by Hilary Haskell
Leachate containing Volatile Organic Compounds (VOCs) may be treated through a pump and treatment system at a landfill site. Following landfill closure, monitoring and extraction wells can be used to determine the effectiveness of this treatment method in preventing groundwater contamination. Martinez and Liu (2014) studied the Chestnut Avenue Landfill in Fresno, California, following its closure to determine if its pump and treat system had adequately reduced VOC concentrations in leachate. This Class III landfill had monitoring wells to collect groundwater data, but lacked a liner. Thus, the landfill provided an opportunity to research the behavior of leachate and its interactions with groundwater, and whether or not pump and treat systems could sufficiently remove VOCs from leachate.
Landfills provide designated locations for waste disposal and monitoring, and thus result in both safer and cleaner disposal in comparison to open dumping. Still, landfills can threaten groundwater quality, especially if they are not properly lined. When water percolates through the landfill via rainfall or other means, it can contaminate groundwater. This contaminated leachate is the product of biological, chemical, and physical processes due to the degradation of waste. The relative share of leachate’s organic and inorganic components depends on solid waste characteristics, precipitation rates, site hydrology, compaction, waste age, cover design, leachate interaction with the environment, sampling procedures, and landfill design and operation. In this study, Martinez and Liu (2014) focus on the volatile organic compounds (VOCs) found in leachate. VOCs are typically found in fossil fuels, plant degradation by-products, chlorinated solvents, and pesticides. The overall impact of a landfill on groundwater quality depends upon landfill thickness, moisture content, temperature, and water density and composition.
Previous studies have focused on how pollutants from solid waste migrate and undergo physiochemical and biological processes. Traditional methods of groundwater analysis have focused on hydrological descriptions and geophysical studies using vertical electrical sounding and ground penetrating radar to study contaminant plumes. Additionally, data are usually collected through monitoring wells and data logging probes that monitor hydrologic behavior. Solid phase extraction is used to determine qualitative and quantitative properties of new compounds found in leachate. Through installing extraction wells perpendicular to regional groundwater flow, researchers have improved the capture zone and minimized drawdown. Many studies have considered specific pollutants such as organic compounds, nutrients, minerals, and heavy metals by collecting leachate and then analyzing its properties. Microorganisms are commonly used to remove organic pollutants from leachate. Simulation-based models are typically used to determine monitoring efficiency, while mathematical models can simulate pollutant transport and eventual destinations.
Martinez and Liu studied the 32-acre Chestnut Avenue Landfill in Fresno, California. This Class III landfill lacked a liner, and had been monitored to provide data on groundwater contamination since its closure. As a Class III landfill, the site accepted non-hazardous materials, consisting mostly of municipal, industrial, and commercial wastes. The landfill is situated near a railroad, agricultural land, and agricultural waste ponds. Beneath the landfill are Holocene alluvial fan deposits from the Sierra Nevada mountain range. Soil near the site is mostly inter-bedded, light brown, micaceous silty sands and sandy silts. The main groundwater sources in the Fresno area come from the upper, unconsolidated, medium to coarse-grained alluvial complex within 60–70 feet of the surface. The unconfined aquifer system extends 400–1,000 feet, and the groundwater gradient is sloped toward the southwest. Water percolating into this landfill turns to leachate, and can eventually come in contact with groundwater. Beneath the landfill, groundwater occurs at approximately 60–90 feet below ground, and flows toward the northwest. Seasonal differences in groundwater flow may be due to standing water recharge in the irrigation recharge basins northwest of the site. Hydraulic conductivity ranges from about 10 to 70 feet each day. Groundwater velocity both below and next to the site is about 0.13 feet to 0.91 feet per day
The landfill closed in September 1993 after approximately 30 years of operation, and was covered in July 1995 with a protective soil layer, surface vegetation, a thick high-density polyethylene geomembrane barrier layer, and a foundation layer. Since its closure, VOCs have been detected, thus prompting installation of a pump-and-treat system in 1993 to reduce detection levels. Despite installation of the system, detections have remained at low, constant levels for the past five years. Continuous detection of these contaminants indicates that the system may need to be replaced, or a different method of remediation should be implemented to decrease the continuous VOC pollution. The authors aimed to determine how effectively the pump and treat system is in treating the contaminated plume, as well as to study migration and fate of VOCs to groundwater surrounding the landfill. By determining which VOCs were most recently abundant and their relative concentrations from monitoring well data, the authors could examine trends and determine if the pump and treatment system in place is sufficient to remediate the contamination plume.
The authors collected groundwater monitoring data from a variety of wells located at and around the landfill site that had previously detected VOCs. Measurements were taken using electronic water-level sounding equipment at each well to determine groundwater flow patterns, water-level fluctuations, and the volume of water in each well so that purge volumes could be calculated. The authors purged the monitoring wells to remove any remaining stagnant water, thus allowing for accurate water formation samples. Water samples were measured for pH, temperature, and specific conductivity. The corrective action monitoring program network of monitoring wells consisted of 12 wells, at depths between 71 and 150 feet below ground surface with a pair of shallow and deep monitoring wells installed using the Air Rotary Casing Hammer method The wells were made of PVC pipes with a protective casing at the surface. Although the landfill site had many monitoring wells, only the seven that had consistently detected concentrations of the chemicals 1,1-Dichloroethene, Cis-1,2-Dichloroethene, Dichlorodifluoromethane, and Trichloroethene were studied. Martinez and Liu focused on these chemicals because they had been detected consistently since the landfills closure, despite installation of a pump-and-treat system. VOC samples were analyzed using EPA Method 8260B–Volatile Organic Compounds by Gas Chromatography/Mass Spectrometry (GC/MS) to determine whether VOCs were present, and at what concentration.
The authors found that down gradient monitoring wells situated either on the landfill or along it received the most contamination. Some of the off site, down gradient wells were used to extract groundwater from the capture zone for the pump-and-treatment system, which then processed the water to remove any VOCs before discharging the water to a percolation basin. The authors created groundwater elevation contour maps using Surfer to demonstrate trends in groundwater elevation changes, which indicated a general tendency for water to flow southwest.
Trichloroethene was the most common compound detected at the landfill, which breaks down into 1,1,1-Dichloroethene, Cis-1,2-Dichloroethane, and Dichlorodifluoromethane. Their concentrations were highest at wells immediately down gradient from the landfill. The contour maps demonstrate that concentrations of these compounds tend to migrate down gradient, as they are initially detected at an up gradient site, and then later at a down gradient site. Fluctuations in the contour maps can in most cases be attributed to interruptions in operation of the pump and treat system. The authors indicated which groundwater monitoring wells detected 1,1,1-Dichloroethene, Cis-1,2-Dichloroethane, and Dichlorodifluoromethane since the landfill’s closure. Deviations from typical levels of 1,1,1-Dichloroethene were most likely due to the system’s failure to allow water percolation into the treatment basin at a compatible rate. Some sporadic detections of this compound are not readily understood based on the pump and treat system’s operations. Wells down gradient from the landfill had lower concentrations of the pollutant, presumably due to dilution. When one of the extraction wells was removed in 1998, this did not impact the detected concentrations of 1,1,1-Dichloroethene. Typically, wells closer to an extraction well detected higher concentrations of the contaminant. 1,1,1-Dichloroethene was last detected was in spring of 2004.
Cis-1,2-Dichloroethene was consistently detected from 1994 until 2005. Fluctuations in detection of this contaminant can also be attributed to the pump and treat system’s operation. For this compound, removal of an extraction well may have influenced higher detection rates. Extraction wells pump out water (contaminated or non-contaminated), and thus VOC levels are detected near monitoring well sites because they share the same groundwater. The last detection of Cis-1,2-Dichloroethene occurred in June 2005.
Dichlorodifluoromethane was not detected when the landfill was first closed. The compound was first found near the end of 1994. This contaminant’s concentration decrease following instillation of the pump and treat system indicates the effectiveness of the system in reducing concentrations of VOCs in leachate. After 1997, the compound was no longer detected.
Finally, Trichloroethene, the main chemical considered in this study, has been detected on a consistent basis for 18 years at the landfill site. There has been a general decrease in detection of the compound in most of the monitoring wells, although one well continued to detect low levels. All of the compounds detected in the study, which are formed from the breakdown of Trichloroethene, were no longer found after some time following the installation of the pump and treat system. Trichloroethene continues to be detected at non-health-threatening levels. Fluctuations in detection levels for Trichloroethene do not follow the same patterns of other down gradient wells. Detection rates of Trichloroethene decreased, and no longer exceed maximum contaminant levels as determined by the EPA. Trichloroethene was found at higher levels at wells down gradient from the landfill, and fluctuations in levels of the contaminant also mirror operation of the pump and treat system.
During the 18 years following the landfill’s closure, it appears that the pump and treat system has effectively removed much of the contamination of Trichloroethene. Levels currently detected do not exceed maximum contaminant levels of 5 μg/L. Therefore, this compound is the only compound still detected at the site, while all other compounds have not been detected since 2005. This finding indicates that the pump and treatment system is an effective remediation method. In the future, the pump and treat system will probably not remove any additional Trichloroethene, as the pump and treat system cannot selectively remove groundwater contaminants. The authors suggest other remediation methods for any remaining Trichloroethene, such as Monitored Natural Attenuation, which uses small insects to consume remaining chemicals and convert them into gaseous compounds following digestion.
Martniez, E., Liu, L., 2014. Investigation of the Migration of Selected Volatile Organic Compounds in Groundwater Under an Unlined Landfill. Energy and Environmental Engineering 2, 55–66. Complete paper: http://bit.ly/1sSi3XG
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