Dispersivity and distribution coefficients in marine sediments using ³H and ²²⁶Ra

¹ Department of Oceanography and Coastal Sciences, Louisiana State University, Baton Rouge, 70803 Louisiana, USA
² Department of Geology and Geophysics, Louisiana State University, Baton Rouge, 70803 Louisiana, USA

Corresponding author: jcable@lsu.edu

Abstract

Continental aquifers flow along topographic gradients toward coastlines in many geologic environments. Wherever aquifers intersect coastlines, ground water mixes with seawater infiltrated in marine sediments. In this mixing zone, sediment-pore fluid interactions create geochemical gradients that migrate back and forth on short (e.g., tides, waves) and long time scales (e.g. seasonal and eustatic sea level fluctuations). We focus on sorption and ion exchange characteristics of radium in these mixing zone sediments using laboratory column experiments. Previous studies have shown salinity affects radium sorption, but this process has not been investigated in porous sediments where solution ionic strength may fluctuate over a range of time scales. Additionally, different sediment types present within this subsurface zone may act as temporally and spatially variable source/sink sites for Ra. These sediments may have been underestimated in Ra mass balances for estimating coastal ground water fluxes. We performed microcosm sediment column experiments to examine Ra behavior and quantify the source/sink capacity of sediments. Longitudinal dispersivity results indicate local flow field variability was minimal during experiments. Radium desorption from sediments occurs rapidly (within 60 to 120 minutes) with exposure to solution and is generally high in the shallow portion of the subterranean mixing zone.