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Master of Science (M.S.)
Year Degree Awarded
Month Degree Awarded
Hydrogeology, Geophysics, Discrete Fractures, Crystalline Rock, Bedrock, Massachusetts
Igneous and metamorphic rock units have long been considered marginal aquifers yet they are a significant source for potable drinking water in many areas worldwide. Additionally, use of these systems is on the rise due to many factors including, contamination and overuse of surficial systems, as well as expanding population and drought. The Nashoba Terrane is a fault-bounded block of high-grade, steeply dipping metavolcanic and metasedimentary rock located in eastern Massachusetts, U.S.A. The Nashoba is northeast trending, extending from Oxford, MA to the Gulf of Maine south of Newburyport, MA. Seventeen previously drilled wells throughout the Nashoba were selected for use in this study. The goal of this study was to characterize the hydrogeologic system of the Nashoba Terrane.
Wells studied were in three bedrock types: granite, schist and amphibolite. Three fracture types were identified: FPF, subhorizontal unloading joints and tectonic joints. Several major fracture orientation sets were also identified including northeast trending FPF, east-west trending and north-south trending tectonic joints as well as northwest trending tectonic joints. Dominant sets varied in the three rock types and the frequency of fractures was found to decrease with depth.
Only four percent of all fractures measured in this study were flowing. Approximately 32% of the flowing fractures were northeast trending, 17% of subhorizontal fractures were flowing and the remaining 51% were of variable orientation and dip. In general, the orientation of fractures was not found to determine whether a fracture flows, nor was rock type a significant determinant of flow. There was no flow identified below 170 meters and the majority of flow in the Nashoba Terrane is constrained to the upper 100 meters. This is most likely due to decreased fracture frequency and permeability with depth. This study is significant to the search for a sustainable groundwater source in bedrock because results show that the few fractures are actually contributing to flow and that flow is primarily occurring near the surface.
David F. Boutt