Seasonal changes in groundwater input to a well-mixed estuary estimated using radium isotopes and implications for coastal nutrient budgets

Abstract

226Ra (half-life 1,600 yr) and 228Ra (half-life 5.75 yr) were used as tracers to determine seasonal changes in groundwater input to the Pettaquamscutt estuary from June 1999 to June 2000. Ra isotopes were observed to be in excess of activities due to input from streams, Rhode Island Sound water, and particle desorption. The source of excess 226Ra and 228Ra within the estuary is attributed to groundwater transport of Ra derived from the weathering of bedrock within the Pettaquamscutt watershed. Short-lived 224Ra (half-life 3.66 d) was used to calculate an average residence time of the estuary of 8 ± 4 d. Box models used to constrain seasonal changes in groundwater input to the estuary indicate highest groundwater input in the summer (6.4-20 L m-2 d-1) and lowest values in the winter (2.1-6.9 L m-2 d-1). These Ra-derived groundwater fluxes are similar to rates of aquifer recharge over the annual cycle. Using these Ra-derived groundwater fluxes, the input of inorganic nitrogen and phosphorus from groundwater to the Pettaquamscutt was estimated at 61-180 mmol m-2 yr-1 and 4.4-13 mmol m-2 yr-1, respectively.