Seasonal changes in submarine groundwater discharge to coastal salt ponds estimated using 226Ra and 228Ra as tracers

Abstract

Submarine groundwater discharge (SGD) to coastal southern Rhode Island was estimated from measurements of the naturally-occurring radioisotopes 226Ra ( t 1/2 = 1600 y) and 228Ra ( t 1/2 = 5.75 y). Surface water and porewater samples were collected quarterly in Winnapaug, Quonochontaug, Ninigret, Green Hill, and Pt. Judith–Potter Ponds, as well as nearly monthly in the surface water of Rhode Island Sound, from January 2002 to August 2003; additional porewater samples were collected in August 2005. Surface water activities ranged from 12–83 dpm 100 L − 1 (60 dpm = 1 Bq) and 21–256 dpm 100 L − 1 for 226Ra and 228Ra, respectively. Porewater 226Ra activities ranged from 16–736 dpm 100 L − 1 (2002–2003) and 95–815 dpm 100 L − 1 (2005), while porewater 228Ra activities ranged from 23–1265 dpm 100 L − 1 . Combining these data with a simple box model provided average 226Ra-based submarine groundwater fluxes ranging from 11–159 L m − 2 d − 1 and average 228Ra-derived fluxes of 15–259 L m − 2 d − 1 . Seasonal changes in Ra-derived SGD were apparent in all ponds as well as between ponds, with SGD values of 30–472 L m − 2 d − 1 (Winnapaug Pond), 6–20 L m − 2 d − 1 (Quonochontaug Pond), 36–273 L m − 2 d − 1 (Ninigret Pond), 29–76 L m − 2 d − 1 (Green Hill Pond), and 19–83 L m − 2 d − 1 (Pt. Judith–Potter Pond). These Ra-derived fluxes are up to two orders of magnitude higher than results predicted by a numerical model of groundwater flow, estimates of aquifer recharge for the study period, and values published in previous Ra-based SGD studies in Rhode Island. This disparity may result from differences in the type of flow (recirculated seawater versus fresh groundwater) determined using each technique, as well as variability in porewater Ra activity.