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Draft (ver. 2) Groundwater Information for Elkhorn Slough
Greg Shellenbarger (gshellen@usgs.gov)
Groundwater – Background
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Elkhorn Slough occupies a small portion of the southern end of the Pajaro Valley
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groundwater basin. Groundwater recharge in this basin comes primarily from
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precipitation, applied water use (e.g., agricultural irrigation) and stream recharge.
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Groundwater is extracted predominately for urban, agricultural and rural residential use.
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This basin has been subject to severe groundwater overdrafts (where extraction exceeds
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recharge) for over 60 years (CDWR 2004). Annual estimates of groundwater extractions
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(~69,000 acre feet) exceed recharge (~61,000 af) by less than 10,000 af but are almost
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three times the sustainable extraction estimate of 24,000 af (RMC 2001 as cited in
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CDWR 2004). The severe, chronic overdrafts have led to falling groundwater levels and
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significant seawater intrusion along the coastal margin for this basin. Overdrafts appear
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to be affecting primarily the confined aquifers at 180’ and 400’ (this is true for the
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Salinas Valley groundwater basin – I have not yet found aquifer depths for Pajaro
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Valley). Rainwater recharge though the porous coastal sand dunes may serve as a
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hydraulic barrier to seawater intrusion in the shallower aquifers (CDWR 2004).
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Potential Subsidence Due to Groundwater Pumping
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(requires analysis of an existing dataset)
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In areas that experience seasonal groundwater withdrawals and recharge (i.e., each
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process happens during a different season), reversible seasonal uplift and subsidence of
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the soil can occur (Bawden et al. 2003). Continued extraction of groundwater at a rate
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that exceeds the recharge rate can lead to regional (on the scale of km to 10s of km)
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ground compaction around wells. The issue of subsidence is a critical one when
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evaluating land elevations in a tidal system. Marsh development and growth requires a
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very narrow range in elevation to provide a proper wetting period for survival (Cornu and
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Sadro 2002).
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Relatively recently, a technique has been developed that enables high resolution
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detection of changes in land surface elevations on a seasonal scale using satellite
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Interferometric Synthetic Aperture Radar (InSAR). By bouncing radar signals off the
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ground at different times but from the same point in space, millimeter-scale accuracy of
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the ground surface elevation changes can be determined (Galloway et al. 1998).
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Interferograms have been developed to show subsidence resulting from groundwater
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pumping, hydrocarbon production and a variety of other anthropogenic activities that
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contribute to uplift and subsidence (Bawden et al. 2003).
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The USGS has about 16 pairs of InSAR images from Elkhorn Slough collected
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between 1996 and 2000. These image pairs are currently unprocessed and of unknown
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quality, but presumably some pairs may be useful to determine seasonal (and perhaps
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interannual) changes in land surface elevation. The image pairs predominately cover
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transitions between summer and winter, so it might only be possible to determine
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seasonal uplift and infer subsidence.
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Tidally Pumped and Terrestrially Derived Groundwater flux
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In the coastal region, groundwater discharge along the ocean or an estuary margin
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rarely consists solely of freshwater. Rather, the near-coast subsurface serves as a reaction
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zone of fresher groundwater and more saline seawater that has been termed a
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‘subterranean estuary’ (Moore 1999). The water in the subterranean estuary has been
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called submarine groundwater (SGW) to emphasize, that it is not only fresh groundwater,
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but can include a significant component of seawater that infiltrates into the subsurface.
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This water can then be subsequently discharged to the coast with changed chemical
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characteristics (Burnett et al. 2002).
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Even in the absence of fresh groundwater, SGW discharge can be explained by the
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physics of the interaction of the waves and tide with the shoreline (Nielsen 1990; Horn
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2002). The face of the estuary margin acts like a highly non-linear filter to the movement
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of water across it. Because of the non-linearity, a tidally averaged over-height of the
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water in the ground relative to the coast can be maintained. This over-height implies that
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tidal physics alone can always provide a hydraulic gradient in the shallow unconfined
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aquifer that drives water from the ground to the sea. The groundwater over-height is
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increased further with the presence of waves (Li et al. 1999).
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In 2002, a group from Stanford University initiated a study to estimate groundwater
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fluxes into Elkhorn Slough using radium isotopes (Misra and Paytan 2002). Naturally
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occurring radium isotopes are bound to soil particles in freshwater but readily desorb via
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ion exchange when in contact with solutions of higher ionic strength (desorption begins
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at salinities around 2 and is complete at salinities around 20). Preliminary results show
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higher radium activities at low tide compared to high tide. This is consistent with the
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above explanation of the dynamics of tides and waves on the shallow unconfined aquifer
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near the shoreline. In addition, the data suggest that there is potentially fresh
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groundwater inputs along the northern reaches of Elkhorn Slough (north of the end of the
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deep channel), although the flux of this groundwater to the slough has not been
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quantified. Supporting data collected from the Elkhorn Slough region also suggest likely
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groundwater fluxes (not quantified) into the Salinas River and/or Moro Cojo Slough.
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Literature Cited
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Bawden, G.W., M. Sneed, S.V. Stork, and D.L. Galloway. 2003. Measuring humaninduced land subsidence from space. U.S. Geological Survey Fact Sheet 069-03, 4p.
Burnett, W., J.P. Chanton, J. Christoff, E. Kontar, S. Krupa, M. Lambert, W.S. Moore, D.
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O’Rourke, R. Paulsen, C. Smith, L. Smith, and M. Taniguchi. 2002. Assessing
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methodologies for measuring groundwater discharge to the ocean. EOS, Trans.
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AGU, 83: 117-123.
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California Department of Water Resources. 2004. Central coast hydrologic region,
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Pajaro Valley groundwater basin, California’s Groundwater Bulletin 118.
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http://www.dpla2.water.ca.gov/publications/groundwater/bulletin118/basins/pdfs_des
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c/3-2.pdf
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Cornu, C.E., and S. Sadro. 2002. Physical and functional responses to experimental
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marsh surface elevation manipulation in Coos Bay’s South Slough. Restoration
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Ecology, 10(3): 474-486.
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Galloway, D.L., K.W. Hudnut, S.E. Ingebritsen, S.P. Phillips, G. Peltzer, F. Rogez, and
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P.A. Rosen. 1998. Detection of aquifer system compaction and land subsidence
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using inferometric synthetic aperture radar, Antelope Valley, Mojave Desert,
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California. Water Resources Research, 34: 2573-2585.
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Horn, D.P. 2002. Beach groundwater dynamics. Geomorph., 48: 121-146.
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Li, L., D.A. Barry, F. Stagnitti, and J.-P. Parlange. 1999. Submarine groundwater
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discharge and associated chemical input to a coastal sea. Water Resour. Res., 35:
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3253-3259.
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Misra, G., and A. Paytan. 2002. Radium isotopes as tracers for groundwater input in
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Elkhorn Slough, California. American Geophysical Union, Fall Meeting 2002,
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abstract #OS22B-0276.
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Moore, W.S. 1999. The subterranean estuary: a reaction zone of groundwater and
seawater. Marine Chemistry, 65: 111-126.
Nielsen, P. 1990. Tidal dynamics of the water table in beaches. Water Resources Res.,
26(9): 2127-2134.
Raines, Melton, and Carella. 2001. Pajaro Valley Water Management Agency – Revised
Basin Management Plan (Draft).