Estimating groundwater discharge into the northeastern Gulf of Mexico using radon-222

Submarine groundwater discharge (SGD) may provide important chemical constituents to the ocean, but the dispersed nature of this process makes locating and quantifying its input extremely difficult. Since groundwater contains 3–4 orders of magnitude greater radon than seawater, 222Rn may be a useful tracer of this process if all other sources of radon to bottom waters can be evaluated. We report development of a SGD tracing tool based on radon inventories in a coastal area of the northeastern Gulf of Mexico. We evaluated factors that influence the concentration of radon in the water column (i.e., production-decay, horizontal transport, and loss across the pycnocline) using a linked benthic exchange-horizontal transport model. Total 222Rn benthic fluxes (≥2420 dpm m−2 day−1) measured with in situ chambers are of the magnitude required to support measured sub-pycnocline 222Rn inventories, while estimates of molecular diffusion show that this input is relatively small (≤230 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 m3 sec−1 into the 620 km2 study area. This discharge, equivalent to an upward advective velocity of approximately 2–10 cm day−1 dispersed over this entire study area, is equivalent to approximately 20 first magnitude springs.