Radium Isotopes as Tracers of Water Mass Age and Groundwater Flux to Shallow Coastal Salt Ponds

Abstract

Water mass age within and submarine groundwater discharge (SGD) to coastal southern Rhode Island was estimated from measurements of the naturally-occurring radioisotopes of 223Ra (t1/2 = 3.66 days), 224Ra (t1/2 = 11.4 days), 226Ra (t1/2 = 1600 yrs) and 228Ra (t1/2 = 5.75 yrs). 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, beginning January 2002 through August 2003. Additional porewater samples were collected in August 2005. Surface water activities ranged from 1 - 78 dpm 100 L-1, 5 - 885 dpm 100 L-1, 12 - 83 dpm 100 L-1 and 21 - 256 dpm 100 L-1 for 223Ra, 224Ra, 226Ra, and 228Ra, respectively. Porewater radium activities ranged from 3 - 715 dpm 100 L-1 for 223Ra, 57 - 4,926 dpm 100 L-1 for 224Ra, 14 - 858 dpm 100 L-1 for 226Ra and 23 - 1,265 dpm 100 L-1 for 228Ra. Measured activities of 223Ra and 224Ra provide water mass ages for the salt ponds ranging from 1 - 13 days, with a clear seasonality. Using a simple box model, average 226Ra-based submarine groundwater fluxes ranged from 8 - 183 L m-2 d-1; 228Ra data provided similar SGD values. These radium-derived fluxes are up to two orders of magnitude higher than results predicted by a freshwater numerical model, estimates of aquifer recharge for the study period, and values published in previous radium-based studies conducted in Rhode Island. This disparity may result from differences in the type of flow measured using each technique, and possibly to a reduced collection efficiency of Mn-fiber when used to sample porewater radium.