METHODOLOGY OF DELINEATING WELLHEAD PROTECTION ZONES IN CRYSTALLINE BEDROCK IN MAINE

摘要

Delineation of contributing areas for wellhead protection around supply wells drilled into bedrock in Maine, USA, is currently achieved by assigning a fixed radius circle around the well. This project develops a methodology that incorporates hydrogeolog-ic processes and ground water modeling (MODPLOW) and accounts for variable data availability to estimate the areas that contribute water to 26 bedrock supply wells. Outcrop fracture mapping and lineament analysis are used to characterize the fracture system. Multiple simulations are constructed of each site using ranges of values for recharge, hydraulic conductivity, and anisotropy. Uncertainty in the delineation process is accounted for by portraying the delineated areas as confidence zones that are constructed by overlapping the capture zones from the multiple simulations. The results are variable and depend on the ability to characterize a site in a way that can be easily modeled. Sites with complex hydrogeolo-gy tend to have larger contributing areas that reflect the greater uncertainty in the parameters. The majority of the sites, however, produce reasonable results that provide a much more accurate depiction of the area likely to contribute to a bedrock well than the fixed radius circle.