BEDROCK CONTROLS ON SUBMARINE GROUNDWATER DISCHARGE (SGD) IN EASTERN CASCO BAY, MAINE, AND NUTRIENT ANALYSES OF SGD SPRINGS

Submarine Groundwater Discharge (SGD) is associated with nutrient loading and subsequent eutrophication of coastal waters. Previous studies have found temporally and spatially persistent SGD in eastern Casco Bay, Maine, where fractured crystalline bedrock is variably overlain by Quaternary marine mud. These studies tentatively suggested that bedrock lithology and fracture patterns play a role in determining where SGD is observed. A side-scan sonar survey in Quahog Bay, coupled with additional hydrogaphic sampling, suggests that this working hypothesis has to be modified to include consideration of whether a specific bedrock unit is covered by a drape of relatively impermeable marine sediments. While SGD is widespread throughout Quahog Bay, the lowest salinity values appear in the deep, relatively sediment-free hole in the channel on the western side of Pole Island, an area of presumed scouring tidal currents. In addition, nutrient sampling in areas of known SGD shows the values for phosphate, nitrate, ammonium, and silica hydroxide near the bottom to be elevated with respect to the water column. This is significant to the understanding of the annual eutrophication of the waters in Quahog Bay.