Abstract

Along tidal coasts, seawater circulated through the intertidal beach contributes to submarine groundwater discharge (SGD) and its associated geochemical signature. The short-lived radium isotopes, 223Ra (half-life = 11.4 d) and 224Ra (half-life = 3.66 d), were used to quantify this component of SGD in a large estuary, Long Island Sound (LIS), New York, United States. The tide is semidiurnal with a range of approximately 2 m. Concentrations in beach pore waters ranged from 97 to 678 disintegrations per minute (dpm) 224Ra 100 L–1, whereas concentrations in open coastal waters ranged from approximately 12 to 69 dpm 224Ra 100 L–1. A simple model based on ingrowth of 224Ra in the pore water of the beach sands was used to determine residence times of 0.6 to 2.5 d for water in the intertidal beach. Both 223Ra and 224Ra showed decreasing gradients and concentration in an offshore transect away from the beach face in Smithtown Bay, whereas the long-lived radium isotopes, 228Ra (half-life = 5.75 y) and 226Ra (half-life = 1,600 y), showed no significant gradients. Based on the 224Ra gradient, the flux across the LIS shoreline was estimated to be 1.79 × 108 dpm m–1 y–1. The 224Ra inventories in two zones, 0–50 m and 0–100 m offshore, were used to estimate total SGD fluxes of 3.1 × 1010 to 6.6 × 1010 m3 y–1 of intertidal seawater to the nearshore of LIS. Comparison of this estimate with hydrodynamic models of fresh groundwater flow in the adjacent coastal aquifer suggests that less than 1% of the SGD is freshwater.

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