NUTRIENT DELIVERY TO WEST FALMOUTH HARBOR (MA) FROM GROUNDWATER: EXAMINING THE CONTRIBUTION FROM THE WASTEWATER TREATMENT FACILITY

West Falmouth Harbor is a small estuary on Cape Cod (MA) that has experienced significant increases in eutrophication in recent years, reducing the value of the harbor for recreational uses. Direct groundwater discharge is most likely an important nutrient source to the harbor, at least in part because the harbor is directly down-gradient of the town's wastewater treatment facility, which discharged high-nutrient effluent into the aquifer until 2005. In order to constrain groundwater inputs into the harbor, and the contribution of groundwater to the harbor's nutrient budget, we undertook a study examining radium isotopes and radon-222 as tracers of groundwater discharge. Radium isotopes and radon-222 have been used in many studies as comparable tracers of groundwater discharge to the coastal ocean because each is strongly enriched in groundwater relative to surface waters and each behaves conservatively once introduced to the ocean. During August of 2006 in West Falmouth Harbor (MA), radon activities were quantified in real time from a small boat using a system that measures the radon activity in air continuously equilibrated with surface water. We observed different regions of radon-222 and radium isotope (Ra-223 and Ra-224) enrichment. Because radon is enriched in both fresh and saline groundwater, while radium is enriched only in saline groundwater, we interpret this to reflect different regions of fresh and saline groundwater discharge. This result has implications for the use of each isotope separately, and of all the isotopes collectively, as tracers of groundwater discharge. Aerial infrared photographs reveal still different regions of cold, presumably fresh groundwater discharge. Analysis of groundwater samples suggests fresh groundwater discharge to the harbor contributes a large portion of the total nutrient supply, while saline exchange does not contribute significantly. Strong radon enrichment is observed in the most eutrophic region of the harbor, which is also adjacent to sites of elevated groundwater nutrient concentrations. This pattern is consistent with the notion that the groundwater plume from the treatment plant is currently discharging at a rapid rate to the harbor and is currently an important source of nutrients.