GROUNDWATER-SURFACE WATER INTERACTIONS ACROSS A SLATE-CARBONATE BOUNDARY IN THE GREAT VALLEY OF EASTERN PENNSYLVANIA

The Great Valley Section of the Valley and Ridge province in eastern Pennsylvania is underlain primarily by two major lithotypes, the Martinsburg shale/slate towards the north and a suite of Cambro-Ordovician carbonate rocks (limestone and dolomite) to the south. The Martinsburg surface is a broad upland surface that is heavily dissected by streams and rivers creating a rugged topography, whereas the carbonate terrain consists of a much more subdued karst topography characterized by closed depressions and sinkholes. Many small watersheds (<300km²) begin in the slate and shale terrain, and then flow south across the carbonate section before discharging into the Lehigh or Delaware Rivers.

We mapped twenty watersheds using ArcGIS to compare drainage densities in the two different lithotypes. Drainage densities were measured using two different data sets, the National Hydrography Dataset (NHD) and also a highly accurate data set produced using aerial photography during a period of significant runoff (NCHydro). We measured discharge at multiple locations through time in four watersheds to determine if there was a lithological control on the groundwater - surface water relationships.

Drainage densities are higher in the slate terrain than in the carbonate terrain for both data sets (NHD and NCHydro), however the drainage densities measured using the high resolution (NCHydro) data were significantly higher than the National Hydrography Dataset. Using the NCHydro data, the average drainage density for the slate segments of the watersheds that flowed through both terrains is 2.10 km/km² and the average drainage density for the carbonate segments of the same watersheds is 1.29 km/km².

Stream discharge data indicates that Mud Run, Oughoughton Creek, and Martin’s Creek change from effluent (gaining) to influent (losing) near the interface between slate/shale and carbonate regions. These streams emerge from fracture and cleavage controlled springs, gain discharge progressively in the downstream direction as they flow across the Martinsburg terrain, and then lose discharge into the subsurface when they cross into the carbonate terrain. The Bushkill Creek also loses flow into the carbonates downstream of the bedrock interface, however the data are more complicated lower in the watershed.