ESTIMATING GROUNDWATER DISCHARGE INTO THE NORTHEASTERN GULF-OF-MEXICOUSING RN-222

Submarine groundwater discharge (SGD) may provide important chemical c onstituents to the ocean, but the dispersed nature of this process mak es locating and quantifying its input extremely difficult. Since groun dwater contains 3-4 orders of magnitude greater radon than seawater, R n-222 may be a useful tracer of this process if all other sources of r adon to bottom waters can be evaluated. We report development of a SGD tracing tool based on radon inventories in a coastal area of the nort heastern Gulf of Mexico. We evaluated factors that influence the conce ntration of radon in the water column (i.e., production-decay, horizon tal transport, and loss across the pycnocline) using a linked benthic exchange-horizontal transport model. Total Rn-222 benthic fluxes (grea ter than or equal to 2420 dpm m(-2) day(-1) measured with in situ cham bers are of the magnitude required to support measured sub-pycnocline Rn-222 inventories, while estimates of molecular diffusion show that t his input is relatively small (less than or equal to dpm m(-2) day(-1) ). Using this model approach, together with measurements of the radon inventory, we estimated a regional subsurface fluid flow ranging from 180 to 710 m(3) sec(-1) into the 620 km(2) study area. This discharge, equivalent to an upward advective velocity of appoximately 2-10 cm da y(-1) dispersed over this entire study area, is equivalent to approxim ately 20 first magnitude springs.