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

Estuarine, Coastal and Shelf Science

By:
and
DOI: 10.1016/j.ecss.2009.03.027

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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, Kh (224Ra = 2.86 ?? 0.7 m2 s-1; 223Ra = 1.32 ?? 0.5 m2 s-1). Because 222Rn (t1/2 = 3.8 day) and 224Ra (t1/2 = 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 (t1/2 = 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-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; PO43-= 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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Additional Publication Details

Publication type:
Article
Publication Subtype:
Journal Article
Title:
Coastal groundwater dynamics off Santa Barbara, California: combining geochemical tracers, electromagnetic seepmeters, and electrical resistivity
Series title:
Estuarine, Coastal and Shelf Science
DOI:
10.1016/j.ecss.2009.03.027
Volume
83
Issue:
1
Year Published:
2009
Language:
English
Publisher:
Elsevier
Description:
13
Larger Work Type:
Article
Larger Work Subtype:
Journal Article
Larger Work Title:
Estuarine, Coastal and Shelf Science
First page:
77
Last page:
89
Country:
United States
State:
California
City:
Santa Barbara