Reports
Simulation of ground-water flow and base flow in weathered crystalline rock, upper Cattail Creek, Howard County, Maryland
1986, Willey, R.E. and Achmad, G.
Report of Investigations 45
Abstract
Population growth and changing land use patterns are increasing water-supply demands in the Maryland Piedmont. Local ground-water resources may be sufficient in much of the area to satisfy these demands; however, there is a need to quantify these resources and assess the potential impacts of ground-water withdrawals and waste-water returns. Digital-simulation modeling of the stream-aquifer system is one method of doing this.
The Trescott, Pinder, and Larson two-dimensional finite-difference model is used to simulate ground-water flow in an 8.4-square-mile area at the headwaters of Cattail Creek, located in Howard County, Maryland. The model is modified to revise the method of decreasing ground-water evapotranspiration with falling water levels and to allow systematic reduction of hydraulic conductivity and specific yield with depth. Most hydrologic parameters are initially approximated using published data from similar hydrogeologic settings. Ground-water levels and base flows from steady-state and transient-flow simulations agree well with values obtained in the field.
Predictive simulations are made to demonstrate the utility of modeling in assessing the potential effects of well pumpage and waste-water returns on the stream-aquifer system. Hypothetical development schemes examine various modes of water supply, waste-water disposal, and popUlation distribution, while holding both total popUlation and water use constant. Impacts to the modeled area are shown as changes in long-term average annual water levels and base flows. Individual subbasin changes vary greatly depending mainly on supply well location, location and manner of waste-water disposal, and subbasin hydrology. Collectively, base flows are reduced essentially by the amount of pumpage not returned as recharge. Base flow and water-level declines are greatest when all pumpage is sewered out of the basin. Predictive simulations with long-term average annual conditions underestimate impacts of development on ground-water resources during seasonal low-water periods and drought years.
