FLOOD-DOMINATED RECHARGE OF A GLACIOFLUVIAL AQUIFER, BIG WALNUT CREEK, GREENCASTLE, INDIANA

The city of Greencastle, Indiana draws an average of 1.5 mgpd from the Big Walnut Creek aquifer. The aquifer is composed of a 52-foot thick sequence of interbedded gravels, sands, and clays deposited in paleoglacial valley carved into Mississippian limestone and siltstone of the Borden Group at the end of the Wisconsian ice age. The drainage basin above the wellfield occupies an area of 216 mi². Transmissivity, storativity, and hydraulic conductivity of the aquifer have values of 273 gpd/ft, 0.25, and 912 ft/day respectively. Effective porosity is estimated at 30% and permeability is approximately 1900 – 2400 gpd/ft² (Hydrophase, 1996; White, 1964; Bieber, 1955). Temperature, conductivity, and del ¹⁸O data collected from 2002 to 2004 from Big Walnut Creek, the aquifer, springs, and precipitation were correlated with stream stage and water table level data to monitor aquifer recharge. Comparison of the conductivity, temperature and del ¹⁸O data with water table elevation data from monitoring wells and stream gage data (USGS 03357500), it was found that the dominant recharge to the aquifer occurs when Big Walnut Creek experiences elevated stages or flooding. Aquifer temperature clearly responds to both long term atmospheric changes with a two month lag-time, and to short term heavy precipitation events such as the Labor Day, 2003 flood event in which 7.5 inches of rain fell in a 24-hour period. Infiltration through the streambed appears to be minimal based on the current data and may be hampered by the deposition of silts and clays due to construction of a three-foot high dam at the south end of the wellfield over 100-years ago (Dale, pers. comm.). Del ¹⁸O data are used to infer that seepage from the regional Mississippian limestone bedrock is also minimal.