Water mass ages of coastal ponds estimated using 223Ra and 224Ra as tracers

Measurements of the naturally occurring radioisotopes 223 Ra ( t 1/2 = 11.4 days) and 224 Ra ( t 1/2 = 3.66 days) in southern Rhode Island salt ponds were combined with a simple model to obtain independent estimates of the age of these coastal waters. 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. Surface water activities ranged from 1–78 dpm 100 L − 1 and 5–885 dpm 100 L − 1 for 223 Ra and 224 Ra, respectively. Porewater radium activities ranged from 3 to 715 dpm 100 L − 1 for 223 Ra, and 57–4926 dpm 100 L − 1 for 224 Ra. Results indicate seasonally varying water mass ages for Ninigret (5–12 days), Winnapaug (2–6 days) and Pt. Judith-Potter Ponds (1–9 days) and, in contrast, relatively constant ages for Green Hill (5–7 days) and Quonochontaug Ponds (3–6 days).