Identification and Assessment of Groundwater and Soil Contamination from an Informal Landfill Site

Abstract

Landfills are a potential source of local environmental pollution of all kinds, and the gradual destruction of seepage-proof structures in informal landfills will lead to contamination of the surrounding soil and groundwater environment. In this study, an informal landfill site in eastern China is used as the research object. Using technologies such as unmanned vessels and monitoring well imaging to delineate the amount and distribution of polluting media, sampling of the surrounding soil, sediment, groundwater, and surface water for testing, analysis, and evaluation is carried out visually and finely for heavy metals, petroleum hydrocarbons, volatile organic compounds (VOCs), semi-volatile organic compounds (SVOCs), and other indicators. The test results show that volatile phenols are the main contaminant species in the shallow groundwater, chlorinated hydrocarbons and benzene were prevalent in the deep groundwater, hexachlorobenzene and lead in the surface soil, and di(2-Ethylhexyl) phthalate in the deep soil (5.5 m), with a maximum exceedance of 1.24 times. Nearly 10 years have passed since the waste dumping incident at the landfill, but characteristic contaminants are still detected in the topsoil of the dumping area, which shows the long-term nature of the environmental impact of illegal dumping on the site. The study recommends that when developing a comprehensive remediation plan, the persistence of the environmental impact of the waste should be considered and appropriate remediation measures should be screened.

1. Introduction

As China’s economy grows and people’s living standards improve, the amount of municipal domestic waste (MSW) generated is increasing at a rate of over 10% per year [1]. In 2016, China generated more than 10% of the world’s total MSW, and in 2019, China’s waste disposal reached 242 million tons [2]. The way MSW is handled is receiving increasing attention [3]. Landfill is the most dominant domestic waste treatment method in China [4,5,6,7]. At this stage, the main methods of domestic waste treatment commonly used in China and the world are landfilling, incineration, composting, sorting, recycling, etc. Landfilling accounts for 53% of the harmless disposal of domestic waste in China [8]. By the end of 2016, the landfill rate of waste in China’s municipal landfills reached 60.3%.

Landfills are a potential source of local environmental pollution [9] and have the potential to produce leachate for hundreds of years after closure, so it is important to ensure the proper operation of all landfill systems during landfill closure [10]. Formal landfills must meet strict design, operational, and containment requirements, separating waste from the surrounding environment through bottom liners and daily soil cover [11]. However, many informal landfills still exist [12,13]. The data shows that the countries with the highest number of illegal landfills in Europe are Albania (9046) and Slovakia (8334). Italy, Russia, and France have similar problems with illegal landfills. In 2020, there were over 2000 existing illegal, “wild” landfills inventoried in Poland, covering a total area of almost 2 km² [14]. In China, most of the simple landfills established in the early years of rapid economic development were informal. These informal landfills were not built and operated by applicable standards, were immature in construction, and lacked environmental protection measures [15,16]. The landfill method used at such sites is mainly open-air disposal [17], and the only measure to prevent seepage is the physical and biochemical action of the seepage zone or clay layer on the various contaminants in the leachate [18,19]. Such landfills usually do not have leachate collection systems, and over time, impermeable structures in informal landfills gradually break down, and landfills adjacent to surface water bodies can discharge leachate into surrounding sediments and watercourses [20], leading to impacts on nearby soil and groundwater quality [21,22,23,24], potentially exacerbating the degradation of the soil environment [25], and posing significant health risks to local residents [26,27]. Based on the perspective of environmental protection and human health, it is crucial to understand the long-term impact of informal landfills on the surrounding soil and water environment, and it is urgent to strengthen the environmental control of landfills [28].

In this study, soil, substrate, groundwater, and surface water were sampled and analyzed around an informal landfill in eastern China, and the amount and distribution of contaminated media were accurately determined using techniques such as unmanned vessels and monitoring well imaging techniques [29] to study the extent of soil and groundwater contamination around the landfill and to analyze the lasting effects of illegal dumping on the study area. This study can provide theoretical support for the analysis, evaluation, and redevelopment of informal landfills.

2. Materials and Methods

2.1. Overview of the Study Area

The project site is located in eastern China, east of Shanghai, in the Taihu Lake Plain of the Yangtze River Delta. In 2007, the site of this study was the excavation area of the original brick kiln factory. The excavation area formed a 7–8 m deep pit, which was then used as a landfill for domestic waste. In 2016, the pit was filled, and the soil began to be covered. The terrain is irregularly square, east, west, south, and north are 8 km, and it has a flat terrain with a small natural slope. The water area of the town is 14 square kilometers, accounting for 21.3% of the total area of the town. The landform of the region is deeply restricted by the cutting of flowing water and the accumulation of hydraulic pushing. The land is mostly in the shape of a peninsula, along the lakeshore, with high topography, extending from the lakeshore to the hinterland, and gradually descending in topography. Most of the land elevation is more than 2 m (3 m in the lakeside area), the soil-forming parent material is lake sediments, heavy texture, and the main soil belongs to the loess soil. There are three kinds of soils, five subclasses, seven genera, and thirteen species in the area. Since 2011, unscrupulous elements have been illegally dumping chemical distillation residues in the region.

2.2. Sampling Point Placement

According to the “Technical guidelines for soil pollution risk management and remediation monitoring of construction land” (HJ 25.2-2019), the sampling points of soil, groundwater, surface water, and sediment were arranged by the method of systematic arrangement and expert judgment. A total of twenty-nine soil sampling sites, fifteen groundwater sampling sites, eight surface water sampling sites, and eight sediment sampling sites. Since the pollution of the groundwater was in the two layers, twelve submersible wells and fifteen pressurized water wells were set up to collect the groundwater samples in the area where there are two aquifers. The depth of soil sampling was 0.5 m below the surface water level, and the depth of sediment sampling was 0.5 m and 1.5 m, respectively.

Due to the presence of waste dumping and possible waste landfills at the site, the investigation was carried out in response to suspected waste landfills, including (1) timely analysis of anomalies found during drilling operations, to ensure that, as far as possible within the scope of the investigation, the landfills of the wastes are disclosed and the boundaries affected by the dumping of the wastes are initially determined; and (2) to drill for samples from the original dumping area to determine its potential waste residue.

Based on the location of the site, soil and surface water samples were collected at control points in the surrounding adjacent farmland area and the surface water area to the north. The layout of the sampling points is shown in Figure 1.

2.3. Evaluation Methods

The single-factor evaluation index method was adopted to evaluate the environmental quality of soil and groundwater around the landfill, and the specific pollution indicators monitored are shown in

Table 1. Monitoring indexes and testing methods of water and soil environmental quality.

Project Category	Test Items	Soils and Sediments	Limit of Detection (mg/kg)	Water	Limit of Detection (×10−3 mg/kg)	Reasons for Selection
General	pH	HJ 962-2018		GB/T 6920-1986		Determining contamination factors
Heavy metals	Lead	GB/T 17141-1997	10	Untested		Soil characteristic pollution factors
	Copper, zinc	HJ 491-2019	1	HJ 700-2014	0.08, 0.67	Soil characteristic pollution factors
	Mercury	HJ 923-2017	0.2 × 10−3	HJ 694-2014	0.04	Soil characteristic pollution factors
	Nickel, arsenic	Untested		HJ 700-2014	0.06, 0.12	Determining pollution factors
	Chromium (hexavalent)	Untested		GB/T 7467-1987	4	Determining pollution factors
Total petroleum hydrocarbons	Petroleum hydrocarbons (C₁₀–C₄₀)	HJ 1021-2019		HJ 894-2017		Soil characteristic pollution factor
	Petroleum hydrocarbons (C₆–C₉)	HJ 1020-2019		HJ 893-2017		Soil characteristic pollution factor
Volatile phenols	Volatile phenols	USEPA 9065-1986		HJ 639-2012		Soil characteristic pollution factor
VOCs	Carbon tetrachloride, chloroform, chloromethane, 1,1-dichloroethane, 1,2-dichloroethane, 1,1-dichloroethylene, cis-1,2-dichloroethylene, trans-1,2-dichloroethylene, dichloromethane, 1,2-dichloropropane, 1,1,1,2-tetrachloroethane, 1,1,2,2-tetrachloroethane, tetrachloroethylene, 1,1,1-trichloroethane, 1,1,2-trichloroethane, 1,1,2 trichloroethylene, 1,2,3-trichloropropane, vinyl chloride, benzene, chlorobenzene, 1,2-dichlorobenzene, 1,4-dichlorobenzene, ethylbenzene, styrene, toluene, m- & p-xylene, o-xylene, naphthalene, bromodichloromethane, tribromomethane, dibromochloromethane, 1,2-dibromoethane, 1,3,5-trimethylbenzene	HJ 605-2011		HJ 639-2012		Determining pollution factors
SVOCs	Nitrobenzene, aniline, 2-chlorophenol, benzo(a)anthracene, benzo(a)pyrene, benzo(b)fluoranthene, benzo(k)fluoranthene, dibenzo(a,h)anthracene, indenol(1,2,3-cd)pyrene, hexachlorocyclopentadiene, 2,4-dinitrotoluene, 2,4-dichlorophenol, 2,4,6-trichlorophenol, 2,4-dinitrophenol, pentachlorophenol, di(2-ethylhexyl)phthalate, o-xylene methylphenyl phthalate, butyl benzyl phthalate, di-n-octyl phthalate, 3,3’-dichlorobenzidine, hexachlorobenzene, hexachlorobutadiene	HJ 834-2017		USEPA 8270E-2018		Determining pollution factors
Waste-related characteristic pollutants	2-bromotoluene, 1-bromo-4-ethylbenzene, 2-bromo-m-xylene, 3-bromo-o-xylene	USEPA 8260D-2018		USEPA 8260D-2018		Waste-related characteristic pollutants

. The single-factor index method participates in the comprehensive water quality assessment of water quality standards, with the worst water quality of a single indicator belonging to the category to evaluate the water quality level. The method is simple and clear, can directly understand the relationship between water quality conditions and evaluation criteria, and gives the evaluation of the factors of the rate of compliance, exceedance rate, and exceedance multiples, and other characteristic values. However, only one water quality factor is considered, and the water quality condition of the river cannot be evaluated comprehensively [30,31]. Its evaluation method is as follows:

$$P_i=\raisebox{1ex}{$C_i$}\!\left/ \!\raisebox{-1ex}{$S_i$}\right.$$

(1)

where P_i is the single-factor pollution index for pollutant i in soil/groundwater/surface water, C_i is the measured concentration of pollutant i in soil/groundwater, and S_i is the evaluation reference value for pollutant i in soil/groundwater/surface water. The reference values for soil evaluation were adopted from the Soil Environmental Quality Soil Contamination Risk Control Standards for Construction Land (Trial) (GB 36600-2018) [32] for screening values for Class II sites and the US EPA Regional Screening Values for industrial land. For characteristic pollutants that lack evaluation indicators, the laboratory detection limit was selected as the screening standard. The sediment was initially evaluated regarding the above criteria. The reference value for groundwater evaluation uses the index limit of Class IV water bodies in the Groundwater Quality Standard (GB 14848-2017) [33], and the surface water evaluation refers to the Class III standard in the Surface Water Environmental Quality Standard (GB 3838-2002) [34]. According to the size of the P_i value, P_i ≤ 1 means that the soil/groundwater/surface water is not contaminated; P_i > 1 means that the soil/groundwater/surface water is contaminated; when P_i is above 5, it is severely contaminated [35].

2.4. Unmanned Vessels and Monitoring Imaging Investigations

Much of the current literature on landfills is mostly groundwater and soil only. This study uses technologies such as unmanned vessels and monitoring well imaging techniques to capture the amount and distribution of contaminated media visually, finely, and accurately [36,37,38], and adds sediment investigation and sample collection to fill the gap in the sediment investigation work in damage identification and assessment work [39].

The unmanned vessel carries out bathymetry through the onboard measurement system, which is mainly composed of digital bathymetry, attitude sensors, GPS receivers, distance sensors, and other precision sensors. The system adopts GPS-RTK dynamic differential positioning, which receives GPS satellite signals through the base station and sends the checkpoint data to the receiver installed on the unmanned vessel to realize the real-time positioning function. The water depth measurement device is completed by the digital dual-frequency depth sounder on board, which mainly uses ultrasonic waves to penetrate the medium and produce emission on the surface of different media, emits ultrasonic waves through the transducer, and measures the time difference between the emitted and reflected waves to measure the water depth [40]. The attitude sensor of the hull is used to correct the collected bathymetry data, eliminating the problem that the oscillation of the hull caused by wind and current during the measurement process makes the bathymetry data collected by the transducer mismatch with the GPS receiver plane data, ensuring that the measurement data is true and reliable [36,41]. The parameters of the unmanned vessel equipment are shown in

Table 2. List of equipment parameters.

Description	Parameters
Model/size	S105/1050 × 550× 350 mm
Resolution	Horizontal 1.25 cm, vertical 1/1000 range
Ship type	Deep V
Image transfer	1080P, distance > 1000 km
Standard weight	20 kg
Load	10 kg
Wind and wave rating	Class 4 wind, 1.5 m wave
Sonar type	Side sweep, down sweep
Manipulation	Mobile app, a distance greater than 1000 km
Data analysis	Cloud platform
Maximum speed	8 m/s

.

The monitoring imager achieves visualization through the collection and storage of multi-media depth parameters and image data from monitoring wells, groundwater, strata, substrates, etc. It is mainly used for the quality control of concealed works for the investigation of key industrial enterprise sites, quality control of monitoring wells (structure, blockage), groundwater contamination properties (color, status), monitoring of landfill leachate conditions, contaminated site investigation work, contaminated site remediation work, observation of the current state of river and pond substrates, observation of landfill leachate wells, measurement of water levels, quality inspection of monitoring well washing, and seabed detection. The main parameters of the instrument are shown in

Table 3. Monitoring imager parameters.

Description	Parameters
Operating temperature	−30–60 °C
Continuous working time	>2 h
Voltage input	5–12 V input, 12 V output
Measuring tape length	10, 20, 30, 40, 50 m
Measuring tape accuracy	0.001 m
Screen	1024 × 600/800 × 480 HD display
Storage	Support 128 GB TF memory card
Battery capacity	7000 mAh/4500 mAh Li-ion battery
Signal input	HD-TVI, HD-CVI, AHD, CVBS mixed signal auto recognition input
Minimum illumination	0.0001 Lux @ F1.2
Digital noise reduction	Support
Backlight compensation	Auto
Auto gain	Auto
Shutter/slow shutter	1/25 s to 1/50,000 s, 16× MAX
Video probe	Ultra-high pixels

.

3. Results

3.1. Unmanned Vessels and Monitoring Imaging Investigations

The total area of the landfill pond area was measured to be approximately 4700 square meters (as determined by site measurements), of which the water surface area during work was approximately 3700 square meters and the deepest depth was approximately 4.3 m deep, as shown in Figure 2. The volume of surface water in the pond from the measurements was calculated to be approximately 14,269 cubic meters. A spot test was carried out on the northern pond near the dumping area to estimate the depth of the waste that might remain in the pond, and the data was calculated that the depth of the waste remaining in the northern pond near the dumping area (within the scope of this unmanned boat survey) should be around 3 m. As the pond is located in a landfill site, it may have been filled with household waste and a large number of branches and leaves over the years, and the underwater conditions are very complex. According to the principle of this survey, the sediment volume measurement is calculated for the difference in reflection characteristics of different media, and due to the complexity of the site conditions, there may be some uncertainty in the depth calculation.

The internal conditions of the monitoring wells were investigated by the imager, mainly to identify the quality conditions of the monitoring wells of the concealed works, such as the location and connection of the screen pipe and white pipe, and the physical properties of the groundwater, and the findings are shown in Figure 3. There was no obvious LNAPL distribution in the groundwater surface of the monitoring wells, the white pipes, and connections of the monitoring wells, and the sieve pipes of the monitoring wells were by the design requirements, the quality of the good washing was good (Figure 3a–c), the groundwater of the monitoring wells had more suspended matter, the groundwater transparency was good, and no obvious traces of oil contamination were found (Figure 3e). In addition, a survey of the internal conditions of the pond’s surface water was carried out to initially map the current state of its bottom, as shown in Figure 3f, with good water clarity and the presence of litter deposits such as leaves and dead branches at the bottom.

3.2. Soil/Sediment Assessment Results

The pollutants for which the soil exceeded the control value and the single-factor indices are shown in

Table 4. Excess pollutants in soil.

Type of Pollutant	Min. (mg/kg)	Max. (mg/kg)	Screening Value (mg/kg)	Control Values (mg/kg)	Detection Rate	Exceedance Rate	Pi
Hexachlorobenzene	ND	30.9	1	10	1.60%	1.60%	30.90
Bis(2-Ethylhexyl) phthalate	ND	271	121	1210	33.00%	1.09%	2.24
Trichloroethylene	ND	4.78	2.8	20	2.70%	1.09%	1.71
Benzo(a)pyrene	ND	1.8	1.5	15	20.90%	0.54%	1.20
Lead	4.5	969	800	2500	100%	0.50%	1.21
Volatile phenols	ND	76.6	0.01 **	——	24.70%	24.70%	7660.00
2-bromotoluene	ND	0.14	0.05 **	——	0.54%	0.54%	2.80
1-Bromo-4-ethylbenzene	ND	1.21	0.05 **	——	1.08%	1.08%	24.20
2-Bromo-m-xylene	ND	4.06	0.05 **	——	1.63%	1.63%	81.20
3-Bromo-o-xylene	ND	0.3	0.05 **	——	1.08%	1.08%	6.00

** Detection limit; ND, below the detection limit.

. There was one contaminant that exceeded the control value: hexachlorobenzene (maximum value 30.9 mg/kg). Four pollutants exceeded the screening value and five characteristic pollutants lacked evaluation indicators and exceeded the laboratory detection limit.

A summary of the contaminants detected in the substrate that exceeded the limit and the single-factor indices is shown in

Table 5. Excess pollutants in sediment.

Type of Pollutant	Min. (mg/kg)	Max. (mg/kg)	Screening Value (mg/kg)	Control Values (mg/kg)	Detection Rate	Exceedance Rate	Pi
Toluene	ND	11,200	1200	1200	86.75%	43.75%	9.33
Ethylbenzene	ND	3270	28	280	62.5%	25%	116.79
m-& p-xylene	ND	1500	570	570	62.5%	25%	2.63
Petroleum hydrocarbons (C₆–C₉)	ND	56,400	4500	9000	86.75%	12.5%	12.53
Benzene	ND	53.3	4	40	25%	12.5%	13.33
o-xylene	ND	311	640	640	25%	12.5%	0.49
1,2-Dichloroethane	ND	212	5	21	25%	12.5%	42.40
Chlorobenzene	ND	6750	270	1000	75%	12.5%	25.00
Chloromethane	ND	22.3	5	47	6.25%	6.25%	4.46
Carbon tetrachloride	ND	4.64	2.8	36	6.25%	6.25%	1.66
Chloroform	ND	7.04	0.9	10	12.5%	6.25%	7.82
Tribromomethane	ND	3.68	0.24	2.4	6.25%	6.25%	15.33
Di(2-ethylhexyl) phthalate	ND	369	121	1210	62.5%	6.25%	3.05
Volatile phenols	ND	84.4	0.01 **	——	62.5%	62.5%	8440.00
2-bromotoluene	ND	22.5	0.05 **	——	12.5%	12.5%	450.00
1-Bromo-4-ethylbenzene	ND	222	0.05 **	——	18.75%	18.75%	4440.00
2-Bromo-m-xylene	ND	709	0.05 **	——	18.75%	18.75%	14,180.00
3-Bromo-o-xylene	ND	10.4	0.05 **	——	18.75%	18.75%	208.00

** Detection limit; ND, below the detection limit.

. Six pollutants exceeded the control value, six pollutants exceeded the screening value, five characteristic pollutants lacked evaluation indicators and exceeded the laboratory detection limit, and the remaining pollutants were below the detection limit.

3.3. Groundwater Assessment Results

The summary of groundwater exceedance pollutants and the single-factor indices are shown in

Table 6. Excess pollutants in groundwater.

Type of Pollutant	Min. (mg/kg)	Max. (mg/kg)	Screening Value (mg/kg)	Detection Rate	Exceedance Rate	Pi
As	0.001	0.14	0.05	100%	4%	2.74
Volatile phenols	ND	11.80	0.01	25%	13%	1180.00
Xylene	ND	2.60	1.00	40%	8%	2.60
Ethylbenzene	ND	1.48	0.60	24%	4%	2.47
Benzene	ND	0.14	0.12	56%	4%	1.21
Toluene	ND	42.00	1.40	52%	4%	30.00
Chlorobenzene	ND	4.47	0.6	32%	4%	7.45
1,2-Dichloroethane	ND	0.066	0.04	12%	4%	1.65
Petroleum hydrocarbons	0.04	59.37	5.00 **	4%	4%	11.87

** Massachusetts Groundwater Standards [42]; ND, below detection limit.

. There are nine pollutants that exceed the relevant screening criteria for groundwater.

3.4. Surface Water Assessment Results

A summary of the surface water test results and the single-factor indices are shown in

Table 7. Excess pollutants in surface water.

Type of Pollutant	Min. (10−3 mg/kg)	Max. (10−3 mg/kg)	Screening Value (10−3 mg/kg)	Detection Rate	Exceedance Rate	Pi
Oil	ND	5.48	1 *	87.5%	50%	5.48
COD	23	526	40 *	100%	50%	13.15
BOD5	2.9	42.4	10 *	100%	50%	4.24
NH4+-N	0.227	67.4	2 *	100%	50%	33.70
TN	1.88	86.8	2 *	100%	75%	43.40
TP	0.11	3.16	0.4 *	100%	50%	7.90

ND, below the outgoing limit; * Class III standard in Environmental Quality Standard for Surface Water (GB 3838-2002) [34].

. The test results show that the concentrations of pollutants such as petroleum, chemical oxygen demand, five-day biochemical oxygen demand, ammonia nitrogen, total nitrogen, and total phosphorus exceeded the limits for surface water category V. There were thirteen organic pollutants detected, including benzene, chlorinated hydrocarbons, and di(2-ethylhexyl) phthalate, but they did not exceed the standard limits for centralized surface water sources for domestic drinking water of the Environmental Quality Standard for Surface Water (GB 3838-2002) [34], the non-conventional indicator limits of the Sanitary Standard for Domestic Drinking Water (GB 5749-2006) [43], and the Class III standard of the Groundwater Quality Standard (GB 14848-2017) [33].

3.5. Residual Waste in the Dumping Area

Samples were collected and tested from the bottom sediment of the previous hazardous waste dumping area and the area of possible impact, the depth, and test results are shown in

Table 8. Results of soil samples and substrate testing in the dumping area.

Testing Indicators	Dumping Area (Unit: mg/kg)			Sediment (Unit: mg/kg)
	GW8-0.2 m	GW8-1.5 m	DN1-0.5 m	DN8-0.5 m	DN8-1.0 m
2-Bromotoluene	0.14	ND	17.0	ND	22.5
1-Bromo-4-ethylbenzene	0.71	ND	8.35	0.59	222
2-Bromo-m-xylene	4.06	0.17	23.7	2.74	709
3-Bromo-o-xylene	0.30	ND	2.66	0.15	10.4

. The test results showed that the samples contained indicators such as 1-Bromo-4-ethylbenzene, a characteristic contaminant consistent with dumped waste.

4. Discussion

4.1. Migration Analysis of Contamination

Pollutant migration pathways mainly include surface runoff, litter and soil leaching and infiltration, and groundwater diffusion migration [44,45]. The pattern of pollutant transport and transformation in the investigation area is closely related to the topography, hydrogeological conditions of the site [46], and the distribution of pollutants and pollution sources [47,48,49].

The sources of pollution in this study area are divided into three main components: (1) residual pollutants from the former hazardous waste dumping area; (2) waste and contaminated substrate that may be present in the ponds affected by the dumping; and (3) the site as a landfill, where landfill material may also be a source of pollution. The topography of the area is uneven, especially in the investigation area around the reservoir there is a waste landfill with a steep slope, high terrain, and steep terrain.

The area is an informal landfill, with large voids and strong permeability in the landfill body. Pollution sources are subject to surface water formed by regional atmospheric rainfall and slope water that stays briefly and then quickly runs off to lower areas, discharging to the pond and surrounding water bodies within the survey area, which is subject to seasonal influences and frequent exchanges between groundwater and surface water in the site. The types of pollutants are mainly organic, the sources of pollution are relatively concentrated (identification of previous waste dumping) and uncertain (possible pollutants in informal landfills), and the dispersion and migration of pollution are influenced by the seasons.

4.2. Analysis of Pollution Indicators

4.2.1. Analysis of Soil/Sediment Evaluation Results

There are seven points where the soil samples exceeded the standard for pollutants. In the horizontal direction, the points where soil pollutants were exceeded were concentrated in S9, GW7, GW8, GW10 in the core area, and GW6, GW14, and GW15 in the focus area. In the vertical direction, three sites, GW7, GW8, and S9, showed exceedances of the HCB standard in the surface soil.

The Pi levels for volatile phenols, 2-Bromo-m-xylene, hexachlorobenzene, and 1-Bromo-4-ethylbenzene in the soil are all higher than five. This is severe contamination. Soil organic matter (SOM) and soil moisture, as well as the acidity of local precipitation, affect the leaching and evaporation of benzene and xylene from the soil [50]. The data shows that the annual average rainfall pH values for the site for 2018–2021 range from 5.51–6.69, with an acid rain frequency of 0–25.9%. Chen et al. [50] indicated that rainfall acidity inhibits the release of xylenes from soils, which may be related to the solubilization of xylenes by dissolved organic matter (DOM). Higher pH has been reported to promote the solubilization of DOM in the soil [51], thereby facilitating the release of chlorophenols from the soil. On the other hand, alkyl and aromatic molecules in soil organic matter (SOM) can act as adsorbers of hydrophobic organic carbon, so their presence can inhibit the leaching of xylenes [52]. Conversely, the higher moisture content may promote the leaching of xylenes from soils. The generally low pH of local precipitation and high organic matter in the soil inhibited the release of organic pollutants from the soil, resulting in serious exceedances of the 2-bromo-m-xylene standard in the soil.

Although nearly 10 years have passed since the waste dumping incident, the characteristic contaminant, 1,3,5-trimethylbenzene, was still detected in the surface soil of GW8 and S9 in the core area (below the screening value), which demonstrates that the environmental impact of illegal dumping on the site is of a long-term nature.

The Pi levels for ethylbenzene, volatile phenols, 2-bromotoluene, 1-Bromo-4-ethylbenzene, 2-Bromo-m-xylene, and 3-Bromo-o-xylene in the substrate were greater than 100, which is a serious exceedance. Regarding the distribution of sediment contamination, organic pollutants such as benzene and halogenated hydrocarbons in the DN1-0.5 surface layer, DN8 surface layer, and lower layer, as well as the concentration levels of characteristic pollutants were significantly higher than other points. The locations where screening values were exceeded included DN1 and DN8 (concerning waste management), DN2, DN3, and DN4.

4.2.2. Analysis of Groundwater Evaluation Results

There are nine pollutants that exceed the relevant screening criteria for groundwater. The Pi level for volatile phenols was as high as 1180, and again high concentrations of volatile phenols were detected in the soil tests in this area. It is possible that this was due to the saturation of volatile phenol adsorption in the leachate, combined with changes in soil moisture, which led to the leaching of volatile phenols.

In the horizontal direction, the groundwater pollutant exceedance points were GW8 in the core area and GW4 in the peripheral area. The greatest level of contamination was found in GW8 (former hazardous waste dumping area), where the maximum values of various pollutant indicators were found. The only point in shallow groundwater that exceeded the standard was GW4, where the pollutant species that exceeded the standard were mainly volatile phenols, while deep groundwater GW8 had exceeded the standard for chlorinated hydrocarbons and benzenes, with associated 2-bromotoluene and other characteristic pollutants detected, and GW4 had exceeded the standard for volatile phenols and benzenes.

4.3. Impacts of Residual Waste in the Dumping Area

Characteristic contaminants were found at the dumping area points GW8-0.2 m and GW8-1.5 m. Two points DN1 and DN8 were selected; DN1 and DN8 were located close to GW8 and S8, respectively, 1 m from the shore, and their profiles showed waste at both 0.5 m and 1.5 m.

In the core area, at the former S8 site, waste made of fiberglass was found buried 1.5 m below ground and with a strong irritating smell. This area is adjacent to the former dumping area and the location of the pond to the west, and the waste is similar to the chemical distillation residue dumped earlier. Two soil (GF1, GF2) and one leachate (FY1) samples were collected on-site, and laboratory tests were conducted on their composition. The results showed that the soil and leachate of the suspected waste landfill area were found to contain characteristic contaminants such as benzene, halogenated hydrocarbons, and 1-bromo-4-ethylbenzene, further verifying that some of the components of the waste buried at this site were consistent with the characteristic indicators of the chemical distillation residue.

A large amount of rubbish with a strong irritating odor was found during the collection of DN8. DN1-0.5 m, DN8-0.5 m, and DN8-1.0 m all contained characteristic pollutants, indicating that the area had been affected by previous hazardous waste dumping and that this effect was of a long-term nature. This is consistent with the findings of other studies, which are presented in

Table 9. Comparison of other site survey results.

Type of Site	Location	Source of Pollution	Characteristic Pollutants	Conclusion	References
Three illegal waste landfill sites	The forest area in the Bydgoszcz commune	Mixed waste, including debris and ceramic waste, glass, plastics, metals, textiles, and used electrotechnical equipment Organic waste from households	Heavy metal	The accumulation of waste on the site inhibits the development of microorganisms and their enzymatic activity. The fact that operating illegal waste dumps are a potential threat to the natural environment is confirmed by this study.	[27]
An illegal dumping site	The boundary of the town of Takko and Ninohe City in Japan.	Ash, waste oils, sludge, waste plastic, and bark	PCE, Dichloromethane, Benzene, Cis-1,2-dichloroethylene, 1,2-dichloroethane	The study proposed a new needs analysis method for developing a conceptual land-use plan following the remediation of illegal dumping sites by considering economic and social aspects based on the potential needs of the region’s residents.	[53]
A landfill	Lagos, Nigeria	Municipal solid waste	Heavy metal (Pb, Cr)	Active sites in landfills are a potential source of toxic lead, cadmium, and zinc, and if the current trend of indiscriminate waste disposal at the site is not controlled, environmental contamination can occur. Waste management and treatment policies should be developed for landfills and waste disposal must be pre-treated prior to disposal.	[14]
Brownfield site	In the eastern part of the Guanzhong plain	Food-grade fumaric acid (anti-succinic acid)	1,2,3-trichloropropane	Most of the soils within this fumaric acid brownfield site were at a severe contamination level. 1,2,3-TCP was the primary exceeded pollutant in the fumaric acid brownfield site, and it could be the focus of subsequent studies on fumaric acid brownfield sites.	[35]
An illegal waste dumping site	In the Tohoku region of Japan	Incinerator ash, sludge, and refuse-derived fuel materials	1,4-dioxane	The study shows that it is possible to predict and remedy pollution from illegal waste dumps and encourages further extensive research into the complex geological structure and groundwater changes at illegal waste dump.	[54]

for other sites, including the three informal landfills. Comparison with the results of this study shows that illegal dumping and informal landfills have long-term impacts on the ecological environment and human health, and attention should be paid to pollution prediction and remediation measures for these sites, as well as proposed land-use options after remediation.

5. Conclusions

The main findings of this study are as follows: (1) A total of five contaminants in the site soil exceeded the screening or control values for soil contamination risk, with 4.34% of the total number of samples exceeding the standard. (2) A total of nine contaminants in the groundwater exceeded the relevant screening standards for groundwater, with 12.5% of the total number of samples exceeding the standards. Among them, the Pi value of volatile phenols was as high as 1180, which is serious contamination. Contaminants were detected at some points in the pressurized water aquifer. (3) The exceedance points are adjacent to the former waste dumping area, which shows that the waste dumping has affected part of the substrate area of the pond. (4) Characteristic pollutants were still detected in the project area. Although nearly 10 years have passed since the waste dumping incident, characteristic pollutants were still detected in the surface soil of the dumping area (below the screening value), which shows the long-term impact of illegal dumping on the site environment.

This study recommends that the persistence of the environmental impact of the waste is considered when developing a comprehensive remediation plan and that appropriate remediation measures be screened.