EVALUATING SUBMARINE GROUNDWATER DISCHARGE AND RESIDENCE TIMES IN WINNAPAUG POND: DRIVERS AND DYNAMICS OF A SOUTHERN RHODE ISLAND COASTAL LAGOON

Author

• Shelby Elin Nicole Gibson, University of Rhode IslandFollow

Date of Award

2026

Degree Type

Thesis

Degree Name

Master of Science in Oceanography

Department

Oceanography

First Advisor

Jaime B. Palter

Abstract

Submarine groundwater discharge (SGD) is a critical pathway for freshwater and nutrient delivery to coastal ecosystems, yet its sensitivity to interannual precipitation variability remains poorly constrained in coastal lagoons. This study investigates groundwater-surface water dynamics at Winnapaug Pond, a barrier lagoon in southern Rhode Island, using continuous physical monitoring across five stations and discrete radium isotope sampling (²²⁴Ra, ²²³Ra, ²²⁸Ra, ²²⁶Ra) in ground and surface waters during 2024 and 2025. Stations captured a gradient from tidally dominated conditions near the inlet (Site 1) to groundwater-influenced conditions in a shallow embayment (Site 3) and the western basin (Site 5), allowing for an evaluation of how precipitation-driven changes propagate through salinity structure and pond circulation into estimates of volumetric flux and water mass residence time.

The 2024 season coincided with the fourth wettest year on record in Westerly, RI since 1999 (129.2 ± 5.9 cm), featuring a wet summer (37.6 ± 1.6 cm) that maintained a westerly salinity gradient and high flushing efficiency (return flow b = 0.28). During this period, multi-site fresh SGD averaged 32.7 ± 14.6 L m^-2 d^-1, while total radium-derived SGD reached magnitudes of 95-167 L m^-2 d^-1 at internal sites. In contrast, summer 2025 was characterized by a 69% precipitation deficit (11.6 ± 0.9 cm), resulting in uniform lagoon salinity and restricted flushing (b = 0.77). While absolute fresh SGD declined system-wide to 13.7 ± 8.1 L m^-2 d^-1, the total SGD flux at internal sites collapsed by approximately 80% (to 13-34 L m^-2 d^-1). Notably, the relative fraction of fresh-to-total SGD increased at these interior stations during the dry year, suggesting that while both terrestrial delivery and saline recirculation declined, the latter was more sensitive to the reduction in lagoon-wide circulation.

Radium-derived water ages, ranging from 0.9-3.2 days across sites in 2024 and 5.0-6.5 days in 2025, were consistently shorter than tidal prism residence times in the wet year but moved toward convergence in the deficit year. At the internal sites (3 and 5), the near-quadrupling of Ra age from approximately 1.4 to 5.4 days reflects the radium clock increasingly tracking the same tidal water mass as freshwater forcing weakened. This convergence was accompanied by a localized seawater intrusion event detected at the Western Well in 2025, suggesting that intrusion-driven desorption likely amplified the porewater ²²⁴Ra/²²⁸Ra activity end-member (from 2.9 to 5.6) during the dry year. Together, these results demonstrate that year-to-year precipitation variability affects not only the volume but also the composition and geochemical character of groundwater exchange, with significant consequences for nutrient delivery and water quality in precipitation-sensitive coastal lagoons.

Recommended Citation

Gibson, Shelby Elin Nicole, "EVALUATING SUBMARINE GROUNDWATER DISCHARGE AND RESIDENCE TIMES IN WINNAPAUG POND: DRIVERS AND DYNAMICS OF A SOUTHERN RHODE ISLAND COASTAL LAGOON" (2026). Open Access Master's Theses. Paper 2710.
https://digitalcommons.uri.edu/theses/2710