Relationship Between Subsurface Landfill Gas and Arsenic Mobilization into Groundwater

Abstract

The maximum contaminant level for arsenic was reduced by the U.S. Environmental Protection Agency (U.S. E.P.A.) for Drinking Water Standards from 50 micrograms per liter (µg/L) to 10 µg/L, effective January 23, 2006. The subject site is a double‐lined sanitary landfill facility located in the mid‐Atlantic region of the United States. Arsenic was reported above the maximum contaminant level in a downgradient monitoring well (MW‐18) in July 2005. Since July 2005, arsenic levels in MW‐18 fluctuated above and below 10 µg/L. This research focuses on determining whether reducing conditions in groundwater, enhanced by subsurface landfill gas emissions, were causing naturally‐occurring arsenic to mobilize from the native variably‐saturated vadose zone soils into groundwater.The groundwater data collected from the impacted well (MW‐18) were compared to an upgradient well (MW‐8) to determine whether significant differences existed during the time period of April 2004 to April 2007. Linear regression analysis was also used to determine whether other parameters had a significant relationship with the arsenic concentrations detected in MW‐18. The groundwater located in MW‐18 was consistently more reduced than groundwater located in the upgradient/background well MW‐8, and this was most likely attributed to the presence of subsurface landfill gas in the area. According to the U.S. E.P.A., oxidation‐reduction potential (ORP) values in groundwater less than 50 millivolts (mv) suggest that a reducing environment may be present. The data presented in this study indicate that arsenic can mobilize into groundwater under moderately reducing conditions, with ORP measurements averaging 53 mv.