Electrical geophysical and hydrogeological investigations of groundwater aquifers in Ruseifa municipal landfill, Jordan

The effect of the Ruseifa municipal landfill on the shallow groundwater aquifers in the area was investigated in two separate sites. The first one was not used since 1994, whereas the other is still being used for dumping. Fourteen electrical resistivity soundings were performed to detect the leachate and its effect on the quality of the groundwater. Results indicated that the solid waste thickness of the landfill was ranged from 3 to 20 m with resistivity value less than 10 Ω m. Based on the resistivity decreases of values less than 5 Ω m, the leachate was detected in the landfill sites at depths ranged from 10 to 50 m. However, the flow direction of the leachate at depth ranging 10–20 m in the terminated site was toward north, whereas the flow direction of the leachate in the site still used for dumping was toward east–northeast which causes the major source of groundwater pollution.     

甘肃某生活垃圾填埋场地下水污染特征及污染途径分析

依据实际调查数据,采用单因子污染指数分析法确定了生活垃圾填埋场地下水中主要污染物、污染程度和污染范围,并进行了土壤和地下水污染耦合性分析,研究结果表明：该生活垃圾填埋场地下水污染物主要为氨氮、总氮、高锰酸盐、锰、挥发酚和氟化物,其污染指数最大值分别为1 407.24、745.00、137.12、97.50、3.60和1.68,污染区总面积0.168 km²;填埋场下游沟谷纵向上,总氮、锰两项指标在土壤中的含量与地下水中浓度变化趋势一致;其污染途径主要为库区防渗层破损,渗滤液垂向沿基岩风化裂隙进入地下含水层,以连续渗漏的方式不断污染地下水,污染物在地下水径流作用下向拦挡坝下游不断迁移。     

Biogeochemistry of landfill leachate plumes

The literature has been critically reviewed in order to assess the attenuation processes governing contaminants in leachate affected aquifers. Attenuation here refers to dilution, sorption, ion exchange, precipitation, redox reactions and degradation processes. With respect to contaminants, focus is on dissolved organic matter, xenobiotic organic compounds, inorganic macrocomponents as anions and cations, and heavy metals. Laboratory as well as field investigations are included. This review is an up-date of an earlier comprehensive review. The review shows that most leachate contamination plumes are relatively narrow and do not in terms of width exceed the width of the landfill. The concept of redox zones being present in the plume has been confirmed by the reported composition of the leachate contaminated groundwater at several landfills and constitutes an important framework for understanding the behavior of the contaminants in the plume as the leachate migrates away from the landfill. Diverse microbial communities have been identified in leachate plumes and are believed to be responsible for the redox processes. Dissolved organic C in the leachate, although it appears to be only slowly degradable when the volatile organic acids are gone, apparently acts as substrate for the microbial redox processes. Several xenobiotic organic compounds have been found to be degradable in leachate contaminated groundwater, but degradation rates under anaerobic redox conditions have only been determined in a few cases. Apparently, observations in actual plumes indicate more extensive degradation than has been documented in the laboratory. The behavior of cations in leachate plumes is strongly influenced by exchange with the sediment, although the sediment often is very coarse and sandy. Ammonium seems to be subject to anaerobic oxidation, but the mechanisms are not yet understood. Heavy metals do not seem to constitute a significant pollution problem at landfills, partly because the heavy metal concentrations in the leachate often are low, and partly because of strong attenuation by sorption and precipitation. Although complexation of heavy metals with dissolved organic matter is significant, the heavy metals are in most cases still strongly attenuated in leachate-polluted aquifers. The information available on attenuation processes has increased dramatically during the last 15 a, but the number of well-documented full scale leachate plumes are still few and primarily from sandy aquifers. Thus, the diversity of attenuation processes in leachate plumes is probably not yet fully understood. Apparently, the attenuation processes in leachate plumes may for many contaminants provide significant natural remediation, limiting the effects of the leachate on the groundwater to an area usually not exceeding 1000 m from the landfill.     

Environmental impact and geochemical behavior of soil contaminants from an industrial waste landfill in Southern Brazil

Industrial waste landfills produce great impacts on soil and groundwater. There are many industrial waste landfills in Vale dos Sinos, Southern Brazil, which were inadequately planned and maintained since the industry started in the first half of the twentieth century. The largest industrial landfill in the Valley, which causes the most severe impacts on soil and groundwater, is the subject of this paper, which studies the environmental impacts and behavior of contaminants in soil. The landfill was carefully mapped on a scale of 1:1,000; 88 samples were collected from soil probes; the leachate of three samples was comprehensively analyzed; and soils mineralogy and chemistry were studied. Few studies have been made on this landfill. This study shows widespread contamination of soil in the surrounding areas of the landfill. Chromium, chloride and ammonium have the highest contamination levels, reflecting their high contents in landfill leachate. Contamination by petroleum hydrocarbons, cyanide and mercury is registered in more than 65% of soil samples with low concentrations. Lead, copper and barium show low contamination restricted to a few soil samples. Soil contamination occurs mainly in the unsaturated zone of the aquifer at the convergence points of stormwater, showing that the preferential transport of contaminants occurs on surface flow followed by soil infiltration. The results of leaching tests indicate high metal sorption capacity of soil. The remediation of contaminated soil must contain at least the following actions: sealing the top of the landfill, installation of geochemical barriers, removal of the liquid waste basins without sealing the base and collection and treatment of the rainwater drainage.     

Experimental and analytical model studies on leachate volume computation from solid waste

An extensive use of solid-waste landfills for disposal of municipal and industrial wastes have prompted increased attention to groundwater pollution caused by leachate generated in such landfills. The potential for groundwater contamination by leachate has necessitated engineering designs for landfills. The quantity of leachate generated from the solid waste and the movement of water through the solid waste depends on water input and the solid-waste characteristics. This paper dealt with the experimental investigations using the laboratory solid-waste leaching column to estimate the total leachate volume/leachate flow for unsaturated and saturated conditions. The hydraulic properties of the solid waste like initial moisture content, field capacity, permanent wilting point, saturation moisture content, effective void ratio, saturation hydraulic conductivity and saturation suction pressure were determined from the small-scale laboratory experiments, which are the input for analytical model study of leachate flow/total leachate volume for both unsaturated and saturated conditions. The result of analytical model study was compared with the results of experimental investigations. Comparisons of measured and computed total leachate volume/leachate flow using Darcy’s law showed reasonable agreement.     

The use of strontium and lead isotopes to identify sources of water beneath the Fresh Kills landfill,Staten Island,New York,USA

A study was undertaken to explore whether the isotopic compositions of Pb and Sr are useful to distinguish mixtures of uncontaminated groundwater, seawater, and landfill leachate at the Fresh Kills landfill, Staten Island, New York. Ratios of ⁸⁷Sr/⁸⁶Sr ranged from 0.7088 to 0.7137 and could be used to distinguish Sr that was derived from seawater from that in uncontaminated groundwater. Lead isotopic abundances did not vary systematically among the different water sources. Plots of ⁸⁷Sr/⁸⁶Sr versus dissolved organic C, B, and NH₄⁺ defined perpendicular trends, documenting where leachate or sea water mixed with uncontaminated groundwater, and demonstrating that leachate has not contaminated groundwater in aquifers beneath the landfill.     

Stability Analyses of Municipal Solid Waste Landfills with Decomposition

A bioreactor landfill is operated to enhance refuse decomposition, gas production, and waste stabilization. Some of the potential advantages of bioreactor include rapid stabilization of waste, increased landfill gas generation, gain in landfill space, enhanced leachate treatment, and reduced post closure maintenance period. Due to the accelerated decomposition and settlement of solid waste, bioreactor landfills are gaining popularity as an alternative to the conventional Subtitle D landfills. However, the addition of leachate to accelerate the decomposition changes the physical and engineering characteristic of waste and therefore affects the geotechnical characteristics of waste mass. The changes in the physical and mechanical characteristics of solid waste with time and decomposition are expected to affect the shear strength of waste mass. The objective of this paper is to analyze the stability of solid waste slopes within the bioreactor landfills, as a function of time and decomposition. The finite element program PLAXIS is used for numerical modeling of bioreactor landfills. Stability analysis of bioreactor landfills was also performed using limit equilibrium program STABL. Finally the results from finite element program PLAXIS and limit equilibrium program STABL are compared. GSTABL predicted a factor of safety of more than 1 in all the cases analyzed, whereas PLAXIS predicted a factor of safety of less than 1 at advanced stages for a slope of 2:1. However, the interface failures between solid waste and landfill liners have not been considered in this paper.     

Increase of vulnerability of karst aquifers due to leakage from landfills

Karst aquifers are very vulnerable to various pollution sources. Among these, landfills can contribute contaminants over long periods. Groundwater quality monitoring is required to assess the impact of a landfill leachate on underlying aquifer water or spring discharge. Tracer tests are useful for selecting the location and frequency of sampling. Three landfill sites in karst areas were studied with tracer tests. Additional insight was accomplished by the comparison of the tracer transfer and the breakthrough curves obtained from these tests with the results of two other tracer tests carried out in the same period in karst areas without a landfill. To explain the differences observed, the hypothesis of increased permeability below the landfill due to the presence of inorganic acids in leachates was further tested. The results of detailed, long-term monitoring of contaminated drip water in the Postojna Cave were used to verify the hypothesis. The analysis of the simultaneously increased content of calcium, magnesium, and contaminants in the drip water indicates a direct correlation between limestone dissolution and contaminants. Increased dissolution increases secondary porosity and thus permeability of the vadose zone resulting in higher vulnerability of underlying aquifers and springs in the vicinity landfills.     

建筑垃圾填埋场周边地下水化学组分来源解析

建筑垃圾是城市市政垃圾的重要组成部分,对其堆填处理可能造成场地及周围一定范围内的地下水污染。传统观念认为,建筑垃圾中多为惰性或无害成分,对地下水影响有限,缺乏关于建筑垃圾堆填对地下水水质造成影响的研究。选取2处建筑垃圾堆填场地,通过对场地周围地下水采样分析,得到场地周边地下水化学指标空间分布特征,并利用正定矩阵因子分解法(PMF)识别研究区地下水化学组分来源,从而定量评价建筑垃圾填埋对周边地下水水质变化的贡献。结果表明,建筑垃圾填埋会显著影响周边区域地下水组分的浓度和质量,尤其是TDS、TH、Ca2+、SO42-等组分显著受到影响,填埋时间越长、填埋体量越大,影响程度越深。两场地区域范围内Na+、Ca2+、Mg2+、SO42-和TDS 5个组分或指标的空间分布整体变化趋势较一致,均表现为垃圾填埋场附近沿地下水流下游方向的点位浓度较高,其他点位浓度较低,验证了建筑垃圾填埋对区域地下水质量的影响。PMF来源解析确定两场地周边地下水化学组分来源有建筑垃圾填埋、岩石风化溶解、水岩相互作用和农业活动,在场地一来源贡献占比分别为29.2%、21.6%、24.2%和15.1%,在场地二贡献占比分别为15.6%、23.2%、28.4%和18.2%。两场地周围地下水质量受人类活动影响程度较深,人为污染源正在成为地下水中离子组分的重要来源。     

Tracing subsurface migration of contaminants from an abandoned municipal landfill

This paper aims at determining of inorganic leachate contamination for a capped unsanitary landfill in the absence of hydrogeological data. The 2D geoelectrical resistivity imaging, soil physicochemical characterization, and surface water analysis were used to determine contamination load and extent of selective heavy metal contamination underneath the landfill. The positions of the contaminated subsoil and groundwater were successfully delineated in terms of low resistivity leachate plumes of <10 Ωm. Leachate migration towards the reach of Kelang River could be clearly identified from the resistivity results and elevated concentrations of Fe in the river downslope toe of the site. Concentration of Fe, Mn, Ca, Na, K, Mg, Cu, Cr, Co, Ni, Zn, and Pb was measured for the subsoil samples collected at the downslope (BKD), upslope (BKU), and the soil-waste interface (BKI), of the landfill. The concentration levels obtained for most of the analyzed heavy metals significantly exceed the normal range in typical municipal solid waste landfill sites. The measured heavy metal contamination load in the subsoil is in the following order Fe ? Mn > Zn > Pb > Cr > Cu. Taking into consideration poor physical and chemical characteristics of the local soil, these metals first seem to be attenuated naturally at near surface then remobilize unavoidably due to the soil acidic environment (pH 4.2-6.18) which in turn, may allow an easy washing of these metals in contact with the shallow groundwater table during the periodic fluctuation of the Kelang River. These heavy metals are believed to have originated from hazardous industrial waste that might have been illegally dumped at the site.     

城市垃圾填埋场甲烷资源量与利用前景

垃圾填埋气的主要成分为CH4、CO2等气体,可严重污染大气、地下水和生态环境,并对全球气候变暖产生一定的影响;同时填埋气也是一种清洁可再生能源和资源,回收和利用垃圾填埋气可实现环境、安全、能源、资源、经济多重效益。目前,垃圾填埋气的利用主要为甲烷利用。本文介绍了填埋气中甲烷资源量的计算方法,采用一阶动力模型对国内城市垃圾填埋气中的甲烷排放量进行了计算和预测,获得了城市生活垃圾填埋气中甲烷的资源量的范围,并分析了国内垃圾填埋气排放的特点和趋势以及国内外对填埋气利用的途径、方法及效果。结合清洁发展机制（CDM）项目和国情分析了垃圾填埋气的利用前景,并提出了填埋气回收利用的主要问题和建议,为国内城市生活垃圾填埋气的利用提供了有益参考。     

Geophysical characteristics of Wadi Hanifah water system, Riyadh, Saudi Arabia

Integrated geophysical techniques including resistivity image, vertical electrical sounding (VES), and seismic refraction have been conducted to investigate the Wadi Hanifah water system. The groundwater in Wadi Hanifah has problems caused by the high volumes of sewage water percolating into the ground. The combination of VES, resistivity image, and seismic refraction has made a valuable contribution to the identification of the interface between the contaminated and fresh water in Wadi Hanifah area. The contaminated groundwater has lower resistivity values than fresh groundwater due to the higher concentration of ions which reduces the resistivity. Resistivity image and sounding in this area clearly identified the nature of the lithological depth and proved useful at identifying water-bearing zones. Fresh groundwater was found in the study area at a depth of 100 m within the fractured limestone. Water-bearing zones occur in two aquifers, shallow contaminated water at 10 m depth in alluvial deposits and the deeper fresh water aquifer at a depth of about 100 m in fractured limestone. The interface between the contaminated water (sanitary water) and fresh water marked out horizontally at 100 m distance from the main channel and vertically at 20 m depth.     

Geoelectrical surveys of the Nanjido waste landfill in Seoul,Korea

Electrical surveys have routinely been taken to map and monitor groundwater contamination. In 1994-1996, various electrical surveys were applied to investigate contaminant distributions in the ground at the Nanjido landfill. The geophysical survey data were compared with other available information, particularly boring data. Interpretations of electrical survey data show low resistivity zones below 10 ohm-m which appear to be zones fully saturated with leachate. Annual variations of resistivity anomalies clearly indicate that resistivities and thicknesses of layers contaminated by leachate become lower and thicker in and around the Nanjido landfill during one year. In particular, mean thickness of saturated layers with leachate increased by about 3-6 m/year and the resistivity of bedrock decreased. It seems obvious that ground contamination by leachate is in progress. In the area northeast of the landfill, no evidence of bedrock contamination is indicated. Soundings made at one year intervals in this area do not show any evidence of further ground contamination by leachate. From these results, it appears that contamination of the weathered zone and bedrock is in progress mainly southwest of the Nanjido landfill.     

Can stable isotopes be used to monitor landfill leachate impact on surface waters?

Bacterially mediated methanogenesis in municipal solid waste landfills has been shown to cause an enrichment of carbon stable isotope ratios of dissolved inorganic carbon and hydrogen stable isotope ratios of water in landfill leachate. In the present study, we investigate the universality of this enrichment in leachate obtained from four diverse landfill sites in New Zealand. At each site, surface water samples upstream and downstream of landfills were analysed to examine the applicability of stable isotope ratios as a tool for monitoring leachate contamination in landfill-associated streams. The design of leachate collection systems, operational history, and landfill location appeared to strongly influence leachate isotopic values and the effectiveness of isotope ratios as an environmental monitoring tool for surface water.     

Permeability of Municipal Solid Waste in Bioreactor Landfill with Degradation

Bioreactor landfills are operated for rapid stabilization of waste, increased landfill gas generation for cost-effective energy recovery, gain in landfill space, enhanced leachate treatment, and reduced post closure maintenance period. The fundamental process of waste stabilization in bioreactor landfill is recirculation of generated leachate back into the landfills. This creates a favorable environment for rapid microbial decomposition of the biodegradable solid waste. In order to better estimate the generated leachate and design of leachate recirculation system, clear understanding of the permeability of the Municipal Solid Waste (MSW) with degradation and the factors influencing the permeability is necessary. The objective of the paper is to determine the changes in hydraulic properties of MSW in bioreactor landfill with time and decomposition. Four small-scale bioreactor landfills were built in laboratory and samples were prepared to represent each phase of decomposition. Then, the changes in hydraulic properties of MSW in bioreactor landfill with time and decomposition were determined. A series of constant head permeability tests were performed on the samples generated in laboratory scale bioreactor landfills to determine variation of permeability of MSW with degradation. The test results indicated that the permeability of MSW in bioreactor landfills decreases with decomposition. Based on the test results, the permeability of MSW at the first phase of degradation was estimated as 0.0088 cm/s at density 700 kg/m³. However, with degradation, permeability decreased to 0.0013 cm/s at the same density, for MSW at Phase IV.     

The Role of Fracture Structure in Leachate Pollution at Landfills: A Case Study of a Landfill in Chongqing City

This paper studies a landfill where there are three faults running through. As serious pollution has occurred to the geological environment, the landfill is to be closed up and renovated. The paper aims to explore the role of fracture structure in leachate pollution at the landfill. The research was carried out in several stages. First, mathematical models of the pre-renovation landfill with three faults running through and the landfill after renovation were established. And then, the boundary conditions and parameters of the two mathematical models were determined. The groundwater level of the landfills was simulated in order to modify the two mathematical models. As a result, a feasible mathematical model was achieved. Based on this model, a comparison was made of the COD concentration variations in the inside leachate and outside leachate between the two landfills. Accordingly, the impacts of the fracture structure on the pollution of leachate at the landfills could be identified. The study results show that while faults contribute to the migration of ieachate, they also serve as a confluence of leachate, thus further deteriorating the environment. The COD concentrations of the inside leachate and the outside leachate of the pre-renovation landfill are respectively 800 mg/L and 220 mg/L higher than those of the post-renovation landfill. Therefore, measures must be taken to handle the ieachate seepage in areas where there are faults as well as the neighboring areas so as to get the environmental pollution under control.     

The Role of Fracture Structure in Leachate Pollution at Landfills： A Case Study of a Landfill in Chongqing City

This paper studies a landfill where there are three faults running through.As serious pollution has occurred to the geological environment,the landfill is to be closed up and renovated.The paper aims to explore the role of fracture structure in leachate pollution at the landfill.The research was carried out in several stages.First,mathematical models of the pre-renovation landfill with three faults running through and the landfill after renovation were established.And then,the boundary conditions and parameters of the two mathematical models were determined.The groundwater level of the landfills was simulated in order to modify the two mathematical models.As a result,a feasible mathematical model was achieved.Based on this model,a comparison was made of the COD concentration variations in the inside leachate and outside leachate between the two landfills. Accordingly,the impacts of the fracture structure on the pollution of leachate at the landfills could be identified.The study results show that while faults contribute to the migration of leachate,they also serve as a confluence of leachate,thus further deteriorating the environment.The COD concentrations of the inside leachate and the outside leacbate of the pre-renovation landfill are respectively 800 mg/L and 220 mg/L higher than those of the post.renovation landfill.Therefore,measures must be taken to handle the leachate seepage in areas where there are faults as well as the neighboring areas so as to get the environmental pollution under control.     

Deterioration of groundwater quality in the vicinity of an active open-tipping site in West Malaysia

There is an urgent need for characterization of leachate arising from waste disposal to ensure a corresponding effective leachate management policy. Field and laboratory studies have been carried out to investigate the impact of municipal landfill leachate on the underlying groundwater at a site in West Malaysia. The solid waste was disposed of directly onto an unprotected natural soil formation. This situation was made worse by the shallow water table. The hydrochemical composition of groundwater in the vicinity of the site (background) is a dilute mixed cation, bicarbonate water. The high ionic balance error of ~13.5% reveals that the groundwater body underneath the site was a highly contaminated leachate rather than contaminated groundwater. Elevated concentration of chloride (355.48 mg/L), nitrate (10.40 mg/L as NO₃), nitrite (14.59 mg/L), ammoniacal-N (11.61 mg/L), sodium (227.56 mg/L), iron (0.97 mg/L), and lead (0.32 mg/L) measured downgradient indicate that the contamination plume has migrated further away from the site. In most cases, the concentration of these contamination indicators, together with the ranges of sodium percentage (66.3–89.9%) and sodium adsorption ratio (10.1–19.7%), were found to be considerably higher than the limit values of safe water for both domestic and irrigation purposes, respectively.     

Containment landfills: the myth of sustainability

A number of major problems associated with the containment approach to landfill management are highlighted. The fundamental flaw in the strategy is that dry entombment of waste inhibits its degradation, so prolonging the activity of the waste and delaying, possibly for several decades, its stabilisation to an inert state. This, coupled with uncertainties as to the long-term durability of synthetic lining systems, increases the potential, for liner failure at some stage in the future whilst the waste is still active, leading to groundwater pollution by landfill leachate. Clay liners also pose problems as the smectite components of bentonite liners are subject to chemical interaction with landfill leachate, leading to a reduction in their swelling capacity and increase in hydraulic conductivity. Thus, their ability to perform a containment role diminishes with time. More critically, if diffusion rather than advection is the dominant contaminant migration mechanism, then no liner will be completely impermeable to pollutants and the containment strategy becomes untenable.

There are other less obvious problems with the containment strategy. One is the tendency to place total reliance on artificial lining systems and pay little attention to local geological/hydrogeological conditions during selection of landfill sites. Based on the attitude that any site can be engineered for landfilling and that complete protection of groundwater can be effected by lining systems, negative geological characteristics of sites are being ignored. Furthermore, excessive costs in construction and operation of containment landfills necessitate that they are large scale operations (superdumps), with associated transfer facilities and transport costs, all of which add to overall waste management costs. Taken together with unpredictable post-closure maintenance and monitoring costs, possibly over several decades, the economics of the containment strategy becomes unsustainable. Such a high-cost, high-technology approach to landfill leachate management is generally beyond the financial and technological resources of the less wealthy nations, and places severe burdens on their economies. For instance, in third world countries with limited water resources, the need to preserve groundwater quality is paramount, so expensive containment strategies are adopted in the belief that they offer greatest protection to groundwater. A final indictment of the containment strategy is that in delaying degradation of waste, the present generations waste problems will be left for future generations to deal with.

More cost-effective landfill management strategies take advantage of the natural hydrogeological characteristics and attenuation properties of the subsurface. The ‘dilute and disperse’ strategy employs the natural sorption and ion exchange properties of clay minerals, and it has been shown that in appropriate situations it is effective in attenuating landfill leachate and preventing pollution of water resources. Operated at sites with thick clay overburden sequences, using a permeable cap to maximise rainfall infiltration and a leachate collection system to control leachate migration, ‘dilute and disperse’ is a viable leachate management strategy. Hydraulic traps are relatively common hydrogeological situations where groundwater flow is towards the landfill, so effectively suppressing outwards advective flow of leachate. This approach is also best employed with a clay liner, taking advantage of the attenuation properties of clays to combat diffusive flow of contaminants. These strategies are likely to guarantee greater protection of groundwater in the long term.     

不同降雨模式下山谷型垃圾填埋场水分运移及其稳定性研究

近10年的运营经验表明,国内南方地区第1批建造的山谷型垃圾填埋场中的渗滤液水位一般较高。同时,现有研究表明,降雨入渗引起渗滤液水位过高是垃圾填埋场失稳的主要诱因之一。因此,研究强降雨条件下山谷型垃圾填埋场的水分运移规律及其稳定性,具有重要的现实意义。基于七子山填埋场浅层、中层和深层垃圾土的土-水特征曲线和Brooks-Corey公式,利用非线性拟合技术得到垃圾土的渗透性函数;运用饱和-非饱和渗流理论,对递减型、中心型、增强型和平均型4种降雨模式下七子山填埋场的水分运移进行了数值计算;利用极限平衡理论,对不同降雨模式下七子山填埋场的稳定性进行了分析。结果表明,降雨模式对山谷型垃圾填埋场内的水分运移规律和填埋场的稳定性有显著的影响,递减型降雨模式下填埋场内孔隙水压的变化最大,同时填埋场稳定系数也下降最为明显,为最不利降雨模式;经历7 d 746 mm的极端强降雨后,七子山填埋场具有极大的失稳隐患。