ASSESSING GROUNDWATER VULNERABILITY AND RELATIVE RISK IN REGIONS OF COMPLEX BEDROCK STRUCTURE AND GLACIAL OVERBURDEN

Many groundwater vulnerability models explicitly or implicitly assume a relatively simple stratigraphy with subhorizontal isotropic units with regular thickness. In New England and other areas of relatively high metamorphic grade and complex structure, these conditions may not apply. Due to the generally low yield of bedrock wells, public water supply wells and other wells requiring high yield are not screened in bedrock but instead in highly permeable overburden. In these conditions, it may be more practical to regard the vulnerability of the bedrock aquifer as primarily a function of overburden thickness and permeability, and to consider not the vulnerability of the permeable overburden aquifers, but to develop measures of relative risk. Beginning as a cooperative project with the Maine Geological Survey twenty years ago, Maine DEP has developed these two approaches into procedures that assess relative risk to high-yield areas of sand and gravel aquifers, based on development density and specific risks posed by certain types of development and land-use practices, and to bedrock aquifers, using recent detailed mapping of glacial deposits and soil hydrologic groups and overburden thickness determinations based on well casing length and other geologic information. However, it is unclear whether regional assessments of vulnerability are meaningful in a hydrogeologic setting with relatively small flow cells in the sections of aquifer used for water supply wells and in which local influences, based on ongoing work by the Department, appear to play a very significant role in groundwater contamination and the impacts on surface waters of contaminated baseflow.