Patterns of contaminant transport in a layered fractured aquifer

We investigated patterns of contaminant transport within the layered and fractured aquifer of a 7.3-km2 upland agricultural watershed in east-central Pennsylvania, USA. Geometry and hydraulic properties of the aquifer had been characterized by field testing and model calibration. These results were extended to simulate flow pathways and patterns of contaminant transport in both areal and cross-section formats within the watershed. The analyses indicated that the ground water flow system at the larger watershed scale is comprised of smaller units of subsurface flow which are self-contained at the scale of first- or second-order streams. For this scale subwatershed or larger, contaminant inputs to ground water from the mix of land use within the subwatershed should translate directly to the quality of nonstorm streamflow. For illustration, recharge water quality from typical land-use distributions were combined with a simple model of contaminant transport to simulate nitrate concentration patterns in ground water in a cross-section format. Land use in the vicinity of the drainage divides between streams was found to control ground water quality within the deeper layers of the aquifer, while land use over the remainder of the watershed area affected water quality only within the shallower layers of the aquifer. Streamflow nitrate data collected during a baseflow survey on the watershed were examined in context of these simulations and found to support the conclusions. Results of the study demonstrate the potential for localized contamination of ground water and nonstorm streamflow by agricultural land use, as well as the potential for managing stream quality and minimizing contamination within targeted zones of the ground water by controlling land use position.