RADIUM ISOTOPES AS TRACERS FOR GROUNDWATER-SURFACEWATER INTERACTIONS IN TIDALLY-INFLUENCED CREEKS

Radium isotopes are key tracers for saline groundwater discharge in coastal settings, but groundwater endmember concentrations are often uncertain because of high variability and low sampling density. We combined hydrogeologic observations and detailed Ra analyses to identify systematic spatial and temporal controls on Ra activities in two salt marshes. The salt marsh sites were chosen because a large proportion of coastal surface-water groundwater interactions occur in these environments on the U.S. East Coast and Gulf Coast and because porewater salinity at the depths we sampled (1-6 m) remained high (~35 ppm) throughout the year, so that Ra did not sorb to sediments. One marsh was a salt marsh island with no upland; the other was a salt marsh on the seaward side of a barrier island. Both sites were characterized by fine-grained marsh muds overlying a sandy confined aquifer, typical of the U.S. Southeast, and both sites were instrumented with monitoring wells.

We found that groundwater exchange at both sites was dominated by spring-neap tidal cycles and seasonal variations in mean sea level. Groundwater discharge correlated with tidal amplitude on spring-neap cycles. On seasonal time scales, groundwater discharge peaked in early spring, when sea level was lowest, and reached a minimum in October, when sea level was highest, consistent with prior numerical modeling experiments. Quarterly Ra sampling could not resolve variations on spring-neap time scales, but seasonal patterns in Ra activities were apparent at the marsh island site. At that site, high Ra concentrations were observed in the fall, consistent with reduced groundwater exchange when sea level was high, and lower Ra concentrations were observed in the spring, consistent with increased groundwater exchange when mean sea level was low. Submarine groundwater discharge (SGD) from the back-barrier marsh was strongly influenced by variations in the tidal signal, but Ra activities showed no obvious seasonal variations at this site. We hypothesize that Ra transport at the back-barrier site is influenced by storm surge and rainfall events, which are unlikely to affect groundwater exchange very strongly in the marsh island. Radium is not transported in fresh groundwater, but the presence of freshwater uplands can nevertheless strongly influence Ra transport.