GROUND WATER CONTROLS ON HYDROLOGY AND WATER QUALITY WITHIN RURAL UPLAND WATERSHEDS OF THE CHESAPEAKE BAY BASIN

Authors: Gburek, William; Folmar, Gordon; Schnabel, Ronald

Submitted to: Chesapeake Research Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 11/26/1994
Publication Date: N/A
Citation: N/A

Interpretive Summary: Recent research of USDA-ARS at University Park, PA directed toward quantifying the role of ground water in the hydrology and water quality of rural upland watersheds of the Chesapeake Bay Basin with their mix of forest, cropland, pasture, and suburban land use is summarized. Geometry and hydraulic properties of the aquifer have been characterized by rock coring, seismic surveys, and hydraulic testing. Modeling of ground water flow using these characterizations provides the basis for flowpath analysis and contaminant transport simulations. Ground water quality is shown to be affected by patterns of land use over the watershed. Land use within upland watershed positions controls ground water quality within the deeper regional aquifer, while patterns of land use over the remainder of the watershed have a direct effect on underlying ground water quality patterns within the shallower fractured layers. In the shallowest layers of near-stream zones, where riparian zone processes can reduce ground water nitrate concentrations from agricultural inputs, water quality is dominated by ground water inflows from land uses immediately upslope with only minimal influence of the regional aquifer discharge affected by more distant land uses. The research provides a means to estimate patterns of contamination of ground water and nonstorm streamflow by agricultural land use distribution, as well as to control ground water contamination by positioning of land use. As a research tool, the methodology provides a framework in which to develop, test, and evaluate detailed water quality investigations within layered and fractured watersheds and their tie to surface water processes.

Technical Abstract: Upland watersheds within the nonglaciated portion of the Chesapeake Bay Basin are commonly underlain by bedrock which is severely fractured at shallow depths as a result of stress-relief fracturing. The highly conductive shallow fracture layer is a major control on ground water flow and its quality both within and from these watersheds. Here we summarize the recent research of USDA-ARS at University Park, PA directed toward quantifying the role of this fractured hydrogeology in the hydrology and water quality of rural upland watersheds with their mix of forest, cropland, pasture, and suburban land use. Results of characterization of the fractured aquifer geometry and hydraulic properties by rock coring, seismic surveys, and hydraulic testing are presented. These techniques are consistent in defining fracture layer depths and hydraulic properties. Using this information, ground water flow and flowpaths can be modeled to illustrate effects of land use distribution on patterns of nitrate contamination within the ground water and its ultimate transport to the stream. The modeling shows that land use over the most upland positions of the watershed controls ground water quality within the regional aquifer, while land use over the remainder of the watershed area affects water quality only within the shallower layers of the aquifer. In the shallowest layers of the near-stream zones, where riparian zone processes can be effective in reducing ground water nitrate concentrations from agricultural inputs, water quality is dominated by ground water inflows from land uses immediately upslope with only minimal influence of the regional aquifer discharge affected by more distant land uses.