UTILITY OF RADON-222 AS A RADIOCHEMICAL TRACER OF SUBMARINE GROUNDWATER DISCHARGE TO A BEDROCK-DOMINATED REGION OF EASTERN CASCO BAY

Submarine groundwater discharge (SGD) has gained recent attention as a source of anthropogenic nutrient discharge to coastal waters. Persistent freshwater anomalies have been observed along the seafloor across eastern Casco Bay that are consistent with SGD. In this study, Radon-222 (t_1/2=3.83 days) was employed as a radiochemical tracer to quantify the distribution and magnitude of groundwater entering Quahog Bay, a sub-estuary of Casco Bay in Harpswell, Maine. ²²²Rn is a naturally occurring isotope in groundwater, enriched by several orders of magnitude relative to saline coastal waters. Surface samples of bay water were collected over the late fall of 2003 and analyzed for radon gas on a closed-loop cold trap extraction line. Average activity across the entire bay was 1.2 disintegrations per minute per liter (dpm l^-1). An area of increased activity (~4 dpm l^-1) was observed at several sampling sites off the north end of Pole Island. This location coincides with persistent freshwater anomalies found during previous hydrographic studies of Quahog Bay, and suggests that groundwater may be discharged through point sources such as subaqueous bedrock fractures or springs.