Author of Response: Dan Shelton, Research Leader, Environmental Microbial Safety Lab, Agricultural Research Service, U.S, Department of Agriculture, sheltond@ba.ars.usda.gov
In general, fertile agricultural soils are relatively effective barriers, sorbing or filtering pathogens/parasites from percolating water, thereby minimizing groundwater contamination. Important exceptions are sandy or rocky soils, which generally allow for greater infiltration of organisms through the soil profile than heavier soils. However, heavy soils (e.g., clay) containing significant cracks or fissures or channels created by decayed plant roots or burrowing worms may allow for significant infiltration through the soil profile. In addition, soils/subsoils throughout the Appalachian region derived from limestone geological formations (know as karst) are highly porous, allowing for the rapid infiltration of pathogens to groundwater. Finally, improper installation of wells can allow for direct contamination of groundwater via the leaching of organisms along the well casing.
Additional reading:
Waterborne Pathogens in Agricultural Watersheds, USDA Watershed Science Institute Technical Note 2, http://forestry.berkeley.edu/rangelandwq/pdfs/Atwillwssitn21.pdf.
