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

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.