KEY INFORMATION FOR DELINEATING CONTRIBUTING AREAS TO PUBLIC-SUPPLY WELLS IN BEDROCK FOR THREE CRYSTALLINE-ROCK SETTINGS IN MASSACHUSETTS

Delineation of contributing areas to public-supply wells is critical for well-head protection. Numerical modeling and associated sensitivity analyses that involved varying model characteristics within reasonable ranges at three crystalline-rock well sites in Massachusetts indicate that knowledge of fracture extent and distribution, recharge rates, and leakage rates to fracture zones are key to confident estimates of contributing areas.

In West Newbury, aquifer tests demonstrated that sub-horizontal and steeply dipping fractures that parallel foliation in phyllite of the Eliot Formation form laterally extensive and elongate transmissive zones in bedrock near two well locations and a close hydraulic connection to surficial materials. Recharge to bedrock is through a thin veneer of till covering bedrock and through thick drumlin tills. Uncertainties in simulated contributing areas for two wells, pumping at a combined rate of 250 gallons per minute, result mainly from uncertainties in recharge rates.

In Maynard, aquifer tests demonstrated that a dense network of fractures in coarse-grained schist of the Nashoba Formation forms a laterally extensive transmissive zone in bedrock that is well connected vertically to surficial materials consisting of sandy till, lacustrine silts, and sand and gravel. Potential leakage rates to bedrock under pumping conditions exceed potential recharge rates to the surficial materials near the transmissive zone. Uncertainties in simulated contributing areas for three wells, pumping at a combined rate of 750 gallons per minute, result mainly from uncertainties in recharge rates to surficial materals.

In Paxton, aquifer tests and borehole logging demonstrated that two deep wells completed in granofels and schist of the Paxton Formation receive much of their water from depths below 500 feet, mostly through parting fractures along foliation dipping gently (10 degrees) eastward. In contrast to the other two sites, these parting fractures are poorly connected vertically to shallow bedrock and till. Uncertainties in simulated contributing areas for these two wells, pumping at a combined rate of 180 gallons per minute, result mainly from uncertainties in the lateral extent of water-bearing zones and in leakage rates to water-bearing zones from overlying and underlying rocks.