Intertidal percolation through beach sands as a source of ^224,223Ra to Long Island Sound, New York, and Connecticut, United States

© 2015 Trevor J. McDougall and Oliver A. Krzysik. 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−1of 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.