224Ra distribution in surface and deep water of Long Island Sound: sources and horizontal transport rates

Measurements of surface water and deep water224Ra (half-life 3.64 days) distributions in Long Island Sound (LIS) were conducted in July 1991. Because the pycnocline structure of LIS had been in place for about 50 days in July (long compared to the half-life of224Ra),224Ra in the surface water and the deep water operate as separate systems. In the surface water, the fine-grain sediments of nearshore and saltmarsh environments provide a strong source of224Ra, which is horizontally mixed away from the shore to central LIS. A one-dimensional model of224Ra distribution suggests across-LIS horizontal eddy dispersivity of 5–50 m2 s−1. In the deep water, the mid-LIS sediment flux of224Ra is enhanced by ∼2x relative to the periphery, and the horizontal eddy flux is from central LIS to the periphery. A second one-dimensional model suggests a cross-LIS horizontal eddy dispersivity below the thermocline of 5–50 m2 s−1.224Ra fluxes into the deep water of the central LIS are likely enhanced by (1) inhomogeneous sediment or (2) a reduced scavenging of224Ra in the sediments of central LIS brought about by low oxygen conditions (hypoxia) and the loss of the MnO2 scavenging layer in the sediments. These rates of horizontal eddy dispersivity are significantly less than the estimate of 100–650 m2 s−1 (Riley, 1967) but are consistent with the transport necessary to explain the dynamics of oxygen depletion in summer LIS. These results demonstrate the use of224Ra for quantifying the parameters needed to describe estuarine mixing and transport.