INTERCOMPARISON OF SUBMARINE GROUNDWATER DISCHARGE ESTIMATES FROM A SANDY UNCONFINED AQUIFER

Recent studies suggest that chemical loading from submarine groundwater discharge (SGD) may rival other major sources such as rivers in many coastal areas. Although SGD is often slow and diffuse, solute concentrations can be several orders of magnitude more enriched in groundwater than in surface water. Given the potential for both fresh and saline SGD to be a significant source of chemical loading to coastal waters, it is important to properly assess and quantify SGD rates and distribution. Many studies have focused on methods to quantify SGD, either through point measurements, hydrogeologic models, or by using geochemical tracers that provide a spatially integrated estimate of total flux. However, each of these approaches provide estimates that vary in both the temporal and spatial scales of integration. In order to better understand these inherent differences, intercomparison experiments have been conducted in which different techniques are used to estimate SGD at the same time and place. Such experiments are vital for identifying reliable field methods and determining a suite of techniques that are effective across a variety of geologic settings. The goal of this study is to investigate spatial variability of submarine groundwater discharge from an unconfined sandy coastal aquifer through a variety of complimentary techniques. In particular, we present and compare physical and geochemical data that are used to estimate SGD, including direct measurement via seepage meters, hydrogeologic estimation using Darcy's law, and tracer-based estimates using radon and radium isotopes The sampling plan was designed in part based on patterns observed in an aerial thermal infrared image of the field site, which defines the spatial variability of discharge. At this particular site, nearshore landward topography appears to exert a significant control on fresh groundwater discharge, presumably through its effects on evapotranspiration rates. The Darcy estimate of fresh SGD is quite similar to the estimate of total SGD (fresh plus saline) made using radon. The radium-based SGD estimate, which primarily measures saline circulation through coastal sediments, is considerably smaller. The major assumptions and difficulties of these estimation methods are presented and discussed.