Coastal groundwater dynamics off Santa Barbara, California: Combining geochemical tracers, electromagnetic seepmeters, and electrical resistivity

This paper presents repeat field measurements of 222Rn and 223,224,226,228Ra, electromagnetic seepage meter-derived advective fluxes, and multi-electrode, stationary and continuous marine resistivity surveys collected between November 2005 and April 2007 to study coastal groundwater dynamics within a marine beach in Santa Barbara, California. The study provides insight into magnitude and dynamics of submarine groundwater discharge (SGD) and associated nutrient loadings into near-shore coastal waters, where the predominant SGD drivers can be both spatially and temporally separated. Rn-222 and 223,224,226,228Ra were utilized to quantify the total and saline contribution, respectively, of SGD. The two short-lived Ra isotopes provided an estimate of apparent near-shore water mass age, as well as an estimate of the Ra-derived eddy diffusion coefficient, Kh ( 224Ra 1⁄4 2.86 0.7 m2 s 1; 223Ra 1⁄4 1.32 0.5 m2 s 1). Because 222Rn (t1⁄2 1⁄4 3.8 day) and 224Ra (t1⁄2 1⁄4 3.66 day) have comparable halflives and production terms, they were used in concert to examine respective water column removal rates. Electromagnetic seepage meters recorded the physical, bi-directional exchange across the sediment/ water interface, which ranged from 6.7 to 14.5 cm day 1, depending on the sampling period and position relative to the low tide line. Multi-day time-series 222Rn measurements in the near-shore water column yielded total (saline þ fresh) SGD rates that ranged from 3.1 2.6 to 9.2 0.8 cm day , depending on the sampling season. Offshore 226Ra (t1⁄2 1⁄4 1600 year) and 222Rn gradients were used with the calculated Kh values to determine seabed flux estimates (dpm m 2 day 1), which were then converted into SGD rates (7.1 and 7.9 cm day , respectively). Lastly, SGD rates were used to calculate associated nutrient loads for the near-shore coastal waters off Santa Barbara. Depending on both the season and the SGD method utilized, the following SGD-derived nutrient inputs were computed (mol per day per meter of shoreline): NH4 þ 1⁄4 0.06–0.29 mol day 1 m 1; SiO4 1⁄4 0.22–0.29 mol day 1 m 1; PO4 1⁄4 0.04–0.17 mol day 1 m 1; [NO2 þ NO3 ] 1⁄4 0–0.52 mol day 1 m 1; dissolved inorganic nitrogen (DIN) 1⁄4 0.01–0.17 mol day 1 m 1, and dissolved organic nitrogen (DON) 1⁄4 0.08–0.09 mol day 1 m 1. Compared to the ephemeral nature of fluvial and marine inputs into this region, such SGD-derived loadings can provide a sustained source of select nutrients to the coastal waters off Santa Barbara, California that should be accounted for in mass balance estimates. Published by Elsevier Ltd.