Factors Influencing Spatial and Temporal Distributions of Submarine Groundwater Discharge (SGD) in the Mississippi Sound

Abstract

The importance of submarine groundwater discharge (SGD) is under‐described in the Mississippi Sound, a subtropical estuary along the US Mississippi/Alabama coastline. A 4‐year study between 2018 and 2021 was conducted within the Sound to quantify SGD water fluxes and determine driving factors behind spatial and temporal variation. Fourteen surveys were conducted during the study period across the Sound using ²²² Rn and the radium quartet to determine SGD water fluxes. We implemented Monte Carlo simulations into mass balance calculations, providing a broad scope of the uncertainty associated with SGD flux estimates. We found there was distinct spatial variation of the magnitude of total SGD flux, with the western Sound having the highest median SGD (avg. 9.0 × 10 ⁶  m ³  d ⁻¹ ), and the eastern Sound the lowest (avg. 1.2 × 10 ⁵  m ³  d ⁻¹ ). We used oxygen isotopes in water to determine the percent composition of terrestrial (fresh) versus marine (circulated) SGD during the wet and dry seasons. Circulated seawater appeared to be the main portion of SGD (70% wet season, 57% dry season); however, the dry season had a larger input of fresh groundwater (43%) compared to the wet season (30%), indicating a lag time between recharge and discharge. Spatial/temporal variations of SGD have been linked to aquifer hydrology, incisional systems, surficial sediment diversity, shipping channel locations, and diversity in waves and tides. For the Mississippi Sound, it appears that aquifer hydrology plays a key role in spatial distribution, and wind forcing may be important for changes in SGD composition seasonally.