Assessment and Seasonal Monitoring of Groundwater Quality in Landfill-Affected Regions of China: Findings from Xiangyang

Abstract

Groundwater pollution in landfill-adjacent regions presents a critical environmental and public health issue. This study evaluates groundwater quality in Xiangyang City, focusing on drinking water sources and key pollution points near landfill sites. The investigation involved a comprehensive field survey, systematic sampling, and laboratory analysis to determine pollutant types, sources, and concentrations. A total of 13 landfill sites were examined, with 178 groundwater samples analyzed for physical, chemical, and biological indicators during both wet and dry seasons. The findings reveal that 27.0% of groundwater samples meet Class I standards, while 46.1% and 27.0% fall into Class IV and V categories, respectively, indicating a significant prevalence of poor-quality groundwater. Seasonal variations were observed, with both wet and dry seasons showing consistent distributions of Class I, IV, and V samples. Heavy metals such as lead and arsenic, along with organic pollutants like polychlorinated biphenyls and pesticides (e.g., hexachlorobenzene), were significant contaminants in several sites. Key indicators such as nitrate, ammonia nitrogen, manganese, and total hardness consistently exceeded standard limits, with the most affected sites including L4 and L5 in Xiangyang. This study identifies leachate infiltration as the primary cause of pollution, exacerbated by geological and agricultural non-point sources. Based on these findings, a robust framework for monitoring and controlling groundwater pollution is proposed, emphasizing stricter regulations, advanced monitoring systems, and cross-regional coordination. The results underscore the urgency of immediate intervention to safeguard groundwater quality in landfill-adjacent regions.

1. Introduction

Landfills, as repositories for municipal solid and hazardous waste, have long been scrutinized for their potential to contaminate groundwater resources [1]. Leachate, a complex liquid byproduct formed through the decomposition of waste and the infiltration of precipitation, is a significant environmental and public health challenge [2]. Leachate composition is notably diverse, encompassing organic and inorganic substances, heavy metals, and emerging contaminants (ECs) such as pharmaceuticals and personal care products [3]. The migration of these pollutants into surrounding groundwater systems depends on landfill design, waste composition, hydrogeological conditions, and the effectiveness of containment measures [4,5].

Xiangyang City, located in northwest Hubei Province, China, represents a pertinent case study for assessing landfill-induced groundwater pollution. Spanning 19,700 square kilometers, the city features a diverse topography, including mountainous areas, hilly plains, and lowlands, which facilitate varied hydrological conditions. The subtropical humid monsoon climate, with annual precipitation ranging from 820 mm to 1100 mm, contributes to significant surface water recharge, thereby intensifying the interaction between landfills and aquifers. Xiangyang’s groundwater resources are extensively utilized, serving as the primary drinking water source for a substantial portion of the population. However, rapid urbanization and the proliferation of poorly managed landfill sites have exacerbated groundwater pollution risks [6].

Globally, landfill-induced groundwater contamination is a pressing concern. A study assessing 62 engineered municipal landfills in Eastern China found that over half of groundwater samples from nearby wells exceeded recommended drinking water limits for inorganic solutes, including chloride (Cl⁻), sulfate (SO₄²⁻), and potassium (K⁺) [7,8,9]. Similarly, investigations in Northeastern China revealed elevated concentrations of emerging contaminants such as perfluorinated compounds (PFCs), antibiotics, and bisphenol A (BPA) in both leachate and adjacent groundwater [1,10]. Internationally, studies from the United States and Europe have highlighted the significant risks posed by aging landfill infrastructure, leaking underground storage tanks, and insufficient waste management practices [11,12].

There were 13 centralized landfill sites identified as key pollution sources for this study in Xiangyang. Many of these sites do not have sophisticated containment systems that would prevent leachate from reaching aquifers. Preliminary surveys have detected high levels of heavy metals including lead and arsenic, as well as conventional pollutants such as nitrate and ammonia nitrogen, in groundwater around these sites. These contaminants have adverse health impacts; for instance, nitrate causes methemoglobinemia, heavy metals lead to neurologic disorders, and organic pollutants cause endocrine disruption [13].

The health consequences of drinking contaminated groundwater are severe. High levels of nitrate, often due to leachate from landfills, are associated with methemoglobinemia, also known as “blue baby syndrome”, in infants, and chronic exposure raises the risk of cancers and thyroid diseases [13,14,15]. Heavy metals, such as lead and arsenic, result in developmental and neurological disorders, and organic pollutants, such as polychlorinated biphenyls (PCBs) and pesticides, lead to carcinogenic and immunotoxic effects [16]. Addressing these risks requires an integrated approach that combines robust scientific research, technological innovation, and stringent policy interventions [17,18].

This study builds upon these global and regional insights by conducting a comprehensive assessment of groundwater quality in Xiangyang City, focusing on the interplay between landfill management and groundwater contamination. The specific objectives of this research are (1) to evaluate the current state of groundwater resources in Xiangyang, both in terms of quality and utilization; (2) to identify the major pollutants and sources of contamination, with an emphasis on landfill-induced impacts; and (3) to propose actionable recommendations for monitoring, mitigation, and restoration of groundwater resources. Drawing from field investigations, systematic sampling, and rigorous laboratory analysis, this research contributes to the growing body of knowledge on sustainable groundwater management and provides a framework for addressing similar challenges in other rapidly urbanizing regions.

2. Materials and Methods

2.1. Study Area

This study was conducted in Xiangyang City, located in the northwest of Hubei Province, China, within the middle reaches of the Han River. Spanning 19,700 square kilometers, the city is geographically positioned between 110°45′–113°43′ east longitude and 31°14′–32°37′ north latitude. Xiangyang’s jurisdiction comprises three municipal districts, three county-level cities, and three counties. Historically known as a significant transportation hub, Xiangyang is referred to as the “thoroughfare of seven provinces”, facilitating extensive trade and cultural exchanges.

The city experiences a subtropical humid monsoon climate with an annual average temperature of 15–16 °C and precipitation ranging between 820 mm and 1100 mm (Figure 1). The hydrological setting is complex, with significant interaction between surface water and groundwater systems. The Han River and its tributaries, including the Xiaoqing River, Tangbai River, and Nanhe River, play a pivotal role in recharging the groundwater aquifers.

Land use patterns in Xiangyang are diverse, encompassing agricultural land, urban zones, industrial sites, and forested areas. The city is also home to 13 centralized landfill sites, identified as key contributors to groundwater pollution. These landfills, distributed across different administrative areas, vary in scale and operational practices, with many lacking modern containment systems. The location map of the study area is shown in Figure 2a, and landfill and groundwater drinking water source distribution is shown in Figure 2b, providing essential context for this study’s sampling strategy. The locations of water sources represented in this figure are centralized groundwater drinking water sources in Xiangyang City, including 13 township-level sources and 47 smaller township-level sources.

2.2. Sampling Strategy

Groundwater sampling was conducted to assess the quality and contamination levels across Xiangyang’s groundwater systems. A total of 178 monitoring wells were strategically selected near 13 landfill sites (

Table 1. Statistical table of monitoring data of landfills in counties and urban areas.

No.	Counties and Urban Areas	Landfill Site (Coded)	Number of Monitoring Points (s)	Sample Data (Group)
1	Laohekou City	L1	7	14
2		L2	7	14
3		L3	8	16
4	Zaoyang City	L4	5	10
5	Gucheng County	L5	5	10
6	Baokang County	L6	4	8
7	Xiangcheng District	L7	8	16
8		L8	5	10
9		L9	8	16
10		L10	8	16
11	Xiangzhou District	L11	8	16
12		L12	8	16
13		L13	8	16
Total		13	89	178

). These sites were chosen based on their operational history, geographical location, and proximity to sensitive groundwater sources.

Following Nilkarnjanakul et al. [19], the sampling design aimed to capture spatial variations in contamination by including wells both upstream and downstream of potential pollution sources. Key parameters analyzed included conventional indicators (e.g., pH, nitrate, total hardness), heavy metals (e.g., lead, arsenic), and organic pollutants (e.g., polychlorinated biphenyls and pesticides) [9]. Duplicate samples were collected to ensure reliability, and blank samples were included to maintain quality control [20].

Groundwater samples were collected via sampling devices, including air pumps, low-flow submersible pumps, inertial pumps, and Beyer tubes. The sampling procedure included pre-sampling preparation of sampling equipment and containers, water level measurement, well washing, on-site monitoring project testing and recording, groundwater sample collection, sample packaging and storage, sample transfer, sampling records, etc. Sampling was performed based on the main requirements of the ‘Technical Specifications for Groundwater Environmental Monitoring’ (HJ164-2020) [21].

To visualize the spatial distribution of sampling points and their relation to potential pollution sources, Figure 3 highlights the locations of the monitoring wells across the landfill sites.

2.3. Field Investigations

Field investigations were conducted to provide a comprehensive understanding of the environmental and hydrogeological conditions surrounding the landfill sites. These investigations involved the following:

• Detailed surveys of landfill infrastructure, including containment systems, leachate collection mechanisms, and surrounding drainage patterns.
• Observations of potential pollution pathways, such as leachate seepage, cracks in landfill liners, and visible discoloration of nearby vegetation.
• Recording hydrogeological data, including aquifer characteristics and groundwater flow directions, to understand the movement of contaminants.

Field observations were complemented by interviews with local stakeholders, including landfill operators, environmental protection authorities, and nearby residents. These discussions provided valuable insights into operational practices, historical pollution incidents, and community concerns about groundwater safety.

2.4. Analytical Methods

Groundwater samples were analyzed in accredited laboratories using standardized methods compliant with the Groundwater Quality Standard (GB/T 14848-2017) [22] and Surface Water Environmental Quality Standard (GB3838-2002) [23]. The analysis covered 62 indicators, categorized into natural control ions, conventional parameters, heavy metals, and organic pollutants (

Table 2. Comprehensive list of monitored groundwater quality parameters near landfill sites in Xiangyang City, categorized by pollutant type.

Indicator Type	Name of Index	Number of Indicators
Natural control ions	Potassium (K), Calcium (Ca), Sodium (Na), Magnesium (Mg), Sulfate (SO42−), Chloride (Cl−), Carbonate (CO32−), Bicarbonate (HCO3−)	8
Conventional indicators	pH, Dissolved Oxygen (DO), Redox Potential (Eh), Conductivity, Color, Odor, Turbidity, Total Hardness, Total Dissolved Solids (TDS), Iron (Fe), Manganese (Mn), Copper (Cu), Zinc (Zn), Volatile Phenols, Synthetic Detergents (anionic), Permanganate Index, Nitrate (NO3−-N), Nitrite (NO2−-N), Ammonia (NH3-N), Fluoride (F−), Cyanide (CN−), Mercury (Hg), Arsenic (As), Selenium (Se), Cadmium (Cd), Chromium (Cr6+), Lead (Pb), Total Coliform Bacteria	29
Organo-Chlorine Pesticides	Hexachlorocyclohexane (HCH), Dichlorodiphenyltrichloroethane (DDT), p,p-DDT, Hexachlorobenzene	4
Halogenated Hydrocarbons	Trichloromethane (CHCl3), Dichloromonobromomethane, Tribromomethane (CHBr3), Carbon Tetrachloride (CCl4), Vinyl Chloride (C2H3Cl)	5
Halogenated Hydrocarbons	Chlorobenzene	1
Monocyclic Aromatic Hydrocarbons	Benzene (C6H6), Toluene (C7H8), Ethylbenzene (C8H10), Xylene (C8H10), Styrene (C8H8)	5
Polycyclic Aromatic Hydrocarbons (PAHs)	Benzo(a)pyrene (BaP)	1
Inorganic Components	Total Phosphorus (P), Bromide (Br−), Thallium (Tl), Total Chromium (Cr)	4
Polychlorinated Biphenyls (PCBs)	Polychlorinated Biphenyl	1
Overall Organic Components	Total Organic Carbon (TOC)	1
Esters	Di(2-ethylhexyl) Phthalate, Di(2-ethylhexyl) Adipate, Di(2-ethyl) Phosphate	3

). The sample analysis was performed by accredited laboratories, such as Hubei Provincial Geological Bureau Eighth Geological Team Experimental Testing Center and Hubei Jingheng Testing Co., Ltd., China. The analytical protocols were standardized across laboratories to ensure data accuracy and reliability.

The groundwater samples were analyzed through standardized instrumental techniques for natural control ions, anions, heavy metals, and organic compounds. Major cations (K⁺, Na⁺, Ca²⁺, Mg²⁺) were analyzed by ICP-OES, and anions (sulfate, chloride, nitrate, and fluoride) were identified using ion chromatography. Heavy metals (Hg, As, Se, and Sb) were measured by atomic fluorescence spectrometry and ICP-MS. Organic contaminants such as volatile organic substances, PCBs, pesticides, and PAHs were analyzed using purge-and-trap GC-MS as well as gas chromatography and high-performance liquid chromatography.

All necessary quality control procedures were applied, including laboratory method blanks, matrix spike recoveries, duplicate analyses, and external calibration verification. Details on specific analytical methods, instruments used, and QC criteria are described in Table 3.

Table 3. Summary of analytical methods, instruments, and quality control parameters used for groundwater quality assessment.

Parameter Category	Analytical Method	Equipment	Method Reference	QC Acceptance Criteria
Natural Control Ions (K, Na, Ca, Mg)	ICP-OES/EDTA Titration	Thermo Fisher iCAP PRO (Thermo Fisher Scientific, Waltham, MA, USA)	HJ 776-2015 [24], DZ/T 0064.13/14-2021 [25]	Recovery: 70–120% (≤10 MDL), 70–130% (>10 MDL)
Anions (SO₄2−, Cl−, NO₃−, NO₂−, F−)	Ion Chromatography	Thermo Fisher AQ-1100 (Thermo Fisher Scientific, Waltham, MA, USA)	HJ 84-2016 [26]	Recovery: 80–120%
Heavy Metals	ICP-MS	Thermo Fisher iCAP RQ (Thermo Fisher Scientific, Waltham, MA, USA)	HJ 700-2014 [27]	Recovery: 70–130%, RSD ≤20%
Mercury, As, Se, Sb	Atomic Fluorescence	Beijing Kechuang AFS-8500 (Beijing Kechuang Haiguang Instrument Co., Ltd., Beijing, China)	HJ 694-2014 [28]	Recovery: 70–130%, RSD ≤20%
Volatile Organic Compounds	Purge-and-Trap GC-MS	Agilent 8860-5977B (Agilent Technologies, Santa Clara, CA, USA)	HJ 639-2012 [29]	Recovery: 60–130%, RSD ≤30%
PCBs and Organochlorine Pesticides	GC	Agilent 8860 (Agilent Technologies, Santa Clara, CA, USA)	SL 497-2010 [30]	Recovery: 70–120%, RSD ≤30%
PAHs	HPLC	Agilent 1260II (Agilent Technologies, Santa Clara, CA, USA)	HJ 478-2009 [31]	Recovery: 60–120%, RSD ≤30%
Conventional Parameters (pH, TDS, DO)	Multiple Methods	Various	Multiple Standards	Method-specific

Table 3. Summary of analytical methods, instruments, and quality control parameters used for groundwater quality assessment.

Parameter Category	Analytical Method	Equipment	Method Reference	QC Acceptance Criteria
Natural Control Ions (K, Na, Ca, Mg)	ICP-OES/EDTA Titration	Thermo Fisher iCAP PRO (Thermo Fisher Scientific, Waltham, MA, USA)	HJ 776-2015 [24], DZ/T 0064.13/14-2021 [25]	Recovery: 70–120% (≤10 MDL), 70–130% (>10 MDL)
Anions (SO₄2−, Cl−, NO₃−, NO₂−, F−)	Ion Chromatography	Thermo Fisher AQ-1100 (Thermo Fisher Scientific, Waltham, MA, USA)	HJ 84-2016 [26]	Recovery: 80–120%
Heavy Metals	ICP-MS	Thermo Fisher iCAP RQ (Thermo Fisher Scientific, Waltham, MA, USA)	HJ 700-2014 [27]	Recovery: 70–130%, RSD ≤20%
Mercury, As, Se, Sb	Atomic Fluorescence	Beijing Kechuang AFS-8500 (Beijing Kechuang Haiguang Instrument Co., Ltd., Beijing, China)	HJ 694-2014 [28]	Recovery: 70–130%, RSD ≤20%
Volatile Organic Compounds	Purge-and-Trap GC-MS	Agilent 8860-5977B (Agilent Technologies, Santa Clara, CA, USA)	HJ 639-2012 [29]	Recovery: 60–130%, RSD ≤30%
PCBs and Organochlorine Pesticides	GC	Agilent 8860 (Agilent Technologies, Santa Clara, CA, USA)	SL 497-2010 [30]	Recovery: 70–120%, RSD ≤30%
PAHs	HPLC	Agilent 1260II (Agilent Technologies, Santa Clara, CA, USA)	HJ 478-2009 [31]	Recovery: 60–120%, RSD ≤30%
Conventional Parameters (pH, TDS, DO)	Multiple Methods	Various	Multiple Standards	Method-specific

2.5. Data Analysis and Evaluation

The collected data were subjected to statistical analysis to determine the extent of groundwater contamination. The pollution index (P) method was employed to classify contamination levels for each parameter [32]. The index is calculated as

$$P_k={\displaystyle \frac{C_0}{C_k}}$$

where $C_k$ represents the observed concentration of parameter k and $C_0$ is the permissible limit defined by the applicable standards. Parameters with P > 1 were categorized as exceeding safe levels.

Comprehensive water quality evaluations were conducted using a weighted average of pollution index scores [33,34], classifying groundwater into five categories from Class I (pristine) to Class V (extremely polluted). Statistical tools were used to identify correlations between contamination levels and factors such as landfill operational age, liner system effectiveness, and proximity to groundwater wells.

3. Results

3.1. Groundwater Quality Assessment

The evaluation of groundwater quality near landfill sites in Xiangyang City revealed a broad spectrum of water quality classifications, ranging from pristine Class I to severely polluted Class V, based on the Groundwater Quality Standard (GB/T 14848-2017). These categories are defined based on their physicochemical parameters (e.g., total dissolved solids, nitrate, ammonia nitrogen, and heavy metals). The specific criteria for this classification are detailed in Supplementary Table S1. A total of 178 groundwater samples were analyzed from monitoring wells located around 13 landfill sites. The single-index evaluation highlighted significant exceedances in several parameters, including nitrate, manganese, and total hardness, which often pushed the water quality into Class IV and Class V categories. The results are presented in

Table 4. Summary of groundwater quality classifications (Class I–V) near landfill sites in Xiangyang City, based on comprehensive pollution index evaluations.

No.	Landfill Site (Coded)	No. of Monitoring Sites	Monitoring Period	No. of Class I Wells (s)	No. of Class IV Wells (s)	No. of Class V Wells (s)	Comprehensive Water Quality Category	Indicators Exceeding Standards (Category IV and V)
1	L1	7	wet season	2	2	3	III	Total hardness, soluble total solids, chloride, manganese, aluminum, nitrate
			dry season	3	1	3
2	L2	7	wet season	3	3	1	III	Total hardness, soluble total solids, iron, manganese, aluminum, oxygen consumption, nitrite, nitrate
			dry season	3	3	1
3	L3	8	wet season	1	6	1	III	Total hardness, iron, manganese, aluminum, ammonia nitrogen
			dry season	0	6	2
4	L4	5	wet season	0	4	1	IV	Total hardness, chloride, iron, manganese, ammonia nitrogen, sodium, nitrite, fluoride, lead, carbon tetracloride, polychlorinated biphenyls (total amount)
			dry season	0	4	1
5	L5	5	wet season	0	4	1	V	Iron, manganese, ammonia nitrogen, nitrite, lead, benzoa (a) pyrene, polychlorinated biphenyl (total amount), 6666 (total amount), DDT (total amount), hexachlorobenzene
			dry season	0	5	0
6	L6	4	wet season	0	2	2	IV	Total hardness, sulfate, iron, manganese, ammonia nitrogen, nitrite, nitrate, lead, polychlorinated biphenyls (total amount)
			dry season	0	2	2
7	L7	8	wet season	3	4	1	III	Total hardness, soluble total solids, sulfate, manganese, aluminum, nitrate
			dry season	3	4	1
8	L8	5	wet season	3	2	0	III	Chlorides, nitrites
			dry season	3	2	0
9	L9	8	wet season	1	3	4	III	Total hardness, soluble total solids, sulfate, chloride, iron, manganese, aluminum, oxygen consumption, ammonia nitrogen, nitrate
			dry season	1	3	4
10	L10	8	wet season	2	2	4	III	Total hardness, soluble total solids, iron, manganese, aluminum, nitrates, iodide
			dry season	2	2	4
11	L11	8	wet season	0	4	4	III	Zinc, aluminum, and nitrate
			dry season	0	4	4
12	L12	8	wet season	2	4	2	IV	Total hardness, soluble total solids, sulfate, aluminum, nitrate, and arsenic
			dry season	2	4	2
13	L13	8	wet season	7	1	0	III	Aluminium
			dry season	7	1	0

.

The comprehensive-index evaluation provided a broader perspective by integrating pollution levels across multiple indicators. This assessment revealed that heavy metals such as lead and arsenic, along with organic pollutants like polychlorinated biphenyls (PCBs), were the dominant contributors to groundwater degradation. The findings indicated that a significant portion of samples were Class I (27.0%), with no Class II or III water observed near the landfill sites. The majority of samples were classified as Class IV (46.1%), followed by Class V (27.0%). These proportions reflect the significant prevalence of poor to extremely poor water quality in landfill-affected regions. The results emphasize the critical need for robust pollution control measures to mitigate groundwater contamination surrounding landfill sites.

The spatial distribution of landfill sites also significantly contributed to variations in contamination. Landfills located near drinking water sources and regions of high aquifer permeability were especially likely to result in severe groundwater pollution. These results highlight the critical need for improved landfill containment systems and stringent monitoring protocols to safeguard groundwater resources in Xiangyang City.

3.2. Pollution Patterns and Key Contaminants

Groundwater quality assessments identified multiple key pollutants and their distribution patterns across the 13 landfill sites. As shown in

Table 5. Pollutants exceeding groundwater standards near landfill sites in Xiangyang City, categorized by frequency and severity.

Pollutant Type	Parameter	Monitoring Period	Number of Wells Exceeding Standard	Exceedance Rate (%)	Range of Exceedance (Times Over Limit)	Number of Affected Landfills	Landfill Site with Maximum Exceedance (Coded)
Conventional indicators	Manganese	Wet season	26	29.21	0.05–18.6	9	L9
		Dry season	26	29.21	0.26–18.3	9	L9
	Nitrate	Wet season	19	21.35	0.045–4.75	8	L6
		Dry season	18	20.22	0.01–4.6	8	L6
	Total Hardness	Wet season	20	22.47	0.0234–2.62	9	L9
		Dry season	20	22.47	0.013–2.55	9	L9
	Iron	Wet season	19	21.35	0.13–12.1	7	L9
		Dry season	21	23.60	0.1–18.47	7	L3
	Soluble total solids	Wet season	14	15.73	0.05–1.82	6	L9
		Dry season	13	14.61	0.01–1.76	6	L9
	Aluminum	Wet season	12	16.00	0.04–12.1	9	L10
		Dry season	12	16.00	0.38–11.5	8	L10
	Ammonia nitrogen	Wet season	10	11.24	0.21–8.63	5	L4
		Dry season	4	4.49	0.334–3.02	2	L9
	Sulfate	Wet season	6	6.74	0.012–1.032	4	L12
		Dry season	5	5.62	0.148–0.956	4	L12
	Nitrite	Wet season	2	2.25	0.5–1.62	2	L5
		Dry season	5	5.62	0.27–10.10	4	L6
	Chloride	Wet season	5	5.62	0.3068–1.448	4	L9
		Dry season	5	5.62	0.284–1.432	4	L9
	Zinc	Wet season	2	2.25	0.09–0.18	1	L11
		Dry season	1	1.12	0.06	1	L11
	Consumed oxygen	Wet season	2	2.67	0.267–2.267	2	L9
		Dry season	2	2.67	0.167–2.1	2	L9
	Iodide	Wet season	1	1.33	0.2526	1	L10
	Sodium	Wet season	1	1.12	0.04405	1	L4
	Fluoride	Wet season	1	1.12	0.04	1	L4
Heavy metal indicators	Lead	Wet season	11	12.36	0.14–7.6	4	L6
		Dry season	2	2.25	0.07–1.7	2	L6
	Arsenic	Wet season	3	3.37	0.38–3.13	2	L12
		Dry season	3	3.37	0.26–2.86	2	L12
Organic pollutants	Polychlorinated biphenyl (Total)	Wet season	8	10.00	0.42–2.82	2	L5
		Dry season	7	8.64	0.1–4.76	3	L5
Pesticides	Hexachlorobenzene	Wet season	4	18.18	0.15–3.84	1	L5
	Benzex (Total)	Dry season	1	4.35	0.319	1	L5
	DDT (Total)	Dry season	1	4.35	0.042	1	L5
Halogenated hydrocarbon	Carbon tetrachloride	Wet season	3	3.37	1.785–4.41	1	L4
Cycloaromatic hydrocarbons	Benzo(a)pyrene	Dry season	2	2.47	3.38–4.72	1	L5

, among the most critical conventional contaminants were manganese, nitrate, total hardness, and iron. Manganese showed the highest exceedance rate at 29.21% in both wet and dry seasons, with levels reaching up to 18.6 times the permissible limit at the L9 landfill site. Nitrate contamination was also significant, exceeding standards in 21.35% of wet season samples and 20.22% of dry season samples, with a maximum exceedance of 4.75 times the limit at the L6 landfill site.

The site L9 emerged as a significant hotspot, showing maximum exceedances for multiple conventional parameters including manganese, total hardness, total dissolved solids, and chloride. Iron levels were particularly concerning at this site during the wet season (12.1 times over the limit) and at site L3 during the dry season (18.47 times over the limit).

Heavy metals presented another substantial concern, particularly lead and arsenic. Lead contamination was more prevalent in the wet season, with 12.36% of samples exceeding standards and maximum concentrations reaching 7.6 times the permissible limit at site L6. Arsenic contamination, while less widespread (3.37% exceedance rate), showed significant concentrations up to 3.13 times the allowable limit at the L12 landfill site.

Organic pollutants were concentrated primarily at the L5 landfill site, which showed the highest levels of several compounds including polychlorinated biphenyls (up to 4.76 times over the limit) and benzo(a)pyrene (4.72 times over the limit). Additionally, hexachlorobenzene, reported as Benzex (total) in wet season monitoring, reached levels of up to 3.84 times over the limit at this site.

Seasonal variations were observed in several parameters, notably ammonia nitrogen, which showed higher exceedance rates in the wet season (11.24%) compared to the dry season (4.49%), and lead, which showed a marked decrease from the wet season (12.36%) to dry season (2.25%) exceedance rates.

3.3. Seasonal Variations in Contamination

The influence of seasonal factors on groundwater contamination near landfill sites was evident from the data. During the wet season, increased precipitation facilitates the infiltration of water through landfill waste, generating higher volumes of leachate. This may result in elevated concentrations of pollutants in the surrounding groundwater. However, as per our findings, during both wet and dry seasons, 27.0% of groundwater samples were classified as Class V, with key contributors including nitrate, manganese, and ammonia nitrogen. The exceedance rates for nitrate reached 21.35% in the wet season and 20.22% in the dry season, while manganese maintained a consistent exceedance rate of 29.21% across both seasons (Figure 4 and Figure 5). While nitrate and manganese contamination are highest during the rainy season, their continued presence implies that these contaminants are not purely seasonal leachate byproducts. Agricultural runoff or residual contamination from previous seasons may also play a role.

Such temporal trends emphasize the need for adaptive monitoring strategies to accommodate hydrological variability. Landfill containment systems can be severely tested during wet seasons when seasonal data reveal a failure to adequately treat the leachate produced.

3.4. Source Attribution and Pollution Dynamics

Identifying pollution sources and contaminant migration mechanisms is essential for developing effective measures to enhance groundwater quality in Xiangyang. According to the landfills studied, the landfill sites in Baokang County, Gucheng County, and Xiangcheng District were identified as major contributors to groundwater pollution. These sites lacked effective containment infrastructure, including absent impervious liners and leaking leachate collection systems.

Here, we found much higher contamination in wells downstream from landfill sites, and our spatial analysis showed clear migration scenarios for pollutants such as nitrate, ammonia nitrogen, and heavy metals. Lead was consistently observed at higher concentrations downstream of Baokang County’s landfill, while arsenic displayed similar trends near the Xiangzhou District’s landfill. The movement of organic pollutants, including PCBs and pesticides, was also noted, particularly in areas with highly permeable aquifers.

The evolving nature of landfill leachate composition is further highlighted by emerging contaminants like TOC and bromide. Although these pollutants are not typically monitored, they provide valuable insights into the impact of landfills on groundwater systems. Their presence underscores the urgent need for improved monitoring frameworks and advanced landfill designs to safeguard groundwater quality in the future.

3.5. Additional Water Quality Indicators

Beyond standard groundwater quality indicators, other metrics were assessed to understand broader contamination patterns.

Table 6. Statistical summary of non-standard water quality indicators in landfills of Xiangyang City.

Metric	Count	Landfill Sites (Coded)
Total Organic Carbon (TOC)	3	L6, L5, L4
Dichloromonobromomethane	2	L6, L4
Bromide	3	L6, L5, L4
Permanganate Index	1	L6
Total Phosphorus	2	L6, L4

presents the occurrence of non-standard indicators across various landfill sites in Xiangyang City. These indicators, while not traditionally monitored under the Groundwater Quality Standard (GB/T 14848-2017), provide critical insights into landfill-related pollution.

The data indicate that TOC, bromide, and dichloromonobromomethane were observed across multiple sites, suggesting pervasive contamination patterns. Notably, the L4 and L6 landfill sites frequently reported elevated levels of these indicators. Such findings emphasize the need for monitoring beyond conventional parameters to capture emerging pollutants effectively.

4. Discussion

Landfill leachate poses significant environmental and public health risks, as evidenced by the data from Xiangyang City. The detection of elevated levels of nitrates, heavy metals, and organic pollutants underscores the urgent need for effective waste disposal and remediation strategies.

Nitrate contamination in over 20% of samples in both dry and wet seasons is especially worrying due to its association with methemoglobinemia, or “blue baby syndrome”, which disrupts infants’ blood from carrying oxygen [13,35,36]. That can pose serious health risks, particularly for communities that depend on contaminated groundwater for drinking water. In addition to its direct health dangers, nitrate contamination is also representative of the larger problem of non-point-source pollution, which is exacerbated by agricultural runoff and poor landfill practices [37,38]. Mitigation will entail coordinated action to handle both agricultural inputs and landfill leachate.

Chronic exposure to high levels of heavy metals, including lead and arsenic, is associated with increased health risks, including neurotoxicity, developmental harm, and cancer risk [8]. In Xiangyang, the exceedance rates for lead during the wet season were over 12%, and the levels of arsenic were higher than allowable in certain hotspots like the Hongshantou landfill. Such findings agree with studies performed in other urbanizing areas, where inadequate containment systems in landfills have been directly associated with increased heavy metal concentrations in underlying groundwater [39,40,41] and South America. Such trends further highlight the urgent need for improved landfill engineering, such as impermeable liners and state-of-the-art leachate collection systems.

There are multiple environmental contaminants from landfills, such as polychlorinated biphenyls (PCBs) and pesticides like hexachlorobenzene, complicating remediation efforts. PCBs were found at concentrations over four times higher than regulatory standards and are of concern as contaminations are persistent and have bioaccumulation potential in aquatic ecosystems. Hexachlorobenzene—a known carcinogen—raises similar concerns. These findings magnify the call for targeted interventions against persistent organic pollutants (POPs), which are commonly not removed by routine treatment methods [42,43].

Seasonal variations have a considerable impact on contamination levels, with leachate migration increasing due to rising precipitation during the wet season. Poorly contained landfills fail to handle this pattern effectively, as seen during costly rainy seasons. Precipitation allows leachate to drift into aquifers, especially in regions with permeable soil or fractured bedrock. Other studies in Southeast Asia [44,45] and South America [46] have shown similar results, as rainfall related to the monsoon season can increase the mobility of pollutants from landfills. Seasonal extremes require adaptive infrastructure to handle higher leachate loads, particularly during peak rainfall periods.

Similar trends have been reported in Bangladesh, where monsoon leachate percolation led to increased contaminant concentrations in adjacent water bodies [47].

Spatial analysis demonstrating greater concentrations of contaminants downstream from landfill sites aligns with findings by Gupta et al. [48], which show that groundwater pollution increases with proximity to landfills. This underscores the importance of proper landfill siting and the creation of buffer zones to safeguard vulnerable water resources [49]. In Xiangyang, downstream wells consistently showed higher levels of lead, arsenic, and organic pollutants compared to upstream locations, showing clear pollutant migration patterns across both locations. This calls for stricter zoning laws that keep landfills away from sensitive aquifers and neighborhoods.

Newly emerging contaminants, such as perfluorinated compounds (PFCs), antibiotics, alkylphenols (APs), and bisphenol A (BPA), present additional challenges due to their stability and potential health risks [50,51]. Studies in China reported high levels of these contaminants in landfill leachates, which subsequently polluted nearby groundwater sources [1,52]. The identification of total organic carbon (TOC) and bromide in numerous landfill sites underscores the increasing complexity of leachate composition. TOC, a marker for emerging contaminants, requires sophisticated analytical methods for detection and treatment. Future research efforts need to focus more on these contaminants so that we can understand their transport and fate in groundwater systems.

Emerging contaminants, such as pharmaceuticals and personal care products, pose a threat to both public health as well as the environment; therefore, the improvement of such treatment technologies should be the focus moving forward. Common existing strategies are activated carbon adsorption, AOPs (advanced oxidation processes), and membrane filtration, like reverse osmosis. Yet, many of these are expensive and need to be developed further to ensure the at-scale execution of such technologies for the treatment of landfill leachate. Promising new pathways for chemical degradation include new approaches within bioremediation, using microbial consortia to degrade persistent organic pollutants and perfluorinated compounds.

The results from Xiangyang City corroborate global observations of landfill-derived groundwater pollution. Poor landfill design, aging infrastructure, and inadequate leachate management exacerbate this issue. A comparative analysis of landfill management practices in developed countries shows that rigorous engineering approaches, including double-liner systems and continuous leachate recirculation, drastically reduce the risk of contamination [53,54]. Similar measures could radically lessen groundwater harm in Xiangyang’s own landfill sites.

Addressing it requires a combination of advanced landfill liners, efficient leachate collection systems, regular groundwater monitoring, and remediation of contaminated sites. Moreover, the integration of geospatial technologies and real-time monitoring systems could improve the detection and tracking of contamination plumes, thereby allowing for more targeted interventions. Equally important for long-term sustainability is an effort to educate landfill operators and local stakeholders on best practices.

Groundwater contamination as a result of landfill activity also poses socio-economic challenges, in addition to direct environmental and health risks. Communities dependent on contaminated water sources end up with higher medical bills for pollution-related diseases, lowered property values, and reduced agricultural production. Polymetallic groundwater pollution in Xiangyang, where Falu’ans occupied a major portion of people using groundwater for daily use, can worsen the economic burdens of households affected by prolonged exposure to polluted water. By introducing financial rewards for a reduction in polluting activities and engaging the community in waste management initiatives, a sustainable approach can be developed.

Overall, this study highlights the critical importance of comprehensive waste management policies and strict environmental regulations to safeguard groundwater resources. Immediate and sustained efforts are essential to mitigate the harmful effects of landfill leachate on groundwater systems, ensuring public health and environmental quality.

5. Conclusions and Recommendations

5.1. Conclusions

This research offers important insights into pollution patterns, significant contaminants, and deficiencies in the current landfill containment system. Over 73% of groundwater samples were classified as poor (Class IV) or extremely poor (Class V) according to the Groundwater Quality Standard (GB/T 14848-2017), with levels of nitrate, manganese, lead, arsenic, and organic pollutants (e.g., polychlorinated biphenyls and hexachlorobenzene) often above allowable limits. Seasonal monitoring showed that pollution was substantially higher in the wet season, mainly due to increased leachate infiltration, especially for nitrates and heavy metals, while manganese and nitrate were consistently above limits in both seasons, pointing to persistent sources of contamination.

The spatial analysis indicates more contamination levels in downstream wells rather than upstream wells, which is a clear reflection of pollutant migration from landfill sites with poor containment measures. The lack of advanced liners and poor leachate management systems resulted in the Zaoyang City and Gucheng County (L4 and L5) landfill sites emerging as significant pollution sources. Moreover, the identification of non-traditional pollutants (e.g., total organic carbon and bromide) in addition to traditional residues indicates the increasing complexity of landfill leachate and the requirement for advanced technologies for monitoring and treatment.

This study highlights the urgent need to improve landfilling infrastructure, including modern liner systems and effective leachate collection mechanisms, to prevent groundwater contamination. Policymakers should also emphasize the need for continuous groundwater monitoring, especially for emerging contaminants, to ensure comprehensive protection. By illustrating these challenges, this study highlights the need for sustainable waste management practices and robust environmental regulations to protect groundwater resources. Future studies should also investigate the socio-economic impacts of groundwater contamination and the potential of advanced remediation technologies for sustained mitigation of landfill leachate risks.

5.2. Recommendations

Future research should consider studying the presence and treatment of newly emerging pollutants like perfluorinated compounds and microplastics in landfill leachate. In addition, long-term follow-up is required for landfill remediation practices to assess the effect of mitigation measures suggested or implemented. To enhance leachate treatment efficiency, advanced remediation methods such as biochar filtration, electrochemical oxidation, and constructed wetlands may be further explored. The results of this study can guide local and national policymaking on landfill governance through stricter regulations on landfill construction standards, leachate treatment, groundwater quality monitoring, and other aspects. Regional-level waste management needs to be consolidated by the cross-sectoral cooperation of environmental agencies, municipalities, and study institutions in order to implement smoothing pollution control.