tecruz Cozapa (2238), Barrio Nuevo (419), San Cristóbal (4267), Santa María (3160), and El Vite (794). Across all these locations, approximately 48% of the population (range: 47.18–49.50%) falls within vulnerable age groups, specifically children under 11 years of age and adults over 60 years, who are considered at greater risk of adverse health effects from PTE exposure.

3.2. PTE Concentrations in Surface Water

Cd and Hg were detected in all surface water samples collected from the MBR. At least one of these elements was quantified in the samples from Barrio Nuevo (BN), San Cristóbal (SC), and Santa María (SM) in a range of 0.002 to 0.021 ppm for Cd and from 0.007 to 0.066 ppm for Hg (

Table 4. PTE concentrations (ppm) in samples from surface water from the MBR.

Sample Site/Element	Site SMA	Site TC	Site BN	Site SC	Site SM	Site EV
Pb	<LOD	<LOD	<LOD	<LOD	<LOD	<LOD
Cd	<LOD	<LOD	0.021 ± 0.001	0.005.06 ± 0.0002	0.002 ± 0.00006	<LOD
Hg	<LOD	<LOD	0.066 ± 0.007	0.045 ± 0.016	0.007 ± 0.004	<LOD

Site SMA, San Miguel Almolón; Site TC, Tecruz Cozapa; Site BN, Barrio Nuevo; Site SC, San Cristóbal; Site SM, Santa María; Site EV, El Vite; <LOD of 0.02 ppm for Pb, 0.0006 ppm for Cd, and 0.002 ppm for Hg.

).

In Mexico, regulations establish a maximum limit of PTE concentrations in natural waters such as rivers [18] and in water for use and human consumption [49]. When compared with the national drinking water standard [49] for Cd (0.005 ppm), the median Cd concentration in the MBR (0.0058 ppm) exceeded the limit by 1.16 times. Notably, Cd concentrations were highest at BN, where they surpassed the limit by 4.2 times, while at SC the value was close to the regulatory threshold. In the remaining sites, Cd levels remained below the national limit. Regarding Hg, across the MBR was 0.0267 ppm, exceeding the Mexican drinking water limit (0.006 ppm) by a factor of 4.4. At individual sites, the concentrations were especially elevated: 11 times higher at BN, 7.5 times at SC, and 1.17 times at SM. These values underscore the potential severity of Hg contamination in specific areas of the reserve. It is important to note that international standards for water quality tend to be more stringent than those established in Mexico. For instance, the U.S. National Recommended Water Quality Criteria sets lower thresholds for both Cd (0.0018 ppm) and Hg (0.00077 ppm) in surface water [20]. Based on these criteria, the median Cd concentration in the MBR exceeded the U.S. limit by 3.22 times, with site-specific exceedances of 11.66 times at BN, 2.77 times at SC, and 1.11 times at SM. Regarding Hg, the median concentration was 34.61 times the U.S. limit. The exceedances at specific sites were particularly striking: 85.71 times at BN, 58.44 times at SC, and 9.09 times at SM. Although Pb concentrations were below the detection limit of the analytical method used (LOD = 0.02 ppm) at all sampling sites, the potential for lead contamination exceeding national or international standards cannot be ruled out, given the known sources of pollution in the region and the limitations inherent in detection thresholds.

BN showed the highest concentrations of at least two of the PTEs evaluated in this study. When comparing the results with the national maximum allowable limits for PTEs in surface waters from natural effluents (Table 1), Cd levels at sites BN, SC, and SM remained below the established threshold. In contrast, Hg concentrations exceeded the permissible limit of 0.02 ppm at BN and SC by 3.3 and 2.25 times, respectively. Pb concentrations were below the detection limit (LOD = 0.02 ppm) across all sample sites. The elevated PTE concentrations at BN may be associated with its geographical position, as water arriving at this site flows through upstream settlements where domestic wastewater and other anthropogenic discharges may contribute to contamination. Furthermore, local testimonies suggest potential sources of PTEs in BN, including the use of agrochemicals (notably pesticides) and the open burning of various types of waste. Despite the high values observed at BN, it was anticipated that similar concentrations would be detected at TC, SC, and SM, given that all these sites share comparable pollution risk index values. Interestingly, this expectation was not fully met. While previous research by López et al. [3] reported a correlation between Cd and Pb levels in surface waters affected by urban areas, the current study did not detect Pb, likely due to the method’s detection limitations. However, the reported trend for Cd aligns with our findings, particularly at BN and SC, which are both situated near urbanized zones and classified as having medium to high pollution risk levels.

In the MBR, potential local sources of PTEs include the use of agrochemicals, open burning of waste, emissions from vehicles and agricultural machinery, and mechanical wear of their components [4,14]. Additionally, contamination may originate from remote locations, as some water bodies in the MBR receive effluents that traverse various regions with potential PTE contamination sources [14].

This study is among the few recent investigations in Mexico to address both the quantification of PTEs in surface water and the associated ecological and human health risks. Similar research conducted in more heavily impacted regions, such as the studies by Pérez et al. [50] and López et al. [3], reported considerably higher PTE concentrations in surface effluents in central and northeastern Mexico. For example, Pérez et al. [50] found Pb concentrations of 0.008 ± 0.007 ppm in spring water and 0.006 ± 0.015 ppm in well water, while López et al. [3] reported Pb concentrations ranging from 0.3 ± 0.17 ppm to 2.98 ± 1.52 ppm and Cd concentrations from 0.02 ± 0.05 ppm to 0.4 ± 0.24 ppm. These elevated levels likely reflect greater anthropogenic pressures in those study areas [47]. In contrast, although anthropogenic activities within the MBR—such as agriculture and small-scale urban development—may contribute to environmental pollution, the semi-protected status of the reserve and its geographic isolation from large industrial zones were expected to result in comparatively lower contamination levels. This expectation was largely confirmed, as the concentrations detected in this study were below or close to national regulatory limits and generally lower than those reported in studies from more polluted regions. International studies conducted in heavily contaminated sites have reported even higher PTE levels, with concentrations that, in some cases, exceeded those obtained in the present study by over 400 times [51,52,53], underscoring the relative moderation of pollution within the MBR.

To support the risk assessment, both the lower and upper bounds of the measured concentrations were considered. For Cd, concentrations ranged from 0.002 ± 0.00006 ppm to 0.021 ± 0.001 ppm, while for Hg, values ranged from 0.007 ± 0.004 ppm to 0.066 ± 0.007 ppm.

3.3. Ecological and Human Health Risk Assessment

The risk assessment of Cd and Hg in the surface water of the MBR provides valuable insights into the potential impacts on both ecosystem integrity and public health. These elements are associated with multisystemic damage in exposed organisms, and have been linked to a wide range of adverse health effects. For instance, Hg can impair the nervous, cardiovascular, and reproductive systems while Cd has been linked to dysfunctions in the respiratory, nervous, and gastrointestinal systems [2]. The detection of these elements at concentrations exceeding national and international safety thresholds highlights the potential health risks for populations within the reserve. These findings underscore the urgency of implementing expanded monitoring strategies across additional sampling sites in the MBR to delineate spatial variability in risk levels and to support the development of targeted mitigation and public health strategies.

3.4. Ecological Risk Assessment

The environmental hazard coefficient (CP) calculated for Cd, based on its median concentration in the MBR (CP = 0.0145), was below the threshold of 1, indicating a low environmental hazard. Similarly, CP values for Cd remained below 1 across all sampling sites (Figure 3), suggesting a consistently low environmental risk from Cd exposure in surface waters of the studied agroecosystems. In contrast, the CP value for Hg, calculated using its median concentration, was 1.33, which corresponds to a low-to-moderate environmental hazard. Notably, at site BN, the CP for Hg exceeded 3, placing it within the range of moderate-to-high hazard (CP = 2–5). Meanwhile, at site SM, the CP for Hg remained below 1, indicating low environmental concern. These findings indicate that, while Cd does not currently represent a significant ecological threat in the evaluated sites, the concentrations of Hg—especially at site BN—warrant increased attention due to their potential to negatively impact aquatic ecosystems within the reserve.

The risk assessment in this study followed a classification in which a CP value greater than 6 indicates high risk, values between 2 and 5.9 indicate medium risk, values from 1 to 1.9 represent low to medium risk, and CP values below 1 indicate low risk [48]. This categorization differs slightly from that used by López et al. [3], who defined CP values below 0.1 as no risk, 0.1–1.0 as low risk, 1.1–10 as moderate risk, and values above 10 as high risk.

In the present study, CP values for Cd remained below 1 at all sites, indicating low environmental hazard. These values are considerably lower than those reported by López et al. [3], who observed CP values for Cd ranging from 0 to 149 in the Marcos River. This discrepancy may be attributed to several factors, including higher levels of pollution from domestic wastewater discharges and reduced river flow reported in that region. Despite the lower concentrations of Pb and Cd in the MBR, an ecological risk cannot be ruled out due to the toxic potential of these elements. Despite the relatively low concentrations of Cd and Pb detected in the MBR, the potential for ecological risk cannot be entirely ruled out, particularly considering the toxic nature of these elements. The ecological risk analysis based on the limits of detection (LOD) and quantification (LOQ) of the method used (GFAAS) (

Table 5. Ecological risk (dimensionless) based on LOD and LOQ of the GFAAS.

LOD				LOQ
	CP	Er	RI	CP	Er	RI
Pb	0.05	0.25	5.5	0.17	0.85	17.8
Cd	0.002	0.05		0.01	0.15
Hg	0.13	5.2		0.42	16.8

Limit of detection (LOD), limit of quantification (LOQ), Environmental hazard coefficient level (CP), Ecological Risk Index (Er), Accumulated Ecological Risk Index (RI).

) suggests a low risk for concentrations below the LOD. However, given that some Hg concentrations were categorized as medium to high risk based on CP values, there is a clear need to identify potential Hg sources and implement regular pollution monitoring in specific areas of the MBR to prevent further contamination.

3.5. Ecological Risk Index (Er)

Considering the toxic response factor (Tr) of each element and the calculated environmental hazard quotient (CP), the potential ecological risk index (Er) associated with Cd was found to be below 40, based on the median Cd concentration in the MBR (Er = 0.44), as well as at sites BN and SM. According to the classification by Hamid et al. [32], this indicates a low ecological risk. In contrast, the Er calculated for Hg using its median concentration in the MBR was 53.3, exceeding the threshold of 40 and indicating a moderate ecological risk. The highest Hg-related Er value was observed at site BN (Er = 132), corresponding to a considerable to high ecological risk. This result is consistent with the environmental hazard coefficient (CP) calculated for Hg at this same site. Conversely, the Er for Hg at site SM was below 40, thus classified as low risk. It is important to note that variability in the methodological approaches and risk indices used across studies complicates the direct comparison of results. For example, López et al. [3] conducted an ecological risk assessment in Mexico, but they did not calculate Er, nor did they incorporate the toxic response factor of each element or consider the potential cumulative effects of multiple PTEs.

In summary, while Cd posed a consistently low ecological risk across all sampled sites, Hg presented a moderate to high ecological risk at site BN, highlighting the need for targeted monitoring and control measures. Furthermore, other studies, such as that of Iqbal et al. [51], have applied alternative methodologies, including the use of toxic units based on aquatic organism sensitivity, which reinforces the idea that results must be interpreted in light of the analytical framework employed.

3.6. Accumulated Ecological Risk Index (RI)

According to the classification proposed by Nozari et al. [31], RI calculated for the MBR (RI 53.74) falls in the category of low accumulated ecological risk (RI < 150) associated with exposure to Hg and Cd concentrations in surface water. Similarly, sites BN and SM also fell into this low-risk category. However, the RI value for site BN was 133.6, which is close to the upper threshold of the low-risk classification, suggesting that this location may be approaching a transition toward a moderate cumulative ecological risk. This relatively elevated RI at site BN is primarily attributed to the higher concentration of Hg detected there, which notably increased both the CP and Er at that site. These findings underscore the need for ongoing monitoring, particularly in areas like BN, where localized contamination may be contributing disproportionately to the overall ecological risk within the reserve.

3.7. Health Risk Assessment

The non-carcinogenic risk was evaluated for Cd and Hg due to their documented association with a range of adverse health effects. A carcinogenic risk assessment was not conducted as none of the evaluated elements are classified as carcinogenic via oral exposure. However, it is essential to note that Cd is classified as carcinogenic to humans through inhalation, whereas Pb and Hg are categorized as possible human carcinogens [5,6,7,34].

According to the previous studies, the presence of PTEs in surface water can contribute to groundwater contamination through the infiltration process [54]. In MBR, the prevalence of sandy soils may facilitate the percolation of these contaminants, thereby increasing the risk of subsurface water pollution. The Cd concentrations detected in surface waters at several localities within the MBR represent a potential exposure hazard for the local population. This concern is underscored by data from the Registro Público de Derechos de Agua (REPDA, for its Spanish acronym) [55], which indicates that part of the water used for domestic consumption in the region originates from surface water sources, with an annual extraction volume of approximately 876 m³. Moreover, groundwater sources used for the same purpose (representing an annual extraction of 4095 m³) could also be at risk of contamination by PTE leaching from surface water bodies [56].

3.8. Non-Carcinogenic Risk (HQ)

The median Cd concentration in the MBR suggests a potential non-carcinogenic risk from oral exposure for infants aged 0–11 months (HQ > 1). For the remaining age groups, the HQ values (ranging from 0.19 to 0.91) indicate no significant non-carcinogenic risk. However, the HQ for children aged 1–3 years (HQ = 0.91) approaches the upper limit of what is considered non-significant, suggesting that this age group may also be vulnerable to Cd exposure through water in the MBR. At site BN, Cd poses a potential non-carcinogenic risk for both children and adults (HQ > 1). Infants aged 0–11 months and children aged 1–3 years are the most susceptible groups, with HQ values of 6.09 and 3.28, respectively. In contrast, at site SC, all HQ values were below 1 for every age group, indicating no significant non-carcinogenic risk.

In terms of Hg oral exposure, the HQ calculated using the median Hg concentration in water across the MBR indicates a potential non-carcinogenic risk for all age groups (HQ > 1). Children aged 0–3 years were the most susceptible, showing the highest HQ values (range: 6.94–12.87), while individuals aged 4–60 years had lower values (range: 1.46–3.01). Similarly, at site BN, all age groups exhibited HQ values above 1. Infants aged 0–11 months and children aged 1–3 years displayed particularly elevated HQs of 31.88 and 17.19, respectively. The remaining age groups had HQ values ranging from 3.1 to 7.46. At site SM, infants (0–11 months) and children aged 1–3 years also showed potential non-carcinogenic risks (HQ range: 1.82–3.38), while the rest of the age groups had HQ values below 1, indicating no significant risk. These results are represented in Figure 4 with a color gradient from green to red that represents lower to higher HQ respectively.

HQ values calculated based on the limits of detection (LOD) of the GFAAS indicated a non-significant non-carcinogenic risk from oral Cd exposure for all age groups (HQ range: 0.02–0.17). Similarly, HQ values calculated based on the limits of quantification (LOQ) also indicated a non-significant risk for most age groups (HQ range: 0.28–0.57), except for children aged 0–3 years, for whom a potential non-carcinogenic risk was observed (HQ range: 1–20.29). For Hg, HQ values calculated at the LOD level revealed a non-significant risk for most age groups. However, infants aged 0–11 months showed a potential non-carcinogenic risk (HQ = 1.45), even at these low concentrations. Conversely, HQ values based on the LOQ for Hg exposure indicated no significant non-carcinogenic risk across all age groups. These results suggest that non-carcinogenic risks associated with Cd exposure at the LOD of GFAAS are negligible. However, vulnerable groups such as infants may still face potential health risks from Hg, even at concentrations close to the LOD. This highlights the importance of analytical sensitivity in risk assessments. Given that Cd and Hg concentrations in sites SMA, TC, and EV were below the LOD of the GFAAS, a low potential non-carcinogenic risk is expected for these locations. Nonetheless, for future monitoring and risk assessment of PTEs in the region, the use of more sensitive techniques such as Inductively Coupled Plasma Mass Spectrometry (ICP-MS) is recommended. ICP-MS can detect PTEs at parts-per-trillion (ppt) levels, providing improved detection limits and enhancing the precision of risk estimation, particularly in areas where concentrations fall below the detection capacity of GFAAS.

Children and older adults are among the most vulnerable groups to PTE exposure (Figure 4). Polluted water can pose a health risk to children under 12 months in addition to other possible sources of PTEs [56]. In Mexico, it is estimated that up to 22.2% of children under 2 years old are exclusively formula-fed with milk from birth [57], raising concerns about the potential for exposure to contaminated water in the MBR. Sharafi et al. [58] reported an HQ values ranging from 0.06 to 0.12 for Cd exposure through breast milk in children aged 1 to 12 months. In comparison, the HQ values calculated in this study for Cd exposure via water for the same age group exceed the highest value reported by Sharafi et al. [58] by a factor of 50. Similarly, the HQ for Hg exposure exceeds the range reported by the same study (HQ: 0.26–0.50) by approximately 63 times. These findings underscore the need to identify and control the sources of Cd and Hg pollution in the water of the MBR. They also support promoting the consumption of purified water, especially among vulnerable populations. It is recommended that awareness campaigns and educational activities be implemented to encourage local communities to adopt healthy water consumption practices and reduce exposure to PTEs. This is particularly important for young children, given the potential non-carcinogenic risks estimated for the 0–3 years age group, even at concentrations equivalent to the GFAAS detection limits. Since this study cannot determine the risk associated with exposure to PTE concentrations below the LOD, future assessments should prioritize the use of more sensitive analytical methods and develop targeted public health interventions.

3.9. Non-Carcinogenic Risk Index (HI)

The non-carcinogenic risk index (HI) was calculated to evaluate the potential health risk associated with simultaneous exposure to multiple PTEs in water, using the median concentrations of Cd and Hg. HI values ranged from 1.64 to 14.56 across all age groups (

Table 6. Non-carcinogenic risk index (HI) (dimensionless) for oral exposure to Cd and Hg at sample sites of the MBR.

Age Group	Age (Years)	Median MBR	Site BN	Site SM
Children	0–11 *	14.56	37.9	3.96
	1–3	7.85	20.47	2.14
	4–8	3.4	8.88	0.93
Adolescent	9–14	2.28	5.95	0.62
	15–18	1.65	4.3	0.45
Adults	18–40	2.7	7.03	0.73
	40–60	2.5	6.55	0.68
Elders	>60	3.35	8.73	0.91

* m months.

), indicating a potential non-carcinogenic health risk. Among the sampling sites, more than one PTE was detected in BN, SC, and SM. HI was specifically calculated for sites BN and SM, representing the highest and lowest levels of PTEs, respectively. In site BN, HI values ranged from 4.3 to 37.97 (Table 6), indicating a potential health risk for all age groups due to chronic oral exposure to Cd and Hg through water. In contrast, at site SM, HI values ranged from 0.62 to 3.96. Here, age groups from 4 to over 60 years presented a negligible non-carcinogenic risk due to chronic co-exposure to Cd and Hg (HI < 1). However, infants (0–11 months) and toddlers (1–3 years) showed a potential health risk related to PTE exposure (HI > 1).

The HI values obtained in this study differ from those reported in previous research, which is expected due to the variability in the HQ values used for their calculation, these, in turn, depend on the specific PTEs analyzed in each study. For example, Iqbal et al. [51] calculated the HI based on HQs for Cu, Cr, Ni, and Pb, while Pérez et al. [50] reported an HI < 1, indicating a low cumulative risk. In contrast, the present study identified HI values corresponding to a potential health risk (HI > 1) in two sampling sites, particularly among children aged 0 to 3 years.

The HI values calculated using the LOD of the GFAAS (

Table 7. Non-carcinogenic risk index (HI) (dimensionless) for oral exposure to Cd and Hg based on the LOD and LOQ of the analytical method used (GFAAS).

Age Group	Age (Years)	HI for LOD	HI for LOQ
Children	0–11 *	1.62	21.26
	1–3	0.88	1.83
	4–8	0.38	0.8
Adolescent	9–14	0.25	0.53
	15–18	0.18	0.38
Adults	18–40	0.3	0.63
	40–60	0.28	0.59
Elders	>60	0.37	0.78

NMonthscinogenic risk index (HI), Limit of detection (LOD), limit of quantification (LOQ), * Months.

) indicated a potential risk for infants aged 0-11 months and a negligible non-carcinogenic risk due to simultaneous exposure to Cd and Hg across age groups from 1 to > 60 years. These results suggest that individuals potentially exposed to PTE concentrations below the detection limits of the analytical method are unlikely to experience adverse health effects from oral co-exposure. However, when HI was calculated using the LOQ values of the GFAAS, results also indicated a low risk for most age groups, except for infants aged 0–11 months and toddlers from 1 to 3 years, for whom a potential risk was identified. These findings emphasize that even low concentrations of PTEs, close to the LOQ, may pose a significant non-carcinogenic health risk to vulnerable populations, particularly infants, when simultaneously exposed to Cd and Hg.

This study provides valuable insights into the concentrations of potentially toxic elements (PTEs) in surface water and their associated ecological and human health risks in the RBBM. Nevertheless, certain limitations must be acknowledged. The sample size was relatively limited (n = 24), and access to some sampling sites was restricted due to their location on private property, requiring prior authorization from local residents. These conditions constrained the ability to perform robust statistical comparisons among sampling sites regarding PTE contamination and the associated risk levels. Additionally, in the health risk assessment, some degree of uncertainty persists due to the lack of consideration of intra-group variability, such as differences in individual metabolism, socioeconomic conditions, and alternative exposure routes. Despite these limitations, the study presents a comprehensive estimation of health risk by integrating vulnerability considerations across different life stages. Overall, the findings contribute meaningful baseline data on PTE contamination in the RBBM and underscore the potential ecological and health risks associated with these elements, thereby supporting future research and public health decision-making.

4. Conclusions

Surface waters in the MBR contain detectable concentrations of Cd and Hg, likely originating from anthropogenic activities such as intensive agriculture and urban development. Given the ecological importance and socioeconomic relevance of the MBR, continued monitoring and risk assessment of potentially toxic elements (PTEs) is imperative. This study is the first to evaluate the ecological and human health risks associated with PTEs in surface waters of the MBR. The results indicate spatial variability in PTE concentrations across the reserve, which may be attributed to the diversity of local and upstream sources of contamination.

Cd and Hg concentrations exceeded both Mexican regulatory limits for water intended for use and human consumption, as well as international standards. Although overall PTE concentrations were lower than those reported in other regions of Mexico and worldwide, Hg still poses a potential low-to-moderate ecological risk throughout the MBR, and a moderate-to-high risk at the BN sampling site. In contrast, the cumulative ecological risk index (RI) indicates a low overall ecological risk from simultaneous exposure to Cd and Hg in surface waters. From a human health perspective, oral exposure to Cd and Hg poses a potential non-carcinogenic risk, particularly for vulnerable populations such as children and older adults. Among these groups, individual or combined exposure to Cd and Hg may lead to health risks even at low concentrations

While some findings are cause for concern, it is essential to note that a relatively small sample size limited the current assessment. Future studies with larger datasets will enable statistical comparisons between sites and facilitate a more accurate identification of areas with elevated ecological and health risks. It is also recommended that future monitoring efforts incorporate more sensitive analytical techniques, such as ICP-MS, to detect PTEs at ppt levels. Finally, the implementation of community outreach programs is recommended to reduce potential exposure, especially among young children, and to promote safe water consumption practices. Preventive measures are essential to mitigate the long-term risks associated with PTE exposure. Moreover, the PTEs examined in this study represent only a subset of many hazardous substances that may continue to accumulate in the environment over time.