The movement from sites of waste-disposal of contaminated groundwater is a problem faced by many cities in world. Solid and liquid waste dumped in landfills generates an atmosphere where inorganic and organic compounds present in the water or which are produced in the landfill become prone of entering the hydrological regime through movement. The movement of contaminants in water bodies through landfills is more distinct in temperate and humid climates as penetration of rainfall is moderate to high, and separation of leachate from the aquifers which are underlying for extended time periods becomes a complex process. In such areas, landfills gets anaerobic leading to decay of organic materials mainly through fermentation, therefore generating many transitional organic constituents that influence the groundwater chemistry in a different way than those initially dumped. The understanding of area hydrogeology and the processes which are physical, like dispersion, dilution and filtration, clarity of the geochemical and biological processes is essential to illustrate and envisage the contaminants movement to the aquifers.
The chief chemical processes and reactions in groundwater at dumping sites comprises of biological decomposition, precipitation and suspension of inorganic compounds and minerals, absorptions of chemical component, sediments leaching, exchange of ions, movement of suspended species and production and dispersion of gases. This paper discusses the groundwater contamination at a landfill and describes the hydrogeologic courses and chemical reaction which impacts the chemistry of groundwater. The landfill considered in this paper is Army Creek landfill, formerly Llangollen landfill, in Delaware New Castle County.
Problem Description
Army Creek landfill is built on 24 hectares area and comprises of refuse of 1,500,000 m3. The refuse was dumped from 1960 to 1968 in a deserted quarry after removal of 6 to 9 metre of gravel and sand which became the landfill. The digging of gravel and sand continued in quarry until the red clay zone and water table was reached. Several types of wastes were dumped, including liquid and solid industrialized wastes and refuse from municipal. In 1960s, chief well fields were created to confine aquifer in 900 to 1200metre east and south of the landfill. Pumping of water from well fields led to lowering of level of water and augmented the downward movement of the water from the landfill towards aquifer. In 1972, a problem in quality of water was identified in a domestic well situated nearby.
The thorough investigation of the causes of contamination led to conclusion that leachate was able to migrate in the aquifer which was situated beneath the landfill, due to either the removal or absence of layer of red-clay in the landfill . Severely contaminated leachate, rich in both inorganic and organic compounds were not sufficiently purified or diluted in filtration before entering aquifer and thus, lead to contaminated water. By the year 1972, the confirmation of extensive spread of leachate all through the restricted aquifer was gained and it was identified that the contamination was spreading to south towards the supply wells.
Hydrology of the Landfill
Atlantic Coastal Plain comprises of the landfill of Army Creek which comprises of both saturated and unsaturated zones. The hydrogeological section of the landfill revealed that sands of Columbia group that is the Pleistocene age were not thick in the direct area to allow for development of supply of water. Beneath Columbia sand, lied the Cretaceous Potomac Formation beneath which were present the Precambrian rocks. Potomac formation comprises of clay and silt was interbedded with gravel and quartz sand . Potomac Formation’s upper sand gets thick towards southeast and makes one of the major industrious confined aquifers as demonstrated in figure 1. Potomac clay, forming the upper layer is thin and is absent near to the landfill. Principle well fields situated at east and south of landfill of Potomac sand have the capability to produce water of 25 x 103 cubic m per day. Pumping of water from well fields reduced the water level, thereby increasing the water movement rate through aquifer and promoted the downward movement of landfill water to Potomac sand.
Inorganic Contaminants
The relative quantities of the principle inorganic components in water, Mg2+, Ca2+, K+, Na+, NO3-, SO42-, and Cl- were found to different at significant level in native groundwater and at anaerobic zone. The average concentrations of major inorganic compounds in groundwater, overlapping zone and anaerobic zone are demonstrated as milliequivalents/liter percentage and their standard deviation is displayed in Figure 2.
Group A demonstrates the local groundwater which has Cl- as chief anion which is due to abundant rainfall in the area. Concentrations of all compounds were found to be quite low in the groundwater. No cation in the groundwater was found to be contributing above 50 per cent of the total cations, which is significant characteristic of Atlantic Coastal Plain water. Group B represents the overlapping zone of groundwater and landfill water and demonstrates mixed cation-anion ratio. The water chemistry is predominated by mixing of groundwater with leachate. Group C represents the leachate water and major cations like Mg2+, Ca2+, K+, and Na+ were found as a result of clays exchange with NH4+ produced in leachate. The high quantity of Mn2+ and Fe2+ was the resultant of water’s contact with metals and minerals with Mn and clay and sand containing ferric oxide due to material lying in the landfill. Fe2+ and NH4+ accounts for almost 50 per cent of the total cations present in anaerobic zone.
Organic Contaminants
The groundwater is richly oxygenated but the water in the landfill is devoid of oxygen due to respiratory processes. The oxidizable material present in landfill was far more in quantity than the amount of oxygen present on groundwater. After the utilization of oxygen, organic matter was reduced and oxidized by anaerobic fermentation process which produces carbon-dioxide and organic species like methane. Volatile organic compounds, BCCE, 1,2-dichloroethane and benzene was found in Army Creek landfill. The majorities of the organic compounds in water were polarized in nature and sediment at a faster rate. High concentration of organic compounds in landfill is the resultant of microbial degradation in anaerobic condition. The gas released initially in landfills is carbon-dioxide substituted by methane at later stage.
Method of Reducing Contamination
With the discovery of contamination in 1970s, the efforts were put to limit the spread of contamination. The pumping from supply wells were reduced and the regime to initiate the pumping of recovery well between the supply wells and landfill. This led to creation of local cone of depression which decreased the movement of contaminated water to supply wells. Moreover, the flow of direction of Potomac aquifer was altered gradually over the years. This along with recovery wells diverted the flow away from supply wells and resulted in the localization of contaminants in the south of system of recovery wells. The efforts were also resulted in gaining funds from federal government and becoming a priority site enlisted in National Priorities List which required long term remedial actions.
In 1990, a mixed funding consent decree was formed between the responsible parties classified as potentially responsible parties which included New Castle County, Du Pont, Chrysler, General Motors and others and initiated the construction over landfill of multi-layer cap. This was followed by building of an on-site facility for water treatment, completed in 1993. The multi-layer cap construction revealed several buried drums, which contained volatile organic compounds. These were removed and shifted to hazardous waste treatment facility. The on-site treatment plant was closed in 2004 from landfill and shifted to Delaware Sand & Gravel Landfill site which was identified as the prime source of contamination of BCEE. The U.S. Environment Protection Agency have been working actively on landfill to reduce the level of contamination and minimize the spread of contaminants. The system has been reported to clean 1.5 billions of contaminated water. The treatment system of contaminated groundwater captures and decontaminates the water from superfund sites adjacent to landfill.
The landfill site, with the construction of protection cap has found its usage as a productive resource. EPA along with trustees of localized natural resources has converted the landfill site into a habitat for wildlife and birds. The protective cap’s integrity is ensured by removal of deep-rooted plants into the landfill.
Impact on Local Population and Wildlife
EPA reported that the current exposure to humans at the landfill is under control and is not a source of any threat. BCEE and benzene are two organic components of special concern as benzene is known to be an effective human carcinogen that is cancer causing organism. Benzene has severe health affects ranging from nose and throat irritation to congestion in lungs. It also has effect on nerves including headaches, slurred speech and dizziness.
Conclusion
In contaminated water, chemical processes and processes do not remain constant and change over time. Early diagnosis of water through landfill causing the contamination aids in quick action and facilitates control of spread of contaminants. The Army Creek landfill causing the contamination was majorly due to the water movement resultant of pumping. The hydrology of landfill plays a significant role in deciding the extent of organic and inorganic contamination. The identification of contaminants in supply well in 1970s led to the establishment of recovery well system which reduced the contamination spread to a greater extent. The anaerobic constituents of the water become threat for human and other lives with the production of methane, benzene and BCEE. Due to efforts of EPA, now the landfill is evolved as a habitat for wildlife and birds and poses no threat to the local environment.
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