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. SGD can occur as terrestrially-derived, typically fresh, groundwater discharge, or by seawater circulation through aquifer sediments. Because both terrestrial and recirculated seawater SGD can be significant sources of chemical loading to coastal waters, appropriate methods are needed to properly assess and quantify the rates and distribution of these different inputs. Although many techniques exist to quantify SGD, each method samples different components of SGD (e.g., fresh versus saline discharge) over different temporal and spatial scales. The result is that confusion exists about how to use the different methods. In this study, we investigate the applicability of a variety of techniques for estimating SGD from an unconfined sandy coastal aquifer. Physical methods are first used to understand the spatial variability of discharge. This information is then used with physical and geochemical data to estimate SGD. Investigative methods include direct measurement via seepage meters, hydrogeologic estimation using Darcy’s law, and tracer-based estimates using radon and radium isotopes. At our field site on Cape Cod, Massachusetts, nearshore landward topography appears to exert a significant control on the spatial variability of fresh groundwater discharge, presumably through its effects on evapotranspiration, and hence recharge, rates. The Darcy estimate of fresh SGD (4.0 m3 m−1 d−1) is similar to a previous estimate made using a seepage meter transect that spans the seepage face. Radon serves as a valuable estimator of total SGD (fresh plus saline) (5.6 m3 m−1 d−1). The difference between the total and fresh SGD estimates is in good agreement with the radium-based SGD estimate (0.56 m3 m−1 d−1), which primarily measures saline circulation through coastal sediments. Thus, we conclude that the hydrogeologic estimate and radon and radium techniques are complimentary for estimating different components of total SGD at our field site.