Geochemical and physical sources of radon variation in a subterranean estuary — Implications for groundwater radon activities in submarine groundwater discharge studies

Submarine groundwater discharge (SGD), from springs and diffuse seepage, has long been recognized as a source of chemical constituents to the coastal ocean. Because groundwater is two to four orders of magnitude enriched in radon compared to surface water, it has been used as both a qualitative and a quantitative tracer of groundwater discharge. Besides this large activity gradient, the other advantage of radon stems from its classification as noble gas; that is, its chemical behavior is expected not to be influenced by salinity, redox, and diagenetic conditions present in aquatic environments.During our three-year monthly sampling of the subterranean estuary (STE) in Waquoit Bay, MA, we found highly variable radon activities (50–1600 dpm L− 1) across the fresh–saline interface of the aquifer. We monitored pore water chemistry and radon activity at 8 fixed depths spanning from 2 to 5.6 m across the STE, and found seasonal fluctuations in activity at depths where elevated radon was observed. We postulate that most of the pore water 222Rn is produced from particle-surface-bound 226Ra, and that the accumulation of this radium is likely regulated by the presence of manganese (hydr)oxides. Layers of manganese (hydr)oxides form at the salinity transition zone (STZ), where water with high salinity, high manganese, and low redox potential mixes with fresh water. Responding to the seasonality of aquifer recharge, the location of the STZ and the layers with radium enriched manganese (hydr)oxide follows the seasonal land- or bayward movement of the freshwater lens. This results in seasonal changes in the depth where elevated radon activities are observed.The conclusion of our study is that the freshwater part of the STE has a radon signature that is completely different from the STZ or recirculated sea water. Therefore, the radon activity in SGD will depend on the ratio of fresh and recirculated seawater in the discharging groundwater.