A Geochemical and Geophysical Examination of Submarine Groundwater Discharge and Associated Nutrient Loading Estimates into

Geochemical tracer data (i.e., 222Rn and four naturally occurring Ra isotopes), electromagnetic (EM) seepage meter results, and high-resolution, stationary electrical resistivity images are used to examine the bi-directional (i.e., submarine groundwater discharge and recharge) exchange of a coastal aquifer with sea water. Our study site for these experiments was Lynch Cove, the terminus of Hood Canal, WA, where fjord-like conditions dramatically limit water column circulation that can lead to recurring summer time hypoxic events. In such a system a precise nutrient budget may be particularly sensitive to groundwater-derived nutrient loading. Shore-perpendicular time-series subsurface resistivity profiles show clear, decimeter-scale tidal modulation of the coastal aquifer in response to large, regional hydraulic gradients, hydrologically-transmissive glacial terrain, and large (4-5m) tidal amplitudes. A 5-day 222Rn time-series shows a strong inverse covariance between 222Rn activities (5- 30 dpm L-1) and water level fluctuations, and provides compelling evidence for tidally-modulated exchange of groundwater across the sediment / water interface in this system. Mean Rn-derived submarine groundwate