A CONCEPTUAL FRAMEWORK FOR PREDICTING SUSTAINABLE WELL YIELDS IN CRYSTALLINE-ROCK AQUIFERS, EXAMPLES FROM MASSACHUSETTS

An intrinsically low porosity limits the quantity of water stored in fractured crystalline rock. Sustainable well yields for bedrock, therefore, may strongly depend on the quantity of water stored in surficial materials that can leak downward into bedrock and on periodic replenishment by recharge. A hydraulic connection to surface-water bodies may also help sustain well yields in some crystalline-rock settings. Results from studies in Massachusetts show that a combination of geologic mapping of bedrock and surficial materials, aquifer testing, and long-term water-level monitoring in surficial materials and bedrock can provide much of the information needed to predict sustainable yields from wells in crystalline-rock aquifers. Geologic information provides a basis for qualitatively assessing leakage to bedrock. A strong vertical hydraulic connection to surficial materials can be expected where the bedrock contains numerous high-angle fractures. A crystalline-rock aquifer in Maynard, Massachusetts, for example, is capable of transmitting nearly 3,000 liters per minute to three public-supply wells because of numerous vertical fractures that connect the aquifer to a thick sequence of surficial silts and sands. These surficial materials readily supply water by leakage and can receive 60 centimeters or more of recharge each year. In another example, a crystalline-rock aquifer in West Newbury, Massachusetts, is capable of transmitting nearly 700 liters per minute because of a high degree of vertical fracturing. At this site, however, limited storage in a thin till layer within the area contributing recharge to public-supply wells and limited surface-water sources may seasonally restrict sustained yields. Where vertical fractures are sparse and surficial materials are thin or poorly permeable, sustainable well yields may be limited. For example, two public-supply wells completed in a water-bearing zone more than 160 meters deep in crystalline rock in Paxton, Massachusetts, combine to produce nearly 700 liters per minute, but the aquifer in the area of the wells can sustain a yield of only half that amount because of limited vertical pathways for water.