Groundwater Information for Elkhorn Slough

Draft (ver. 2) Groundwater Information for Elkhorn Slough

Greg Shellenbarger ()

Groundwater – Background

Elkhorn Slough occupies a small portion of the southern end of the PajaroValley groundwater basin. Groundwater recharge in this basin comes primarily from precipitation, applied water use (e.g., agricultural irrigation) and stream recharge. Groundwater is extracted predominately for urban, agricultural and rural residential use. This basin has been subject to severe groundwater overdrafts (where extraction exceeds recharge) for over 60 years (CDWR 2004). Annual estimates of groundwater extractions (~69,000 acre feet) exceed recharge (~61,000 af) by less than 10,000 af but are almost three times the sustainable extraction estimate of 24,000 af (RMC 2001 as cited in CDWR 2004). The severe, chronic overdrafts have led to falling groundwater levels and significant seawater intrusion along the coastal margin for this basin. Overdrafts appear to be affecting primarily the confined aquifers at 180’ and 400’ (this is true for the SalinasValley groundwater basin – I have not yet found aquifer depths for PajaroValley). Rainwater recharge though the porous coastal sand dunes may serve as a hydraulic barrier to seawater intrusion in the shallower aquifers (CDWR 2004).

Potential Subsidence Due to Groundwater Pumping

(requires analysis of an existing dataset)

In areas that experience seasonal groundwater withdrawals and recharge (i.e., each process happens during a different season), reversible seasonal uplift and subsidence of the soil can occur (Bawden et al. 2003). Continued extraction of groundwater at a rate that exceeds the recharge rate can lead to regional (on the scale of km to 10s of km) ground compaction around wells. The issue of subsidence is a critical one when evaluating land elevations in a tidal system. Marsh development and growth requires a very narrow range in elevation to provide a proper wetting period for survival (Cornu and Sadro 2002).

Relatively recently, a technique has been developed that enables high resolution detection of changes in land surface elevations on a seasonal scale using satellite Interferometric Synthetic Aperture Radar (InSAR). By bouncing radar signals off the ground at different times but from the same point in space, millimeter-scale accuracy of the ground surface elevation changes can be determined (Galloway et al. 1998). Interferograms have been developed to show subsidence resulting from groundwater pumping, hydrocarbon production and a variety of other anthropogenic activities that contribute to uplift and subsidence (Bawden et al. 2003).

The USGS has about 16 pairs of InSAR images from Elkhorn Slough collected between 1996 and 2000. These image pairs are currently unprocessed and of unknown quality, but presumably some pairs may be useful to determine seasonal (and perhaps interannual) changes in land surface elevation. The image pairs predominately cover transitions between summer and winter, so it might only be possible to determine seasonal uplift and infer subsidence.

Tidally Pumped and Terrestrially Derived Groundwater flux

In the coastal region, groundwater discharge along the ocean or an estuary margin rarely consists solely of freshwater. Rather, the near-coast subsurface serves as a reaction zone of fresher groundwater and more saline seawater that has been termed a ‘subterranean estuary’ (Moore 1999). The water in the subterranean estuary has been called submarine groundwater (SGW) to emphasize, that it is not only fresh groundwater, but can include a significant component of seawater that infiltrates into the subsurface. This water can then be subsequently discharged to the coast with changed chemical characteristics (Burnett et al. 2002).

Even in the absence of fresh groundwater, SGW discharge can be explained by the physics of the interaction of the waves and tide with the shoreline (Nielsen 1990; Horn 2002). The face of the estuary margin acts like a highly non-linear filter to the movement of water across it. Because of the non-linearity, a tidally averaged over-height of the water in the ground relative to the coast can be maintained. This over-height implies that tidal physics alone can always provide a hydraulic gradient in the shallow unconfined aquifer that drives water from the ground to the sea. The groundwater over-height is increased further with the presence of waves (Li et al. 1999).

In 2002, a group from StanfordUniversity initiated a study to estimate groundwater fluxes into Elkhorn Slough using radium isotopes (Misra and Paytan 2002). Naturally occurring radium isotopes are bound to soil particles in freshwater but readily desorb via ion exchange when in contact with solutions of higher ionic strength (desorption begins at salinities around 2 and is complete at salinities around 20). Preliminary results show higher radium activities at low tide compared to high tide. This is consistent with the above explanation of the dynamics of tides and waves on the shallow unconfined aquifer near the shoreline. In addition, the data suggest that there is potentially fresh groundwater inputs along the northern reaches of Elkhorn Slough (north of the end of the deep channel), although the flux of this groundwater to the slough has not been quantified. Supporting data collected from the Elkhorn Slough region also suggest likely groundwater fluxes (not quantified) into the SalinasRiver and/or Moro Cojo Slough.

Literature Cited

Bawden, G.W., M. Sneed, S.V. Stork, and D.L. Galloway. 2003. Measuring human-induced 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. O’Rourke, R. Paulsen, C. Smith, L. Smith, and M. Taniguchi. 2002. Assessing methodologies for measuring groundwater discharge to the ocean. EOS, Trans. AGU, 83: 117-123.

California Department of Water Resources. 2004. Central coast hydrologic region, PajaroValley groundwater basin, California’s Groundwater Bulletin 118.

Cornu, C.E., and S. Sadro. 2002. Physical and functional responses to experimental marsh surface elevation manipulation in CoosBay’s South Slough. Restoration Ecology, 10(3): 474-486.

Galloway, D.L., K.W. Hudnut, S.E. Ingebritsen, S.P. Phillips, G. Peltzer, F. Rogez, and P.A. Rosen. 1998. Detection of aquifer system compaction and land subsidence using inferometric synthetic aperture radar, Antelope Valley, Mojave Desert, California. Water Resources Research, 34: 2573-2585.

Horn, D.P. 2002. Beach groundwater dynamics. Geomorph., 48: 121-146.

Li, L., D.A. Barry, F. Stagnitti, and J.-P. Parlange. 1999. Submarine groundwater discharge and associated chemical input to a coastal sea. Water Resour. Res., 35: 3253-3259.

Misra, G., and A. Paytan. 2002. Radium isotopes as tracers for groundwater input in Elkhorn Slough, California. American Geophysical Union, Fall Meeting 2002, abstract #OS22B-0276.

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).