SR, PB, AND U ISOTOPIC BASELINE MEASUREMENTS FOR THE LITTLETON, MA, WATER SYSTEM

Littleton, MA, is planning to bring a new series of bedrock groundwater supply wells into production. The source area is near new subdivisions, lands held by private landowners, and the town boundary with Concord. Concern about possible impacts of large volume production on other local water users motivated this study to determine baseline isotopic compositions for bedrock groundwater and two local surface waters. The two surface bodies include Cobb's Pond, a shallow pond with a small catchment in Littleton, and Nagog Pond, a municipal water supply source for Concord, both of which are within three miles of the well site. As part of a comprehensive evaluation, isotopic compositions of Sr, Pb, and U were examined as tracers of potential mixing between water sources. The data indicate that the three systems studied, (bedrock groundwater, Cobb’s Pond surface water and Nagog Pond surface water) are isotopically distinct. The data also indicate that at least two water sources, distinguishable in Sr isotopic composition, contribute to Cobb’s Pond. Bedrock groundwater contains somewhat higher concentrations of Sr, Pb, and U than the surface waters. Uranium content in surface waters is low compared with the bedrock groundwater; ²³⁴U activity in surface waters is at or near secular equilibrium, but in well water, it is above secular equilibrium, and likely reflects interaction with metamorphic bedrock. Littleton plans to move forward with new source permitting on the bedrock wells and is prepared to continue to monitor isotopic compositions in the bedrock-supplied groundwater and nearby surface waters in order to identify any potential future mixing between water sources.