MIXING OF FRESH WATER AND SALT WATER IN A COASTAL AQUIFER: INSIGHT FROM ²³⁸U SERIES RADIONUCLIDES

In coastal aquifers where the hydraulic gradient supports an offshore seepage face, two mixing zones occur as a lateral mixing of fresh and saline groundwater and a vertical mixing of fresh groundwater and saline surface water. Contrasts in pH, Eh, and fluid chemistry among these water sources affect the transport and transformation of dissolved constituents. We examine mixing effects on the spatial and temporal distribution of ^234,238U, ²²⁶Ra, and ²²²Rn. Pore water and surface water were sampled seasonally from November 2004 to October 2006 along a transect extending 30 m offshore into Indian River Lagoon, Florida, USA. The transect consists of eight multilevel piezometers, each of which have eight regularly-spaced ports extending to depths of 1.15 to 2.30 m below seafloor (bsf). Pore water salinities outline the two mixing zones. The lateral mixing zone occurs between 10 and 22.5 m offshore; the vertical mixing zone is restricted to the upper 35 to 55 cm bsf. Pore water ²²²Rn and ²²⁶Ra activities show limited temporal variability. However, ²²²Rn activities decrease two to three orders of magnitude across the salinity gradient as a result of decreased fresh groundwater input and/or decreased sedimentary production of ²²²Rn. Vertical ²²⁶Ra profiles suggest it is being released to pore water within the vertical mixing zone; the position and thickness of this Ra release zone increases offshore. ²²⁶Ra release occurs between salinities of 3.5 and 12, suggesting exchange of radium with major seawater cations (e.g. Na). Similarly, ^234,238U are also being released to pore waters in the vertical mixing zone. The U peak is generally restricted to 7 to 15 cm bsf for all sampling locations and is probably mediated by changes in redox potential and/or increases in anion complexes (e.g. carbonate, phosphate, or sulfate). In 2004 and 2005, little to no input of U from groundwater occurred at the shoreline site; however, an influx of U was observed during the 2006 sampling trips suggesting a change in fluid chemistry and/or rock-water interaction between 2005 and 2006. Further investigation through geochemical modeling and comparison with other geochemical parameters may help quantify temporal and spatial variability of these radioisotopes within these two mixing zones.