EFFECTS OF BEDROCK LITHOLOGY ON GROUNDWATER LEVELS AND GROUNDWATER/SURFACE WATER INTERACTION IN SLATE AND CARBONATE WATERSHEDS IN EASTERN PENNSYLVANIA

Previous work in multiple watersheds in the Great Valley of Eastern Pennsylvania demonstrated that groundwater/surface water interactions are strongly influenced by bedrock geology. Watersheds included in the study flow from their sources in the Ordovician age Martinsburg slate and then traverse Cambro-Ordovician age carbonates before discharging into the Delaware River. Discharge data from four streams that traverse the slate/carbonate interface show that, during baseflow conditions, streams switch from gaining to losing across the boundary between the two lithotypes. During near drought conditions that prevailed late in the water year during several of the last few years, several of the streams studied were actively flowing in the slate terrane and completely dried up lower in the watershed where the streams lost flow into the carbonates.

In this paper we present groundwater data from wells in both the slate and carbonate terrane to develop a model that explains how lithology affects the hydrology and groundwater/surface water interactions in this landscape. Many of the first-order tributaries in the slate watersheds are topographically amphitheater-shaped, and are initiated at active springs. In some cases springs form from seepage at the surface cover/bedrock interface, or at bedrock seeps. In contrast, many of the first-order tributaries in the carbonate terrain are dry during baseflow conditions. Although there is a significant amount of variation in the area, groundwater data in the two terranes indicate that groundwater levels tend to be deeper in the carbonates and at lower absolute elevations than the slate which we believe reflects higher hydraulic conductivities in the carbonates. Higher hydraulic conductivity in the carbonates make it easier for the groundwater table to grade to the regional baselevel with lower hydraulic gradients. In contrast the lower hydraulic conductivity in the slate results in steeper hydraulic gradients and higher water levels. Because streams are more deeply incised in the slate terrane, which averages approximately 300 feet higher than the carbonate terrane, stream beds are more likely to be lower than the local groundwater levels. In contrast, in the carbonate terrane, many segments of the drainage network are perched above the bedrock groundwater level.