Radioactive and stable isotope measurements reveal saline submarine groundwater discharge in a semiarid estuary

Abstract

There is little information on submarine groundwater discharge (SGD) in hypersaline estuaries in semi-arid climates where SGD may be the dominant nutrient source. Here, we assess the spatial and temporal variability of SGD in the hypersaline Baffin Bay (Texas) using radon [222Rn], radium [226Ra, 224Ra, and 223Ra], and water isotopes [δ18O and δD]. Continuous electrical resistivity surveys revealed potential SGD at nearshore serpulid reefs. High spatial and temporal resolution radon measurements revealed slightly higher SGD inputs at the shoreline near course-grained sediments and relic serpulid reefs. Mass balance models of 222Rn and 226Ra produced equal SGD estimates within the range of uncertainty, while 223Ra yielded substantially higher SGD rates. Baywide SGD rates from 226Ra, 223Ra, and 222Rn ranged from 1.6 ± 0.2 to 41.3 ± 4.1. Larger SGD rates obtained from short-lived isotopes imply higher recirculated/saline SGD rates over short time scales. Radiogenic and stable isotopes as well as resistivity indicate that saline rather than freshwater accounts for most SGD in this hypersaline estuary. Because saline porewater exchange can play a significant role in coastal biogeochemical budgets, SGD inputs should be considered in management strategies in semi-arid areas where surface inflows are almost absent.