Application of 222 Rn and CH 4 for assessment of groundwater discharge to the coastal ocean

Groundwater discharge is a neglected source of freshwater and dissolved constituents to the ocean. It can occur via diffuse seepage and point source spring discharge. Two naturally occurring trace gases, 222 Rn and CH 4 , are present in groundwater at concentrations that are elevated by several orders of magnitude relative to seawater, and they may be useful in tracing groundwater inputs to surface waters. Water samples collected near a submarine spring in the northeastern Gulf of Mexico displayed radon and methane concentrations inversely related to salinity and considerably greater than those found in surrounding waters. Coastal water 222 Rn and CH 4 inventories varied directly with groundwater seepage rates. The integrated quantities of 222 Rn and CH 4 in the nearshore waters overlying a seepage meter transect showed a significant positive relationship (95% C.L.) to direct measurements of seepage. Diffusive fluxes (178±56 dpm m ‒2 d ‒1 ), obtained by three different approaches at the same site, showed that these waters receive only a small contribution of 222 Rn by diffusion. In contrast, benthic flux chamber measurements revealed a total advective‐diffusive contribution of 5,200±1,800 dpm m ‒2 d ‒1 ( n = 14). Radon inventories in these shallow nearshore marine waters are consistent with the input of radon‐bearing groundwaters.