Application of Rn-222 and CH4 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, Rn-222 and CH4, 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 Rn-222 and CH4 inventories varied directly with groundwater seepage rates. The integrated quantities of Rn-222 and CH4 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 Rn-222 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.