RECOGNIZING THE ROLE OF SATURATED EPIKARST IN SOLUTE TRANSPORT: AN EXAMPLE FROM THE LEHIGH VALLEY, PENNSYLVANIA

In karst systems where the epikarst lies submerged beneath the phreatic surface, understanding the unique permeability structure of the epikarst can be vital for evaluating the fate and transport of contaminants. While the epikarst’s functions of contaminant mass storage and downward transmission are widely acknowledged, particularly as applied to the vadose zone, the unique role that a saturated epikarst may play in lateral groundwater transport is rarely reported in the literature. This omission may stem from the difficulty of recognizing and characterizing a saturated epikarst as a distinct hydrostratigraphic interval. The epikarst, like partially-weathered rock in Piedmont terrains, is a transition zone that exhibits hydrogeologic characteristics that are a hybrid of the residuum above, and bedrock below. Intense solution weathering in the epikarst creates a highly complex subsurface environment that is heterogeneous, often anisotropic, and where the high density and interconnectedness of conduits imparts a high bulk transmissivity and porosity. Particularly at the scale of a single industrial site, the dynamics of flow in the epikarst, and not the karst aquifer in general, can be the principal control on contaminant fate and transport.

We present an analysis of field data for a site in Pennsylvania’s Lehigh Valley where chlorinated solvents are present within in a 20-meter thick saturated epikarst formed in structurally complex Paleozoic dolomite. Observations drawn from monitoring wells, continuous water-level monitoring and a tracer study indicate that the epikarst is highly transmissive and supports lateral flow rates that are feasible only via conduits. We hypothesize that the epikarst is poorly drained, and that groundwater within it is inefficiently coupled to a bedrock karst drainage network governing basin-scale drainage. The epikarst is interpreted to form a discrete cell within the groundwater basin that is contained by physical or hydraulic boundaries and flux-limited by its connections to the bedrock karst system.