Discovery of submerged springs: a step forward to effectively remediate and manage contaminated groundwater in a karst aquifer

Abstract

Groundwater is contaminated with chlorinated volatile organic compounds (CVOCs) including trichloroethene (TCE), tetrachloroethene (PCE), and their degradation products at a 230-acre formerly used defense site in Pennsylvania. Unconsolidated overburden materials ranging in thicknesses from 15 to 60 feet overlie two types of bedrock: quartzitic sandstone and phyllite on the eastern sloping hills and limestone and dolomite on the western flat-lying terrane. Karst is well developed in the carbonate rocks and is characterized by a pinnacle-and-cutter bedrock surface, solution-enlarged fractures/conduits at depths up to 450 feet below ground surface (bgs), caverns in tens of feet, and frequent occurrence of sinkholes. The PCE and TCE concentrations in groundwater samples suggest that dense non-aqueous phase liquid penetration extended to depths on the order of 400–450 feet bgs in the carbonate bedrock. A dye trace study was conducted to help refine the conceptual site model. The study consisted of five fluorescent dyes that were injected in five wells that encountered karst conduits/caverns at various depths from 32 to 216 feet bgs. The dyes were monitored at 40 monitoring wells and 16 points along a stream to the west of the site. The dye tracing results established positive dye detections at seven monitoring points in the stream. Field observations, infrared thermography imaging, and water quality survey confirmed that three of them, i.e., Spring-15, Spring-17, and Spring-26, are groundwater discharge points in the stream, i.e., submerged springs. Discovery of the submerged springs significantly contributed to the refinement of the conceptual site model and provided an opportunity to effectively remediate and manage the contaminated groundwater. These springs confirmed the exposure pathways that connect the site-impacted groundwater to surface water receptors, and the final remedy included measures to address potentially unacceptable risks to human beings and the environment in the stream and ensure that the state-regulated ambient water quality criteria are met for the site CVOCs at established compliance monitoring stations. These springs also helped optimize an existing pump and treat system that has been active since 1994. Shutdown and restart-up tests were conducted to evaluate the effectiveness of different extraction scenarios in the removal of the CVOCs at the springs under wet and dry seasons. The results led to a new configuration of the extraction wells by including a previously inactive extraction well CW-20.