The 416-km2 Mahantango Creek Watershed is located about 40 km north of Harrisburg, PA; the watershed outlet is near the town of Malta, PA. Mahantango Creek is a tributary of the Susquehanna River, which contributes 50% of the streamflow input to the Chesapeake Bay. Mahantango Creek Watershed is typical of upland agricultural watersheds within the nonglaciated, folded and faulted Appalachian Valley and Ridge Physiographic Province. Elevation ranges from about 160 to 490 m (msl), and climate is temperate and humid. Average precipitation is approximately 1090 mm yr-1 and streamflow about 650 mm yr-1. Ground water provides up to 60-80% of the annual streamflow. Subsurface flow is controlled by an unconfined, highly transmissive, highly fractured, shallow bedrock layer with low storativity. Overland flow occurs primarily on expanding and contracting areas of ground water discharge and on areas underlain by low-permeability clay (fragipan) layers. Shallow residual soils, mostly silt loams less than 1.5 m deep, cover the underlying bedrock. Mature forest covers the dominant ridge to the north, while cropland and pasture dominate the rolling hills of the watershed interior. Total area is approximately 44% cropland or pasture, 55% forested, and 1% urban and residential.

WE-38 is a 7.3-km2 intensive-study subwatershed of Mahantango Creek. This subwatershed is located on the north flank of the major anticline underlying Mahantango Creek Valley, so all strata dip to the north with a general east-west strike. There are no major faults mapped within WE-38, but local bedding-plane faults are evidenced in rock cores by thin zones of intense fracturing and extensive oxidation staining. Two bedrock formations occur within the subwatershed. The Trimmers Rock formation (Late Devonian) is predominantly shale and outcrops at the watershed outlet (south) in a near-horizontal position. The overlying Catskill (Late Devonian-Early Mississippian) consists of interbedded shales, siltstones and sandstones that become increasingly coarse-grained to the north. The dip of the Catskill strata increases to approximately 30°, where a relatively pure quartz-sandstone-conglomerate outcrops to form the northern watershed divide. No limestone or karst geology occurs within the basin.

Agricultural land use dominates the Susquehanna River basin and most tributaries including the Mahantango Creek watershed. Agricultural land use has also been identified as a major source of nutrients to the Chesapeake Bay. Specifically, within the Mahantango Creek Watershed the following issues are of concern:

Nutrients: Ground water and first-order stream nitrate concentrations in the basin are correlated to adjacent land use. Consequently surface and subsurface water concentrations around high-density poultry and swine operations and intensively cropped fields are usually elevated above acceptable limits. High soil phosphorus levels in fields susceptible to surface runoff often result in significant amounts of the nutrient being exported to adjacent water bodies.

Soil erosion: Lack of stream bank stabilization and heavy agriculture has led to soil erosion and sediment-laden discharge in streams.

ARS has had a presence in the Mahantango Creek Watershed since 1966. However, the majority of the research and monitoring has been conducted within WE-38. Primary research objectives in the past have included determination of subsurface pathways and travel times and an estimation of runoff generation and nutrient export mechanisms. Future research objectives in the basin include further refinement of both subsurface and surface transport mechanisms, impacts of frozen soil and snow on nutrient transport, and BMP assessment.

Gburek, W.J. and Folmar, G.J, 1999, A ground water recharge field study: site characterization and initial results: Hydrological Processes, vol. 13, p. 2813-2831.

Gburek, W.J., and Folmar, G.J., 1999, Flow and chemical contributions to streamflow in an upland watershed: a baseflow survey, Journal of Hydrology, v. 217, issue 1-2, p. 1-18.

Gburek, W.J., Folmar, G.J., and Urban, J.B., 1999, Field data and ground water modeling in a layered fractured aquifer: Ground Water, v. 37, no. 2 p. 175-184.

Gburek, W.J. and Urban, J.B., 1990. The shallow weathered fracture layer in the near-stream zone. Ground Water, 28:875-883.

Gburek, W.J., Urban, J.B. and Schnabel, R.R., 1986. Nitrate contamination of ground water in an upland Pennsylvania watershed. Proceedings of Agricultural Impacts on Ground Water - A Conference. NWWA, Omaha, Nebraska, pp. 352-380.

Lindsey, B.D., Gburek, W.J., and Folmar, G.J., 2001, Watershed scaling effect on base flow nitrate, Valley and Ridge Physiographic Province: Journal American Water Resources Association, vol. 37, no. 5, p 1103-1117.

Pionke, H.B., W.J. Gburek, A.N. Sharpley, and R.R. Schnabel. 1996. Flow and nutrient export patterns for an agricultural hill-land watershed. Water Resour. Res. V. 32, no. 6, pp. 1795-1804.

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