Evaluation of Ra-224 as a tracer for submarine groundwater discharge in Long Island Sound (NY)

摘要

The approach to quantify submarine groundwater discharge using Ra isotopes generally involves developing a Ra mass balance in an estuary, bay or lagoon. In this work we present a Ra-224 mass balance used to evaluate the importance of the submarine groundwater discharge (SGD) in Long Island Sound (NY, US), the third most important estuary in US, located between Long Island and Connecticut that is usually affected by summertime hypoxia in the western basin. Three surveys were conducted between April 2009 and August 2010 where 25 water stations were sampled for Ra isotopes, oxygen and Mn. Stations were oriented along 4 transects: one axial extending from the western to the eastern Sound and three longitudinal transects in the western, central and eastern Sound. The inventory of Ra-224 in the water column in summer was circa 2 times greater than in winter, suggesting an increased Ra-224 flux to the Sound in summer. A mass balance for Ra-224 was constructed considering tidal exchange, inputs from rivers, desorption from resuspended particles, diffusive fluxes (including bioirrigation) from bottom sediments and radioactive decay in the water column. Fluxes of Ra-224 from bottom sediments were measured by incubating cores under oxic conditions in a continuous flow mode such that the overlying water was circulated through a Mn-oxide fiber to maintain a constant activity of Ra-224. Fluxes from muddy sediments (comprising similar to 67% of the Sound bottom) ranged from 127 to 312 dpm m(-2) d(-1) and were similar to 60 dpm m(-2) d(-1) in sandy sediments (33% of the Sound). Incubations under hypoxic conditions showed variable fluxes depending on reduction and mobilization of Mn. The Ra-224 mass balance shows a net input of Ra to the Sound of 106 +/- 50 x 10(12) dpm y(-1) in spring and 244 +/- 112 x 10(12) dpm y(-1) in the summer that is attributed to SGD. Elevated Ra-224 values were observed near shore and in the pore fluids of the coarse beach sands along the Long Island and Connecticut coasts, suggesting that SGD driven by tidal recirculation through the beach face is a major source of Ra-224 to the Sound. Seasonal variation in this source seems unlikely, and the calculated Ra-224 SGD fluxes for spring and summer overlap within the uncertainties. Nevertheless we conclude that variations in the Ra-224 water column inventories could be produced by seasonal changes in bioirrigation and/or redox cycling of Mn as well as sediment resuspension and desorption of Ra-224 from resuspended particles, and that our mass balance underestimates these terms, particularly in the summer. Ra-224 fluxes from sediments in estuaries, especially those with significant areas of muddy sediments and seasonal hypoxia, are important and should be well constrained in future uses of this isotope as a tracer for SGD. (C) 2014 Elsevier Ltd. All rights reserved.