Measurements of Dispersivity and Retardation Factors in Marine Sediments using tritiated calcium chloride solution and Radium-226 Jaye E. Cable 1 , Christopher G. Smith 1 , and William J. Blanford 2 1 Department of Oceanography and Coastal Sciences, Louisiana State University, Baton Rouge, Louisiana 70803 USA, jcable@lsu.edu; 2 Department of Geology and Geophysics, Louisiana State University, Baton Rouge, Louisiana 70803 USA INTRODUCTION One of the most commonly applied environmental tracers is radium, with four naturally occurring radioisotopes, 223,224,226,228 Ra (t 1/2 = 11 d, 3.6 d, 1620 y, 5.7 y). Radium is a ubiquitous alkaline earth metal found in the crystal lattice of most silicate and carbonate minerals as well as sorbed onto clays and Mn- and Fe-hydroxide sediment coatings. It is chemically non-conservative and must be well-characterized for the successful application of Ra mass balances in environmental systems. Nevertheless, its relative ease of measurement and its common occurrence in a variety of geologic environments makes Ra highly useful for studying water transport and circulation patterns, particularly if all sources and sinks are quantified. Within freshwater-salt water mixing zones of coastal aquifers, or the subterranean estuary (Moore, 1999), the sorptive behavior of Ra is poorly defined and sediment Ra contributions to a budget are often considered negligible. Sediment composition and physical properties, redox conditions, and pore water ionic strength affect the sorptive behavior of radium. Poor quantification in turn affects the reliability of the mass balance calculations. This sorption behavior may also impede the consistency of 222 Rn, a 226 Ra daughter, as a geochemical tracer. Both the Ra quartet and 222 Rn are used to evaluate groundwater inputs to coastal systems such as estuaries, wetlands, and continental shelf (e.g. Cable et al., 1996; Moore 1996; Krest et al. 2000; Martin et al, 2007). Numerous geochemical studies have previously recognized the release of radium from particles in estuarine environments (e.g. Li and Chan, 1979; Elsinger and Moore, 1984; Webster et al., 1994, 1995; Moore et al., 1995). In the subterranean estuary, Bokuniewicz et al. (2004) have shown salt penetration into seafloor sediments greatly affects the pore fluid composition. Such salt water penetration is temporally and spatially variable, thus altering the sorption capacity of sediments as pore water ionic strength fluctuates. We derive here the dispersivity of permeable marine sediments and evaluate effects of salinity on Ra sorption in these sediments. Breakthrough curves for 226 Ra and tritiated CaCl 2 solution (conservative tracer) were produced to determine how strongly Ra transport behavior in coastal sediments is coupled to solution ionic strength. MATERIALS AND METHODS A series of sediment column experiments were designed by homogenously packing 2.2-cm inside diameter columns with 226 Ra impregnated, fine-medium quartz sands with less than 1% mud/organic matter (Fig. 1). Column length and volume were 36.8 cm and 95 cm 3 , respectively. Porosity (φ) ranged between 0.30 and 0.38. Permeable marine sediments were soaked for two weeks in 226 Ra-spiked deionized water, dried, and counted via gamma-ray spectrometry prior to packing. After packing the columns, they were capped, oriented vertically on a stand with the inlet at the bottom, and flushed with deionized water (using a HPLC pump) against gravity for 2 to 3 days to ensure full saturation and to remove highly