USE OF FRACTURED GLACIAL TILL WITH THE DRASTIC SYSTEM

The Ohio Department of Natural Resources (ODNR), Division of Water (DOW) began a statewide, county Ground Water Pollution Potential (GWPP) mapping program based upon the DRASTIC system. The mapping process has evolved since its inception in the late 1980's. The impact of the vadose zone media is a critical component of DRASTIC. Initially, vadose zone media for till areas were characterized as silt and clay or sand and gravel with silt and clay and assigned a relatively narrow range of values. In the early 1990's, glacial till was determined to be a unique vadose zone material. This increased focus on till allowed a better evaluation of the impact that till has on the relative vulnerability of an aquifer in glaciated Ohio. For areas of Ohio where till stratigraphy had been developed, till units could be evaluated upon primary characteristics such as texture. The importance of secondary characteristics of till, such as fracturing, was also considered. Areas of thin till cover are considered to be fractured and weathered and therefore have a higher vadose zone rating. In areas of very thick till, the majority of the till is considered to be less fractured and more compacted thus a lower vadose zone rating is applied. Ranges of recharge rates, another critical component of DRASTIC, could also be adjusted based upon the nature and thickness of till overlying aquifers. Manipulating values of vadose zone media and recharge allowed a means to evaluate semi-confined aquifers and overlying “leaky: aquitards. DRASTIC originally required an aquifer to be considered as either confined or unconfined. This led to pollution potential index values that were either too high or low.

Beginning in 2001, GIS technology was used to create GWPP maps. GIS greatly expedited both the compilation and the production of maps and allows easy incorporation of available glacial mapping data. GIS also represents a methodology in which older county GWPP produced before till was considered to be a vadose zone media could be easily reevaluated