GEOCHEMICAL PATHWAYS IN FRACTURED BEDROCK AT AN ARSENIC-CONTAMINATED LANDFILL SITE

High arsenic concentrations (0.030-0.130 mg/L) in domestic bedrock wells abutting a closed landfill in coastal Maine instigated an investigation of leachate-enhanced release of metals to the groundwater. The primary question is whether arsenic release occurs during reducing conditions from natural consumption of oxidants as groundwater passes from recharge to discharge areas or from the enhancement of bacterial reduction by organic-rich leachate. The complicated nature of the geochemical pathways at this site required multiple deep bedrock monitoring wells and multi-pronged geochemical analysis of groundwater.

There are several avenues for investigation: (1) assessment of interconnectivity of fracture networks between the landfill and the domestic wells and (2) geochemical analysis of groundwater. At this site, passive hydraulic tests did not reveal direct hydraulic connection between landfill monitoring wells and domestic wells, but chemical analysis of groundwater revealed a complex pattern of groundwater interconnectivity.

Downgradient location of monitoring wells was insufficient for predicting plume location. Comparison of Stiff diagrams helped delineating flowpaths. Graphs of Cl/Br ratios versus chloride revealed both road salt and leachate sources. High sulfate (up to 420 mg/L), methane (up to 3.3 mg/L), and nitrite (2.6 mg/L) concentrations indicated landfill influence. In graphs of Na versus Cl, landfill-influenced bedrock groundwater plots close to a 1:1 line whereas groundwater influenced by natural reactions along long flowpaths plots above the Na/Cl>1 line. A strong positive nitrite-Cl correlation suggests binary mixing of landfill waters with background water may be useful for assessing leachate influence.

A downhole video camera revealed considerable oxyhydroxide growth on the upper walls of one domestic well, but not in its hydraulically-connected neighbor. One monitoring well had deposits, but another leachate-impacted one did not. High Fe and Mn concentrations in the groundwater coupled with methane are hypothesized to be responsible for these bacterially-activated (?) deposits. Oxyhydroxides probably precipitate under oxidizing conditions during drawdown (~40 ft) in the domestic well, but under uninterrupted anoxic conditions in the monitoring well.