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

Author

Andrea L. Hougham, University of Rhode Island

Date of Award

2006

Degree Type

Thesis

Degree Name

Master of Science in Oceanography

Department

Oceanography

First Advisor

S. Bradley Moran

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 (t_1/2 = 3.66 days), ²²⁴Ra (t1/2 = 11.4 days), ²²⁶Ra (t1/2 = 1600 yrs) and ²²⁸Ra (t_1/2 = 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.

Recommended Citation

Hougham, Andrea L., "Radium Isotopes as Tracers of Water Mass Age and Groundwater Flux to Shallow Coastal Salt Ponds" (2006). Open Access Master's Theses. Paper 2073.
https://digitalcommons.uri.edu/theses/2073