Delineation and Characterization of the Furnace Brook Watershed in Marshfield, Massachusetts: Potential Impact of Water Supply Extraction

The Furnace Brook watershed in Marshfield, Massachusetts supplies a coastal community of 25,132 residents with nearly 45% of its town water supply (Marshfield 2012a). As in many other coastal communities, development pressure in Marshfield has increased in recent years, creating a growing demand for freshwater extraction. It has been observed, however, that portions of the stream and Furnace Pond disappear entirely at certain times of year, generally between June and August, depending on the rate of groundwater extraction. This has created a conflict between protecting the designated wetland areas and meeting public pressure for water resources, even within what is traditionally viewed as a humid region. Questions have arisen as to whether the town’s water extraction is excessively lowering the water table and potentially endangering the health of the stream. This study set out initially to characterize the lowered water table and identify possible anthropogenic and natural influences acting upon the watershed, including stream flow obstructions, water extraction, and geologic conditions. Water-table data were correlated with town pumping information, previous geologic surveys, and meteorological data. Previous analyses indicated that the stream behaved in an anomalous manner by decreasing in discharge, even while flowing downstream, despite normal precipitation inputs. The behavior within this particular watershed appears to be influenced by four primary factors resulting in the stream “running dry” during the June-August period. These factors include: (1) A losing gradient induced by well pumping; (2) Obstructions to stream flow from the upper reaches to the lower reaches of the watershed; (3) A highly anisotropic layer of lower conductivity material regulating infiltration rates; and (4) Evapotranspiration that results in a deficit situation during the summer. Additionally, relationships between well pumping and decreasing discharge, seepage flux loss rates, and hydraulic gradients, have demonstrated that even within humid regions, it cannot be assumed that aquifer recharge is sufficient to avoid conflict between surface water protection and groundwater utilization in watersheds. Timing of precipitation events combined with geological governance of aquifer recharge play critical roles in managing the conjunctive use of water resources and cannot be assumed to have a negligible effect, even within relatively humid regions