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

Swarzenski, P. W.; Izbicki, J. A.

Abstract:

This paper presents repeat field measurements of ²²²Rn and ^{223,224,226,228}Ra, 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 ^{223,224,226,228}Ra 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 m² s^-1; ²²³Ra = 1.32 ?? 0.5 m² s^-1). Because ²²²Rn (t_1/2 = 3.8 day) and ²²⁴Ra (t_1/2 = 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_1/2 = 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): NH₄⁺ = 0.06-0.29 mol day^-1 m^-1; SiO₄ = 0.22-0.29 mol day^-1 m^-1; PO₄^3-= 0.04-0.17 mol day^-1 m^-1; [NO₂^- + NO₃^-] = 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.

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Article

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70034683

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Swarzenski, P. W.; Izbicki, J. A.

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English

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Estuarine, Coastal and Shelf Science

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Coastal groundwater dynamics off Santa Barbara, California: Combining geochemical tracers, electromagnetic seepmeters, and electrical resistivity; 2009; Article; Journal; Estuarine, Coastal and Shelf Science; Swarzenski, P. W.; Izbicki, J. A.