APPLICATION OF RN-222 AND CH4 FOR ASSESSMENT OF GROUNDWATER DISCHARGETO THE COASTAL OCEAN

Groundwater discharge is a neglected source of freshwater and dissolve d constituents to the ocean. It can occur via diffuse seepage and poin t source spring discharge. Two naturally occurring trace gases, Rn-222 and CH4, are present in groundwater at concentrations that are elevat ed by several orders of magnitude relative to seawater, and they may b e useful in tracing groundwater inputs to surface waters. Water sample s collected near a submarine spring in the northeastern Gulf of Mexico displayed radon and methane concentrations inversely related to salin ity and considerably greater than those found in surrounding waters. C oastal water Rn-222 and CH4 inventories varied directly with groundwat er seepage rates. The integrated quantities of Rn-222 and CH4 in the n earshore waters overlying a seepage meter transect showed a significan t positive relationship (95% C.L.) to direct measurements of seepage. Diffusive fluxes (178 +/- 56 dpm m(-2) d(-1)), obtained by three diffe rent approaches at the same site, showed that these waters receive onl y a small contribution of Rn-222 by diffusion. In contrast, benthic fl ux chamber measurements revealed a total advective-diffusive contribut ion 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.