GROUNDWATER FLOW AND COMPOSITION CHARACTERIZATION USING OUTCROP FRACTURE ANALYSIS FOR A BEDROCK AQUIFER

The fracture network, developed in Acadian-aged rocks, immediately east of the Taconic-Acadian structural boundary, known as the Richardson Memorial Contact (RMC) or Dog River Fault Zone (DRFZ) in central Vermont, appears to locally respond to the presence of intrusive rocks over the regional strike. This complex fracture network is developed orthogonal to the regional S-main orientation. The RMC/DRFZ network has a regional trend that is reflected as a NNW/SSE fracture set. The overall outcrop pattern of regional intrusive rocks suggests a NW/SE trend. The orientations of the host rock fabric change in proximity to the RMC/DRFZ, mimicked by the fracturing preserved in granite plutons. The complexity of the fracture network in this region mirrors components of both the regional structural trend and the trends of the plutonic bodies. Samples of groundwater from thirty locations, which were approximately co-located with bedrock outcrops sampled for whole rock geochemistry, were collected to quantify the degree of water-rock interaction. To estimate bulk permeability, outcrop observations of fracture orientation, fracture lengths, and fracture spacing were compiled throughout the study area. The goal was to relate the geological framework with variations in groundwater chemistry in a fractured bedrock aquifer. Initial analysis suggests that the type of bedrock exposed at the surface can be correlated with trends in groundwater chemistry. However, variation, especially in constituents more readily dissolved, such as Na and Ca, appear to respond to the relative concentration of cross-cutting fractures, estimated from exposures of nearby bedrock. The importance of regional ductile deformation fabrics in meta-sedimentary rocks, which play a major role in orientation of fracture orientation, especially in areas away from fault-zone along the DRFZ or where granitic intrusions are not exposed, also appear to have an influence on groundwater chemistry. The integration of whole rock major and trace element geochemistry with fracture characterization to produce estimates of bulk permeability as well estimates of groundwater flow direction appears to be a functional approach to understanding groundwater chemistry in a fractured bedrock aquifer.