Submarine groundwater discharge (SGD), in form of 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 richer in radon than surface
water, it has been used as both a qualitative and a quantitative tracer of groundwater
discharge. Besides this large activity gradient, the other perceived 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
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 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 bay-ward 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.