Multi-Indicator Assessment of Heavy Metals Contamination and Ecological Risk Around the Landfills of the Boruta Zgierz Dye Industry Plant in Central Poland

Abstract

This study assesses the extent of heavy metals (HMs) contamination and the associated ecological risks in soils surrounding waste landfills at the former Boruta Dye Industry Plant in Zgierz, Poland. Soil samples were collected during two sampling campaigns (summer 2023 and winter 2024) from 13 locations. Concentrations of Cu, Ni, Zn, Pb, and Cd were measured, and contamination levels were evaluated using several indices: geoaccumulation index (I_geo), pollution index (PI), pollution load index (PLI), Nemerow integrated pollution index (NIPI), ecological risk factor for a single metal (E_rⁱ), index of potential ecological risk (ERI). The highest I_geo value (10.95) was recorded for Cu in the area of the old landfill, which had been in operation for 90 years. The average PI values were Cu—120.97, Pb—52.46, Cd—46.70, Zn—22.19, and Ni—5.38, indicating considerable (3 ≤ PI < 6) to high (PI ≥ 6) contamination levels. The NIPI values, in descending order, were Cu (2102.2) > Pb (270.7) > Zn (88.3) > Cd (62.8) > Ni (21.5), all reflecting high (NIPI >3) contamination levels. The highest PLI was 5.10, with all remaining values exceeding the contamination threshold (PLI >1). The E_rⁱ value for Cu reached 14,852.75, indicating an extremely high (E_rⁱ ≥ 320) ecological risk. The average ERI value across the study area was 1347.2, suggesting a severe (ERI ≥ 600) ecological threat. These findings confirm that the industrial landfills associated with the dye plant constitute a critical pollution hotspot. The results underscore the urgent need for ongoing environmental monitoring, risk mitigation, and site remediation to prevent further environmental degradation and potential contamination of nearby water bodies.

1. Introduction

The generation of waste from industrial activities poses significant and multifaceted environmental challenges, particularly with regard to the contamination of soil and water resources. Industrial landfills, often containing complex and poorly segregated waste materials, serve as long-term sources of pollution. Among the primary mechanisms facilitating the spread of contaminants beyond landfill boundaries is water infiltration, which acts as a transport medium for hazardous substances, enabling their migration into deeper soil horizons and groundwater reservoirs [1].

When assessing the environmental impact of landfills, it is crucial to consider not only direct contamination but also the broader, transformative effects these sites exert on surrounding landscapes and ecosystems. The development, operation, and, often, inadequate post-closure management of landfill sites impose considerable ecological pressure, resulting in diverse forms of environmental degradation. The contaminants can affect multiple environmental compartments—soil, air, surface water, and groundwater—either individually or through synergistic interactions. Among the most critical emissions from landfills are leachate, airborne particulates, volatile and semi-volatile organic compounds (VOCs and SVOCs), microbial agents, and odorous substances [2].

The generation of leachate is primarily driven by complex chemical, biological, and physical processes within the landfill body. This liquid effluent, composed of dissolved organic matter and minerals resulting from microbial degradation, often contains elevated concentrations of heavy metals (HMs), such as Cd, Pb, Cu, Zn, and Ni. These metals pose a substantial ecological risk, as they can be mobilized in aqueous forms and transported beyond landfill confines, where they accumulate in soils and biota [3].

In addition to leachate-related pathways, airborne transport of contaminants represents a significant environmental concern. Dust emissions, particularly under dry and windy conditions, enable the dispersion of fine particles laden with pollutants. Industrial processes such as coal combustion, metal smelting, biomass fuel production, and dye manufacturing contribute to the formation of fine-grained waste, which is especially prone to wind-driven dispersal [4]. Once airborne, these particles may travel considerable distances and ultimately deposit on surrounding soils, crops, or water bodies, amplifying the spatial extent of contamination.

Landfills are also known sources of bioaerosols-airborne particles containing microorganisms, including both saprophytic and pathogenic species. Their release and dispersal, facilitated by wind, raise concerns about biological contamination of nearby ecosystems and potential public health risks [5,6]. Furthermore, the high flammability of certain industrial wastes increases the likelihood of landfill fires, which can release toxic fumes, HMs, and particulates into the atmosphere, exacerbating both short-term and chronic exposure risks [7].

The HMs are of particular concern due to their persistence, toxicity, and potential for bioaccumulation. Defined by their high atomic weight and density (typically >4.5 g/cm³), these elements are widely used in numerous industrial processes, including electroplating, dye and pigment production, agrochemical synthesis, and construction material manufacturing. Among these applications, dye and pigment production is particularly notable for its substantial contribution to environmental contamination. The HMs are commonly used as integral components of synthetic dyes or as catalysts in production processes. The resulting industrial waste often contains persistent and toxic residues, which, if inadequately treated or illegally disposed of, can accumulate in soils and sediments. This makes former dye industry sites key areas of concern for long-term monitoring due to their high potential for ecological risk and HMs mobility. Given their persistence in the environment and tendency to accumulate in biological tissues, HMs released from industrial sources, such as dye production, pose significant risks not only to ecosystems but also to human health and food safety. Excessive or prolonged exposure to HMs in humans can result in serious health consequences such as neurotoxicity, developmental disorders, and carcinogenic effects. In plants, the HMs disrupt nutrient uptake and metabolic functions, leading to phytotoxicity and reduced biodiversity [8].

While landfills can release a variety of pollutants, the present study concentrates solely on HMs contamination in the surrounding soil. The aim was to analyze the state of contamination and potential ecological risks of selected HMs by using several indices. The focus on HMs is justified by the industrial profile of the study area, which is closely linked to the manufacture of dyes. The HMs are frequently used as ingredients in pigment formulations in such processes or may be introduced inadvertently as contaminants during production. Due to their non-biodegradable nature, high toxicity, and tendency to accumulate in soils and biota, HMs pose a serious, long-lasting environmental threat. Consequently, investigating their presence and distribution is essential for understanding the scale of pollution and potential ecological risks resulting from prolonged industrial activity in dye-producing regions.

This study’s novelty lies in its focus on landfills characterized by complex and heterogeneous waste types. This distinguishes it from the majority of existing research, which predominantly addresses municipal solid waste (MSW) landfills, including three distinctly different landfill types—ranging from inert and non-hazardous municipal waste sites to industrial and hazardous waste deposits that have been disposed of in an uncontrolled manner over several decades—offers a unique foundation for evaluating contamination patterns and ecological risks.

By integrating contamination levels with ecological impact metrics, this work contributes valuable data for long-term environmental monitoring and offers a replicable approach for evaluating similar post-industrial sites and the development of sustainable remediation strategies. Furthermore, this work aligns with several United Nations Sustainable Development Goals (SDGs), including inter alia SDG 3 (Good Health and Well-Being) by identifying hazards that may affect surrounding communities, SDG 11 (Sustainable Cities and Communities) through responsible post-industrial land management, SDG 15 (Life on Land) by promoting soil conservation and pollution prevention.

2. Materials and Methods

2.1. Characteristics of the Study Area

The landfill sites are located in the western part of Zgierz, Poland (Figure 1).

The area surrounding the landfills exhibits significant environmental degradation, primarily resulting from long-term anthropogenic pressures associated with the operations of the former Boruta Dye Industrial Plant (DIP) and the currently functioning Boruta Zgierz Industrial Park. This industrial complex encompasses a municipal wastewater treatment facility, a heating plant, and multiple private enterprises, all contributing to the cumulative environmental impact. Pursuant to the Act of 16 June 2023 on large-scale degraded areas [9], the landfills, along with the broader territory historically occupied by the Boruta Dye Factory in Zgierz, have been formally classified as a large-scale degraded area. This designation necessitates immediate environmental remediation measures. Specifically, the landfill sites require urgent reclamation and the removal of both industrial and municipal waste deposits. Failure to implement such interventions may result in a severe ecological crisis, including the contamination of both surface and groundwater resources, as well as the soils.

To the west, the landfills border on the S14 expressway. One of the investigated landfills is located directly on the banks of the Bzura River. On the eastern side of Miroszewska Street are the former Boruta DIP buildings, now used for private business purposes. Site no. 1, marked on the map (Figure 1), is a landfill for non-hazardous and inert waste owned by the company “Waterworks and Sewers”. Landfill no. 2 is a landfill for hazardous and non-hazardous waste. It is a post-production landfill for fly ash and gypsum waste, commonly referred to as a “dry landfill,” which has been in operation since 1960. After the official cessation in 1986, the landfill continued to be used for approximately another 10 years for the disposal of gypsum and calcium salt by-products from the production of 2,7-naphthalenedisulfonic acid, as well as gypsum from the manufacture of H-Acid. According to historical waste classification systems, these materials were categorized as hazardous, falling under classes II and III of harmfulness. It is estimated that from 1990 onward, the annual input of waste amounted to approximately 1500 Mg per year. The disposal of gypsum waste at this site was officially discontinued in 1996. In addition to the registered waste, post-production waste of unknown origin was buried in barrels and containers covered with rubble, municipal waste, and soil. Between 2012 and 2015, significant quantities of waste, including hazardous waste and other types not permitted for disposal at this facility, were illegally deposited. The waste accumulated in this landfill area also included personal protective equipment used by the military or civil defense, barrels with unidentified contents, and MSW. Furthermore, asbestos-containing waste was deposited without proper containment, posing a serious risk to human health and the environment [10]. Due to the scale of illegal waste disposal at this site, the presence of hazardous substances, and the lack of adequate environmental safeguards, the landfill has been referred to in media reports and public discourse as the “Polish Chernobyl”.

Landfill no. 3 is the oldest, close to the Bzura River, called “Behind the Bzura”. It operated for 90 years, was closed in 1995, and is now overgrown with vegetation. This landfill site was used to dispose of industrial waste generated by the former Boruta DIP. Based on the technological processes carried out at the facility, the types of raw materials used, and the results of waste characterization studies, the deposited materials can be classified as both hazardous and non-hazardous waste according to current standards. Due to the historical nature of the disposal activities, the total volume of waste deposited prior to 1990 remains unknown. However, since systematic waste documentation was introduced in 1990, the recorded annual waste input has amounted to approximately 270 Mg per year [10]. The landfill lacked the technical infrastructure required to protect it against erosion or flooding during periods of high water levels in the nearby river. Furthermore, the site was not protected against precipitation infiltrating the waste layers, which could contaminate both surface and groundwater. The landfill was not equipped with any systems for collecting and draining stormwater or leachate. Additionally, neither the top surface nor the landfill’s slopes were sealed, significantly increasing the potential for uncontrolled leachate migration into the environment [10].

2.2. Soil Sampling and Laboratory Analysis

Soil sampling was carried out in a publicly accessible area during two distinct periods: late summer 2023 and winter 2024. To evaluate the level of contamination, samples were collected from the topsoil layer at a depth of 0.15–0.20 m. The sampling locations designated Z1 through Z13 (Figure 2) were strategically distributed around the periphery of former landfill sites to enable an assessment of the spatial extent of HMs occurrence.

The sampling points were selected to cover the areas surrounding all three of the investigated landfill sites. Only unfenced locations that allowed free access were considered to ensure legal and safe entry for sample collection. The field-collected samples were subsequently prepared for analysis in the laboratory and tested for the HMs (Zn, Cd, Cu, Ni, Pb) content by atomic absorption spectrometry (AAS) method, following the procedure described in a previous study [11]. At each sampling location, three subsamples were collected within a radius of 1 m and combined to create a homogenized composite sample, thereby minimizing the effects of small-scale spatial variability. The soil samples were stored in clean polyethylene containers to prevent contamination. Prior to analysis, all samples were air-dried at room temperature, sieved through a 2 mm mesh, and thoroughly homogenized. Each composite sample was analyzed in triplicate. Moreover, blank samples were analyzed in each analytical run to identify any background contamination from reagents, glassware, or laboratory conditions. As all blank values were below the method detection limits, this indicated the absence of significant contamination and confirmed the validity of the analytical procedure. All reagents used were of analytical grade purity. Before measurements, the analytical instrument used for HMs analysis was calibrated using multi-point standard calibration curves consisting of four to five concentration levels for each HMs. Calibration standards were prepared from certified stock solutions to ensure accuracy and traceability.

2.3. Indicators of Heavy Metal Contamination

The following indicators were considered to assess the levels of contamination:

Geoaccumulation index—I_geo

The I_geo is used to assess the degree of anthropogenic accumulation of HMs in the soil regarding the geochemical background (

Table 1. Geochemical background for most areas of Poland [12].

HMs	Limits of Geochemical Background [mg/kg]	The Value of the Geochemical Background Used for Calculations [mg/kg]
Zn	32–40	36
Cd	0.1–0.6	0.3
Cu	2–30	14
Ni	1–6	3
Pb	8–25	8

), which corresponds to the concentrations of HMs in the natural environment.

The I_geo was calculated according to the formula [13]:

I_geo = log₂ (C_n/1.5 B_n)

(1)

where:

C_n—concentration of a given HM in the soil [mg/kg],

B_n—geochemical background (natural content in the earth’s crust) [mg/kg],

1.5—correction factor, which reflects natural variations in the concentration of a given HM in the soil.

The presence of HMs in soil originates naturally from the weathering of rocks that contain these elements at specific concentrations. However, elevated levels of HMs often signal external inputs, particularly from anthropogenic activities such as industrial emissions, waste disposal, or agricultural practices. The geochemical background is essential in distinguishing between naturally occurring concentrations and those resulting from human activity, thereby enabling more accurate identification of contamination sources [9]. In this context, the I_geo, developed by Müller [13], is commonly used to assess the degree of HMs pollution. This index categorizes contamination into seven distinct classes, as outlined in

Table 2. Classes of the I_geo index.

Class	Igeo	Degree of Contamination
0	≤0	Uncontaminated
1	0–1	Uncontaminated to moderately contaminated
2	1–2	Moderately contaminated
3	2–3	Moderately to heavily contaminated
4	3–4	Heavily contaminated
5	4–5	Heavily to extremely contaminated
6	>5	Extremely contaminated

.

Pollution index—PI

The pollution index is the ratio of the average HMs pollution to the geochemical background value, calculated according to the formula [14]:

PI = C_n/B_n

(2)

The degree of contamination for each soil sample may be determined based on the reference values provided in

Table 3. PI values and specific degree of contamination [14].

Value	Degree of Contamination
PI < 1	Low contamination
1 ≤ PI < 3	Moderate contamination
3 ≤ PI < 6	Considerable contamination
PI ≥ 6	High contamination

.

Pollution load index—PLI

The PLI is used to assess the degree of contamination of soils and sediments. It is calculated as the nth-degree root of the number of n-multiplied CF values [15]:

PLI = (CF₁ × CF₂ × CF₃ × … × CF_n) ^1/n

(3)

where:

CF—contamination factor,

n—number of elements.

PLI can be classified according to the following scale: no pollution if PLI  < 1, moderate pollution if 1  ≤  PLI  <  2, heavy pollution if 2  ≤  PLI  <  3, and extremely heavy pollution if PLI  ≥  3 [16].

To calculate the CF value, the following formula is used [15]:

CF = C_S/C_RefS

(4)

where:

C_S—total HM concentration [mg/kg],

C_RefS—reference content of the HM in uncontaminated soil [mg/kg].

Nemerow integrated pollution index—NIPI

The NIPI, developed by Nemerow [17], was used to assess soil quality:

NIPI = ((PI²_ave + PI²_max)/2) ^0.5

(5)

where:

PI²_ave—average value of the pollution index for an HM squared,

PI²_max—maximum pollution index value for the HM squared.

Based on the obtained results, specific soil contamination degrees were assigned according to the classification criteria outlined in

Table 4. Characteristics of the NIPI value [17].

Value	Degree of Contamination
NIPI ≤ 0.7	No contamination
0.7 < NIPI ≤ 1	Warning of contamination
1 < NIPI ≤ 2	Low level of contamination
2 < NIPI ≤ 3	Moderate level of contamination
NIPI > 3	High level of contamination

.

2.4. Indicators of Ecological Risk

Studies on ecological risk indicators were conducted to determine how the HMs may affect the environment. The ecological risk was classified according to a five-degree class (

Table 5. Metal toxicity index [17] and degrees of ecological risk [13].

HM	Toxicity Index Tj	Value Eri	Degree of Ecological Risk
Ni	5	<40	Low risk
Cu	5	40 ≤ Eri < 80	Moderate risk
Zn	1	80 ≤ Eri < 160	High risk
Pb	5	160 ≤ Eri < 320	Very high risk
Cd	30	≥320	Extremely high risk

).

Ecological risk factor for a single metal—E_rⁱ [18]:

E_rⁱ = T_j ∙ CF

(6)

where:

T_j—toxicity index of a given HM (Table 5),

CF—contamination factor.

Index of potential ecological risk—ERI

The ERI—the sum of all the ecological risk coefficients of the HMs—was calculated using the formula [14]:

ERI = ∑E_rⁱ

(7)

The ecological risk grades based on the ERI values are shown in

Table 6. Ecological risk grades for ERI values [14].

Value ERI	Ecological Risk
<150	Low risk
150 ≤ ERI < 300	Moderate risk
300 ≤ ERI < 600	High risk
≥600	Very high risk

.

3. Results and Discussion

3.1. Analysis of HMs Contamination in Soil

3.1.1. Geoaccumulation Index—I_geo

The calculated I_geo values for Ni ranged from −0.07 to 4.32, indicating variation from unpolluted to heavily polluted conditions. The lowest value was recorded in sample no. 8 during the first sampling campaign, suggesting minimal anthropogenic influence at that location. In contrast, the highest value, recorded in sample no. 12 (also during the first sampling), may reflect localized inputs from historical industrial activity.

For Cu, the I_geo values exhibited a wider range, from −2.26 to 10.95, spanning from unpolluted to extremely polluted classifications. The minimum value, again in sample no. 8 (first sampling), aligns with the low Ni concentration at that location, reinforcing the notion of limited external contamination in that area. The maximum in sample no. 12 suggests a concentrated anthropogenic source of Cu, likely associated with industrial residues.

For Zn, the I_geo values ranged from −1.17 to 6.36. The lowest value, recorded in sample no. 6 during the second sampling period, indicates background or slightly elevated levels potentially influenced by natural geochemical conditions. The highest value in sample no. 11 (second sampling) may be attributed to runoff or windborne deposition from nearby industrial infrastructure or storage areas.

Pb showed the I_geo values from −0.37 to 7.98. The lowest value, found in sample no. 1 (first sampling), suggests limited anthropogenic input, whereas the highest value, observed in sample no. 12 during the second sampling, implies a significant accumulation of Pb.

Cd concentrations were only quantifiable in samples 9 through 13 due to the detection limits of the atomic absorption spectrometer (AAS). Among these, the highest I_geo value was recorded in sample no. 12 during the second sampling campaign, pointing to a localized source of Cd contamination. Potential contributors include industrial waste leaching or prior use of Cd-containing dyes or stabilizers. The lowest I_geo for Cd was found in sample no. 13 (first sampling), indicating a relatively lower degree of contamination or more limited mobility of Cd in that zone.

To contextualize the findings of this study, the results were compared with those reported for other landfill sites. For instance, a study conducted at a municipal landfill in Nakhonluang District, Phra Nakhon Si Ayutthaya Province, Thailand, reported average I_geo values of 2.01 for Cu, 1.98 for Zn, and −0.33 for Pb [19]. Compared to these values, the present study reveals a broader range and, in some cases, substantially higher I_geo values, particularly for Cu and Pb. This suggests a more pronounced degree of HMs contamination at the studied post-industrial site, likely due to the legacy of intensive industrial activity—especially related to dye and textile production—rather than predominantly municipal waste, as in the Thai example.

Notably, while the average I_geo values for Cu and Zn were higher at the Thai municipal landfill, the Pb contamination level in Zgierz exceeded that of the Thai site by nearly three units, indicating a more significant accumulation of this element in the Polish site. Further comparison was made with the Enyimba landfill in Aba, southeastern Nigeria, where reported I_geo values were Cu: 1.10, Zn: −2.51, Pb: −3.10, and Cd: 1.83 [20]. In this case, the Cu values at the Enyimba landfill were higher than those recorded at some sampling points in the present study, whereas Zn and Pb levels were substantially lower, suggesting site-specific differences in waste composition and disposal practices.

A noteworthy pattern at the Zgierz site was the consistent occurrence of the highest I_geo values in samples collected from the oldest sections of the area (samples no. 10–12). This trend likely reflects the cumulative effect of prolonged HMs deposition over time. In contrast, soil samples obtained from peripheral locations exhibited markedly lower levels of contamination, highlighting the importance of landfill age and waste legacy in shaping current contamination profiles.

The combination of prolonged exposure, poor isolation, and the nature of the deposited materials, therefore, likely explains the elevated I_geo values in the zone of landfills no. 2 and no. 3.

In comparison with the elevated concentrations of HMS in soils, reflected in high contamination indices, a study performed by Janas and Zawadzka [21] has also reported increased levels of HMs in groundwater in the vicinity of Zgierz landfill sites. Notably, a marked deterioration in groundwater quality was observed following the closure of the landfill, with a significant rise in HMs concentrations documented in 2015–2016. These findings support the notion that improperly secured or remediated landfills can serve as long-term sources of subsurface pollution, posing ongoing risks to water quality and public health.

The evaluation of the I_geo for Ni conducted at a reclaimed landfill of MSW in central northern Bulgaria revealed a minimum value of 0.81, notably higher than the lowest value recorded at the Zgierz site (−0.07). Conversely, the maximum I_geo for Ni at the Bulgarian site was 1.70, which is more than two times lower than the highest value observed in Zgierz (4.32). Despite these differences in range, the mean I_geo values for Ni at both sites were relatively similar (1.43 for Zgierz and 1.24 for the Bulgarian MSW landfill), indicating comparable overall contamination levels across these two locations.

For Cu, the I_geo values at the Bulgarian MSW landfill site were below 0, indicating that the soil was practically not contaminated. Most of the samples from Zgierz also have I_geo values less than 0, while higher values are particularly evident for sampling at sites numbered Z10–Z12, correlating with the oldest and most historically burdened sections of the landfill. It is important to note that the Bulgarian site was a reclaimed landfill, which may account for the reduced levels of contamination, likely due to remediation measures [22].

Contamination by Cd and Pb was also investigated in the vicinity of a municipal landfill in Gorgan (southwestern Iran). The reported maximum I_geo values at that site were 2.42 for Cd and 2.67 for Pb, while the minimum values were −0.58 and −0.95, respectively [23]. In comparison, the maximum I_geo values at the Zgierz landfill were significantly higher, particularly in the older sections of the landfill (samples Z10–Z12). These elevated values (

Table 7. Values of the I_geo for all collected samples.

Sample No.	Element
	Ni		Cu		Zn		Pb		Cd
	I	II	I	II	I	II	I	II	I	II
1	0.74	3.00	−1.75	2.64	−0.64	1.72	−0.37	2.47	n.d.	n.d.
2	2.45	1.55	1.55	−1.06	2.59	−0.38	2.79	0.71	n.d.	n.d.
3	1.11	1.22	−1.20	−1.25	−0.39	−0.39	0.29	3.74	n.d.	n.d.
4	0.58	1.45	−1.05	−1.18	0.28	−0.46	1.07	1.06	n.d.	n.d.
5	−0.07	0.87	−2.16	−1.61	−1.01	−0.52	0.49	0.99	n.d.	n.d.
6	0.29	0.80	−1.61	−1.46	−0.82	−1.17	0.63	1.12	n.d.	n.d.
7	0.25	0.82	−0.98	−0.91	−0.52	−0.15	1.05	1.45	n.d.	n.d.
8	0.22	1.06	−2.26	−1.38	−0.82	−0.01	0.18	1.20	n.d.	n.d.
9	1.20	1.56	−1.47	−1.43	0.20	0.15	0.24	0.77	n.d.	5.15
10	1.53	1.88	2.40	3.19	5.65	6.17	6.09	6.69	5.06	4.78
11	1.79	1.93	3.39	4.17	5.95	6.36	6.52	7.07	4.73	4.83
12	4.32	3.16	10.95	3.89	6.11	4.37	7.61	7.98	5.00	5.87
13	1.80	1.61	5.42	−0.73	2.50	0.26	3.18	1.70	2.83	n.d.
Average	1.43		0.54		1.35		2.57		4.78
Legend:
≤0	Uncontaminated
0–1	Uncontaminated to moderately contaminated
1–2	Moderately contaminated
2–3	Moderately to heavily contaminated
3–4	Heavily contaminated
4–5	Heavily to extremely contaminated
>5	Extremely contaminated
n.d.	Not detected

) underscore the cumulative impact of long-term industrial waste deposition.

3.1.2. Pollution Index—PI

The PI for Ni indicated medium to high levels of contamination in the majority of analyzed soil samples. The lowest PI value for Ni was observed in sample no. 5 from the first sampling campaign (1.43), whereas the highest was recorded in sample no. 12, also from the first sampling, reaching a value of 30.00. For Cu, most samples collected in the vicinity of the landfills along Miroszewska Street exhibited low PI values. However, several exceptions were noted, including sample no. 1 from the second sampling (9.36), sample no. 2 from the first sampling (4.39), and sample no. 13 from the first sampling, which exhibited a markedly elevated PI value of 64.28. Notably, all samples collected from the oldest section of the landfill demonstrated very strong contamination, with sample no. 12 from the first sampling round recording a PI value as high as 2970.55 (

Table 8. The pollution index (PI) values for all samples collected.

Sample No.	Elements
	Ni		Cu		Zn		Pb		Cd
	I	II	I	II	I	II	I	II	I	II
1	2.51	11.99	0.45	9.36	0.97	4.95	1.16	8.31	n.d.	n.d.
2	8.22	4.40	4.39	0.72	9.02	1.16	10.38	2.46	n.d.	n.d.
3	3.23	3.50	0.65	0.63	1.15	1.14	1.83	20.06	n.d.	n.d.
4	2.24	4.10	0.72	0.66	1.82	1.09	3.16	3.13	n.d.	n.d.
5	1.43	2.74	0.34	0.49	0.74	1.05	2.11	2.98	n.d.	n.d.
6	1.84	2.61	0.49	0.54	0.85	0.67	2.32	3.26	n.d.	n.d.
7	1.78	2.64	0.76	0.80	1.05	1.35	3.11	4.10	n.d.	n.d.
8	1.75	3.12	0.31	0.58	0.85	1.49	1.70	3.45	n.d.	n.d.
9	3.45	4.42	0.54	0.56	1.72	1.67	1.78	2.57	n.d.	53.40
10	4.34	5.52	7.92	13.70	75.55	108.18	101.85	154.96	50.16	41.22
11	5.19	5.72	15.74	26.97	92.59	122.85	137.97	200.93	39.83	42.64
12	30.00	13.41	2970.55	22.20	103.76	31.08	292.61	379.28	48.00	87.66
13	5.23	4.58	64.28	0.91	8.51	1.80	13.64	4.88	10.66	n.d.
Average	5.38		120.97		22.19		52.46		46.70
Legend:
<1	Low contamination
1–3	Moderate contamination
3–6	Considerable contamination
≥6	High contamination
n.d.	Not detected

).

A similar pattern was observed for Zn, Pb, and Cd, where samples collected from the oldest landfill displayed very strong contamination levels. In the case of Zn, most soil samples from the non-hazardous and inert landfill, the hazardous and non-hazardous landfill, and the “dry landfill” indicated moderate contamination. The lowest PI value for Zn (0.67) was recorded in sample no. 5 from the second sampling, while the highest (122.85) was found in sample no. 11 from the same sampling (Table 8). These findings highlight the significant accumulation of HMs in the oldest parts of the landfill complex, reflecting both the legacy of industrial activity and the temporal persistence of these pollutants in the soil environment. The study showed that most samples exhibited moderate to high contamination, with PI values ranging from 1.15 in sample no. 1 during the first sampling campaign to 379.28 in sample no. 12 during the second campaign. These results indicate substantial Pb enrichment in the older sections of the landfill, consistent with the accumulation trends observed for other HMs. For Cd, PI values could not be calculated for the majority of samples due to concentrations falling below the detection limit of the analytical method. However, in the subset of samples where Cd concentrations were quantifiable, the PI values indicated a high degree of contamination. The highest PI value for Cd (87.66) was recorded in sample no. 12 during the second sampling, while the lowest (10.66) was observed in sample no. 13 from the first sampling. These elevated values highlight the localized but significant presence of Cd in specific zones of the landfill, particularly in its oldest sections. Moreover, the study conducted by Zhou et al. [24] confirmed that landfills and industrial zones are often characterized by high levels of contamination and ecological risk, with Cd identified as one of the main contributors to the environmental burden. A comparative analysis with data obtained from a landfill located in Omuooke-Ekit, Nigeria, further underscores the elevated levels of HMs contamination observed in the present study. According to Afolagboye [15], the average PI values recorded over three sampling periods at the Nigerian MSW dumpsite were 0.12 for Cu, 0.47 for Zn, and 1.95 for Pb. In contrast, the average PI values for the samples collected in Zgierz, Poland, were substantially higher: 120.97 for Cu, 52.46 for Pb, 46.70 for Cd, 22.19 for Zn, and 5.38 for Ni (Table 8).

These differences clearly indicate a significantly greater degree of HMs pollution at the Zgierz site, likely resulting from long-term waste disposal practices related to dye and textile production and nearby operating industries. Additional comparative data from two landfill sites located in the Kahrizak region of southern Tehran, specifically the Aradkoh landfill, revealed very low PI values - each below 0.06 for Pb, Cd, Ni, and Cu [25]. These minimal PI values likely reflect both low concentrations of HMs in the soil and elevated geochemical background levels. The contrast between these findings and the significantly higher PI values recorded at the Zgierz landfill may confirm the influence of historical industrial activity and landfill management practices on HMs accumulation.

3.1.3. Pollution Load Index—PLI

All PLI values calculated in this study exceeded the threshold value of 1.0, indicating significant soil contamination across the sampled locations. The lowest PLI value (1.36) was recorded in samples no. 5 and no. 8 during the first sampling campaign, while the highest value (5.10) was observed in sample no. 12 from the same period. These results suggest a consistent and widespread accumulation of pollutants, with particularly severe contamination in the older sections of the landfill. A detailed distribution of PLI values across the sampling sites is illustrated in Figure 3.

For comparison, soil samples collected from the Laogang landfill located on the outskirts of Shanghai exhibited PLI values ranging from 0.7 to 1.4 [26], which are considerably lower than those observed in the present study. This comparison further emphasizes the elevated pollution burden at the industrial Zgierz site.

At the Gorgan landfill in Iran, the maximum PLI value reached 8.74, exceeding the highest value recorded at the Zgierz landfill by more than three units. Conversely, the minimum PLI value at the Gorgan site was 0.88, which is lower than the lowest value observed at the Zgierz site (1.36). These findings suggest greater variability in contamination levels at the Iranian site. It is important to note, however, that the PLI assessment at the Tehran and Gorgan landfills focused solely on Pb and Cd, whereas the evaluation at the Zgierz landfill included a broader suite of HMs —specifically Cu, Ni, and Zn in addition to Pb and Cd [23].

3.1.4. Nemerow Integrated Pollution Index—NIPI

The analyzed soil samples from the Zgierz landfill were characterized by a high degree of contamination, as evidenced by the NIPI. Notably, the minimum NIPI value for Ni recorded in Zgierz was nearly six times higher than the maximum value (3.66) reported in a study investigating the environmental impact of an abandoned landfill in Kaifeng, China [27]. The high value of the NIPI for Cu is primarily attributed to the extremely elevated concentration of the element in samples no. 12 and 13, collected from the oldest section of the landfill area. This finding aligns with Lokhande et al. [28], who reported that dye manufacturing is a major source of Cu contamination, with concentrations reaching up to 33.3 mg/L in industrial effluents. Given that the landfill in Zgierz received waste from facilities involved in dye production, it is plausible that historical industrial discharges significantly contributed to the elevated Cu levels detected in the soil, thereby influencing the high NIPI values (Figure 4).

Comparative data from two landfills located within Volgograd, Russia, provide further context. At landfill No. 1, NIPI values ranged from 25.24 to 43.90, while at landfill No. 2, they ranged from 10.37 to 40.38 [29]. These results similarly reflect high contamination levels in the soil-like fractions of both sites, aligning with the severe pollution observed in Zgierz. Further comparisons were made with three landfill sites in Nigeria—Etegwe, Azikoro, and Biogbolo. At Etegwe, the NIPI values were: Cd = 4.13, Cu = 3.05, Pb = 0.34, Ni = 1.45, and Zn = 3.19. At Azikoro, values were: Cd = 4.44, Cu = 3.70, Pb = 0.27, Ni = 1.54, and Zn = 0.79. At Biogbolo, they were: Cd = 2.51, Cu = 2.86, Pb = 1.03, Ni = 1.86, and Zn = 2.81 [30]. While these values indicate moderate contamination, they remain significantly lower—by several orders of magnitude in some cases—than those recorded at the Zgierz site. This disparity emphasizes the exceptional pollution burden associated with the industrial legacy of Zgierz.

Furthermore, significant Cd contamination has been documented in industrial and mining wastelands in Yangxin County, China, showing the widespread nature of Cd-related environmental risks and the need for continued global monitoring and remediation efforts [31].

3.2. Analysis of the Ecological Risk

3.2.1. Ecological Risk Factor for a Single Metal—E_rⁱ

For samples no. 1–9 located at Miroszewska and Łukasińskiego Streets, almost all show low ecological risk with some exceptions in samples 1, 2, 3, 9, and 13 (

Table 9. Ecological risk index values for HMs.

Sample No.	Elements
	Ni		Cu		Zn		Pb		Cd
	I	II	I	II	I	II	I	II	I	II
1	12.53	59.93	2.24	46.80	0.97	4.95	5.18	41.54	n.d.	n.d.
2	41.09	22.00	21.96	3.60	9.02	1.16	51.91	12.29	n.d.	n.d.
3	16.17	17.53	3.27	3.15	1.15	1.14	9.15	100.29	n.d.	n.d.
4	11.18	20.52	3.61	3.32	1.82	1.09	15.79	15.67	n.d.	n.d.
5	7.15	13.71	1.68	2.46	0.74	1.05	10.56	14.89	n.d.	n.d.
6	9.19	13.03	2.46	2.72	0.85	0.67	11.59	16.31	n.d.	n.d.
7	8.89	13.20	3.80	4.00	1.05	1.35	15.54	20.50	n.d.	n.d.
8	8.75	15.58	1.57	2.88	0.85	1.49	8.48	17.26	n.d.	n.d.
9	17.27	22.12	2.71	2.78	1.72	1.67	8.88	12.83	n.d.	1602.00
10	21.70	27.62	39.62	68.48	75.55	108.18	509.26	774.80	1504.90	1236.63
11	25.93	28.60	78.72	134.85	92.59	122.85	689.85	1004.63	1195.00	1279.21
12	149.98	67.03	14,852.75	111.01	103.76	31.08	1463.03	1896.41	1440.00	2629.70
13	26.17	22.90	321.41	4.53	8.51	1.80	68.18	24.40	319.80	n.d.
Average	26.92		604.86		22.19		262.30		431.05
Legend:
<40	Low risk
40–80	Moderate risk
80–160	High risk
160–320	Very high risk
≥ 320	Extremely high risk
n.d.	Not detected

).

Soil samples collected from sites 10 through 12 revealed moderate to significant ecological risk for Ni and Zn, while Cu posed a very high ecological risk in these same locations. The areas surrounding the old landfill also showed very high ecological risk levels for Pb and Cd. The maximum Eri value was recorded for Cu in sample no. 12 during the first sampling campaign. Conversely, the lowest E_rⁱ value was observed for Zn in sample no. 6 during the second sampling. These are consistent with Bhutiani et al. [32], who reported that high ecological risks associated with Cd are often observed in industrial areas. These are mainly associated with activities such as electroplating, dye and pigment production, and the discharge of industrial effluents and sewage, compounded by the uncontrolled dumping of solid waste. Similarly, Akter et al. [33] found that significantly elevated concentrations of Cd, Pb, and Ni were present in areas associated with the textile and paint industries. Their study highlighted that one of the primary sources of Cd and Pb contamination is the use of various dye complexes employed in textile dyeing, which contribute substantially to the accumulation of these toxic elements in the environment. The results of the presented study were compared with ecological risk assessments from other landfills in Central Europe. At the Radiowo landfill in Warsaw, Poland, the maximum E_rⁱ value for Cd was reported as 744, while at the Zdounky landfill in the Czech Republic, the maximum value was significantly lower, at 25 [11]. In contrast, the Zgierz landfill exhibited a maximum E_rⁱ of 14,852.75, indicating an extreme ecological risk level and a markedly higher contamination burden than those observed at the reference sites. Soil samples collected from 15 landfill sites in Kinshasa, the capital of the Democratic Republic of Congo, also revealed very high ecological risk levels. In that study, triplicate soil samples were taken from each site, with between two and six samples collected per landfill, depending on site size. The highest recorded E_rⁱ was 1820.41 for Cd, while the lowest was 0.26 for Cr [34]. Overall, the E_rⁱ values obtained from the Kinshasa landfills are largely comparable to those observed at the Zgierz landfill. However, the maximum E_rⁱ (14,852.75), calculated at Zgierz, exceeds the highest value reported in Kinshasa by more than eight times. This highlights the extreme ecological threat posed by historical contamination at the Zgierz site, particularly in relation to Cd.

The pollution indices applied in this study indicate that the concentrations of several HMs in soils surrounding the examined landfill sites exceed typical geochemical background levels. These findings suggest moderate to high levels of contamination and are consistent with results reported in studies from other regions, where similar indices have been used to evaluate the impact of landfill activities on soil quality. Nevertheless, it is essential to recognize the inherent limitations of such cross-regional comparisons. Differences in waste composition, climatic conditions, and landfill management practices significantly influence the behavior and accumulation of HMs in the environment. For instance, landfills in tropical or arid climates may exhibit different leaching dynamics compared to those in temperate zones due to variations in rainfall intensity and seasonal moisture regimes. Moreover, the chemical composition of the waste, particularly the proportion of industrial, municipal, or hazardous materials, can substantially affect the type and extent of contamination. The remediation status of the site, including the presence or absence of protective liners, leachate collection systems, and surface sealing, further modifies the extent to which HMs are retained or mobilized. Although such comparisons offer a broader perspective on landfill-related contamination, the findings must be interpreted with caution, and conclusions drawn from them should be supported by locally relevant data and conditions.

3.2.2. Index of Potential Ecological Risk—ERI

The majority of the analyzed samples exhibited low ecological risk, particularly those collected from the non-hazardous and inert waste landfill and the so-called “dry landfill” (samples no. 1–9), as summarized in

Table 10. The values of the potential ecological risk index.

Sample No.	I Sampling	II Sampling
1	21.53	153.22
2	123.97	39.04
3	29.74	122.10
4	32.40	40.60
5	20.14	32.11
6	24.09	32.73
7	29.27	39.05
8	19.65	37.21
9	30.59	1641.39
10	2151.04	2215.72
11	2082.09	2570.13
12	18,009.51	4735.24
13	744.07	53.63
Average	1347.32
Legend:
<150	Low risk
150–300	Moderate risk
300–600	High risk
≥600	Severe risk

.

However, deviations from this trend were observed. For instance, sample no. 1 from the second sampling campaign registered a moderate ecological risk with an ERI value of 153.22. Similarly, sample no. 9 from the same campaign indicated a very high ecological risk, marking a significant anomaly within that zone. A comparable situation was noted in sample no. 13, which reached an ERI value of 744.07 during the first sampling—classifying it under the highest risk category according to standard ecological risk scales. Soil samples collected in the vicinity of the landfill area adjacent to the Bzura River demonstrated consistently very high ecological risk. The maximum ERI value across all samples was recorded in sample no. 12 during the first sampling, reaching 18,009.51, while the lowest was found in sample no. 8 from the same period, at 19.65. These findings suggest a spatial gradient of contamination severity, with particularly acute risks in zones directly adjacent to surface water bodies.

A comparison of ERI values calculated from soil samples collected at landfill sites in Zgierz and Tehran demonstrates a significant disparity, with the Polish results markedly exceeding those of Iran [35], where ERI values ranged from 67.4 to 154.7 Further comparative insight comes from studies conducted at former paint and varnish production sites in Belgrade, Serbia, a relevant reference point given the similar industrial activities once carried out at the Boruta plant in Zgierz. At the Belgrade site, ERI values ranged from 77 to 349, with an average of 164, indicating moderate ecological risk for all analyzed HMs [36]. In contrast, the average ERI value across all HMs at the Zgierz site was 1347.32, more than eight times higher than the Serbian average, highlighting the extreme contamination associated with the historical industrial operations in Zgierz.

The elevated values of pollution indicators observed in this study are primarily attributable to the high concentrations of HMs that exceeded established threshold levels in the analyzed soil samples. Grain size distribution analysis further revealed that the predominant soil types were clayey sands and silty sands [37]. Notably, fine fractions constituted, on average, over 20% of the total soil composition across all sampling locations [38]. These fine-grained fractions are characterized by high surface area and sorption capacity, which facilitate the adsorption and long-term retention of HMs. As such, the granulometric properties of the soils likely played a significant role in promoting the accumulation and persistence of HMs in the vicinity of the Zgierz landfill sites.

In addition to soil texture, physicochemical properties such as pH and organic matter content play a crucial role in controlling the mobility and bioavailability of HMs in soil environments. Lower pH levels typically increase the solubility of HMs, thereby enhancing their mobility and potential uptake by organisms. In contrast, higher pH conditions promote sorption processes, as the increased negative surface charge of soil particles at elevated pH enhances the attraction of cations [39,40]. Moreover, under alkaline conditions, HMs can precipitate as hydroxides, oxides, phosphates, and carbonates, which could reduce their bioavailability [41].

Furthermore, Sherene et al. [39] emphasize the critical role of soil organic matter in the retention of HMs by reducing their mobility and bioavailability. It is important to note, however, that different HMs exhibit varying affinities for organic matter. For instance, Cu²⁺ demonstrates a stronger binding affinity compared to Cd²⁺, Pb²⁺, and Ni²⁺, which facilitates its preferential association with organic components in the soil matrix [42].

Climate change, through shifts in temperature and precipitation patterns, plays a critical role in shaping the mobility and distribution of HMs in soil systems. These climatic factors directly affect soil moisture and temperature regimes, which are key drivers of microbial activity and organic matter dynamics. Elevated temperatures tend to accelerate microbial decomposition processes, leading to a decline in soil organic matter content. Furthermore, increased soil moisture, resulting from higher rainfall, can enhance the accumulation of organic matter by reducing the rate of decomposition [41].

The concentration patterns and risk indicators identified in this study provide a valuable basis for future predictive modeling. These data could be used to simulate the long-term behavior of HMs in landfill-affected soils under varying environmental scenarios (e.g., increased precipitation or temperature). Such models could inform decision-making processes by projecting contaminant migration and estimating future ecological risk, as well as informing targeted remediation or land management strategies. Incorporating temporal dynamics into future studies would improve understanding of the immediate and delayed impacts of legacy pollution.

4. Conclusions

The findings of this study clearly demonstrate that inadequately secured and historically unmanaged industrial landfills represent a significant and persistent environmental threat. The detailed geochemical and ecological risk assessments conducted in the vicinity of the landfill sites in Zgierz have revealed extensive contamination of the soil matrix with HMs, including Cu, Ni, Zn, Pb, and Cd. The calculated pollution indices consistently classified the site as heavily or extremely contaminated, with the most severe values recorded in the oldest landfill sectors, where prolonged accumulation and migration of pollutants have occurred.

The novelty of this research lies in its integrative, multi-indicator approach, which combines several geochemical and ecological risk indices with comparative analyses. By benchmarking the results against data from other international landfill sites, this study provides a broader context for interpreting the severity of contamination in Zgierz. Notably, the observed ecological risk index values for certain metals in Zgierz exceeded those of all compared sites, in some cases by several orders of magnitude, which underlines the exceptional level of pollution and environmental degradation at this location.

Given the documented ecological risks, it is imperative to extend the current research to include comprehensive human health risk assessments, using toxicological reference values, exposure modeling, and biomonitoring to quantify the impacts on vulnerable populations. This should include the development of exposure scenarios involving ingestion, inhalation, and dermal contact with contaminated media.

Furthermore, the granulometric analysis highlighted the dominant presence of fine-textured soil fractions (silt and clay), which are known to facilitate the retention of HMs. This finding warrants further study of the physicochemical interactions between soil components and contaminants, particularly in relation to HMs mobility and bioavailability.

This research provides a foundational dataset that can inform environmental policy, spatial planning, and land rehabilitation strategies. The results should guide the prioritization of remediation and containment measures, particularly in high-risk zones. Suggested actions may include isolation and stabilization of the most contaminated sectors, introduction of vegetative cover to limit erosion and dust dispersion, as well as community engagement and awareness campaigns to communicate health risks.

Future research should explore the effectiveness of remediation technologies and assess temporal changes in contamination levels under different land use and climate scenarios. Expanding the monitoring network to include water, air, and biota will provide a more holistic understanding of ecosystem impacts.

In conclusion, this study contributes significantly to the growing body of knowledge on post-industrial contamination and provides a replicable methodological framework for assessing similar legacy sites. It emphasizes the urgent need for proactive, science-based environmental governance to mitigate historical pollution and prevent future harm.