GROUND WATER FLOW MODELING IN DELAWARE COUNTY, IN: A GIS BASED STUDY

This study was undertaken to improve understanding of the geologic and hydrogeologic framework of Delaware County, Indiana. Arc View GIS 3-D and Spatial Analysts along with VISUAL MODFLOW were used to study ground water flow patterns by developing a 3-D model of major aquifers. The county has Silurian carbonate bedrock overlain by glacial drift that is interbedded with eight horizontal and somewhat discontinuous sand and gravel layers. More than 3000 located water wells are available as a data source in the county with roughly half of the wells in confined sand and gravel aquifers and the remaining half in carbonate bedrock. The bedrock well logs include mention of preglacial karst features, which can indicate areas of higher transmissivity, adding further interest to modeling this region.

The simulated region is an area of 398 square miles. The initial model consisted of a 60 x 60 grid and employed a regular grid spacing of a half kilometer. Several simplifying assumptions were made for the conceptualization and simulation of flow in the basins. The model boundaries used constant head conditions. Two major rivers were included along with the two principle tributaries and discharge to pumping from high capacity wells. Soils association maps were used to map four zones of higher or lower recharge across the region’s surface. Hydraulic conductivity and streambed conductance were adjusted during successive simulations until the flow pattern matched the regional flow direction as computed from the potentiometric surface that is from SE to NW. The water budget for the calibrated model indicates that 16.5% of the inflow to the modeled ground water system is flow across model boundaries, 83% is from effective recharge from precipitation, and the rest is a small contribution from streams. Areas of upward and downward gradients implied from differences in static water levels in shallow versus deeper wells are compared with areas of higher versus lower recharge in different soil associations and with flow paths in the model. Areas of known bedrock solution features mentioned in well logs are modeled as possible areas of higher transmissivity. The results of the modeling study can be used as a predictive tool for long-term management and monitoring of water resources in the region.