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

摘要

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 ~(222)Rn, radium ~(226)Ra, 224Ra, and ~(223)Ra, and water isotopes δ~(18)O 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 ~(222)Rn and ~(226)Ra produced equal SGD estimates within the range of uncertainty, while ~(223)Ra yielded substantially higher SGD rates. Baywide SGD rates from ~(226)Ra, ~(223)Ra, and ~(222)Rn 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.