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

Abstract

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 224,223Ra isotopes provided an estimate of apparent near-shore water mass age, as well as an estimate of the Ra-derived eddy diffusion coefficient, K h ( 224Ra = 2.86 ± 0.7 m 2 s −1; 223Ra = 1.32 ± 0.5 m 2 s −1). Because 222Rn ( t ½ = 3.8 day) and 224Ra ( t ½ = 3.66 day) have comparable half-lives 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 + fresh) SGD rates that ranged from 3.1 ± 2.6 to 9.2 ± 0.8 cm day −1, depending on the sampling season. Offshore 226Ra ( t ½ = 1600 year) and 222Rn gradients were used with the calculated K h 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 −1, 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): NH 4 + = 0.06–0.29 mol day −1 m −1; SiO 4 = 0.22–0.29 mol day −1 m −1; PO 4 3−= 0.04–0.17 mol day −1 m −1; [NO 2 − + NO 3 −] = 0–0.52 mol day −1 m −1; dissolved inorganic nitrogen (DIN) = 0.01–0.17 mol day −1 m −1, and dissolved organic nitrogen (DON) = 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.