Geohydrology of Recharge and Seawater
Intrusion in the Pajaro Valley, Santa Cruz
and Monterey Counties, California

U.S. Geological Survey Fact Sheet 044-03

August 2003

This fact sheet is also available in pdf format 1.1MB

The U.S. Geological Survey (USGS) in cooperation with the Pajaro Valley Water Management Agency (PVWMA), has completed the collection and analyses of geologic, hydrologic, geophysical, and geochemical data in the coastal aquifer systems of the Pajaro Valley (fig. 1). These data were collected to delineate the geohydrologic framework of seawater intrusion, as well as, the source, age, and movement of ground water in the coastal aquifer systems (Hanson, 2003).

Figure 1. Location of Pajaro Valley Water Management Agency, Santa Cruz and Monterey Counties, California.

GEOLOGY

(1) Geophysical logs indicate confining beds that occur within older alluvium and in the upper and lower Aromas Sands (fig. 2).

(2) The layered terrestrial and marine deposits restrict seawater intrusion to zones of coarse-grained deposits (fig. 2).

Figure 2. Cross section of the coastal aquifers showing seawater intrusion, chloride values in wells, perforated depths and generalized geology, Pajaro River watershed, Santa Cruz and Monterey Counties, California.

HYDROLOGY

(1) Pajaro River streamflow and local runoff are the two sources of surface water available for ground-water recharge or additional water supply.

(2) Long-term water-level declines, climatic cycles of 2 to 19 years, and seasonal pumping all suppress water levels below seawater pressures and cause the landward flow of seawater (seawater intrusion) (fig. 3).

(3) The Pajaro River (Group 6) and local runoff (Group 7) (fig. 4) provide natural recharge to the ground-water flow system.

Figure 3. Water-level altitudes and chloride concentrations in well PV-3, Pajaro River watershed, Santa Cruz and Monterey Counties, California.

GEOCHEMISTRY

(1) Samples from coastal monitoring wells represent 5 groups of ground-water that differ from surface waters (groups 6 and 7) (fig. 4):

(1) Recent fresh ground water;

(2) Older fresh ground water;

(3) Recent seawater intrusion;

(4) Older sea water; and

(5) Very old ground water.

(2) Stable isotopes indicate a mixture of older ground water and surface water in samples from coastal monitoring wells (Groups 1 and 2) in the upper-aquifer system (fig. 5).

Figure 4. Chemical evaluation of water from wells and surface sites in the Pajaro River watershed, Santa Cruz and Monterey Counties, California.

Figure 5. Isotope values for selected wells and surface water sites in the Pajaro River watershed, Santa Cruz and Monterey Counties, California.

GEOHYDROLOGIC FRAMEWORK

(1) Shallow wells are pumping water from the upper-aquifer system that consists of recently recharged shallow ground water. This water is a renewable resource as indicated by the presence of tritium (fig. 2) and younger carbon-14 ages (table 1).

(2) Deeper wells are pumping water from the lower-aquifer system that consists of older ground water that was recharged thousands of years ago and may represent a nonrenewable resource in the coastal region .

(3) Alternating layers of fine-grained and coarse-grained sediments retard the vertical movement of recharge and result in water-level differences that have persisted for many years (fig. 3).

(4) The primary structures of the aquifers includes a fault-bounded region adjacent to the Santa Cruz Mountains between the San Andreas and Zayante-Vergeles fault zones (Dupre, 1975) (fig. 1). The relation of faults such as the Corralitos fault and Zayante Faults and ground- water flow remains uncertain (fig. 1).

SEAWATER INTRUSION

(1) Two types of saline water occur within the aquifers of the Pajaro Valley—recent and older seawater.

(2) Recent seawater intrusion contains tritium (<50 years old) and is present in basal layers of coarse-grained sediments of the recent and older alluvium and within the upper Aromas Sands (figs.2, 4, and 6). The stable isotope signature of water from wells in the Pajaro Valley with seawater intrusion differs from that in the adjacent Salinas Valley (fig. 5).

(3) Recent seawater intrusion at PV-1 was estimated to be as large as 60 percent of total seawater on the basis of chloride and stable-isotope mixtures (table 1, fig. 5). The vertical extent of seawater intrusion has increased at PV-1 between 1988 and 1998 (fig. 6).

(4) Older seawater in the lower Aromas Sand (figs. 2 and 4) is saline ground water that is not recent seawater intrusion.

Figure 6. Geophysical logs, well construction, and lithology for monitoring well PV-1, Pajaro Valley, Santa Cruz County, California.

REFERENCES CITED

Davis, S.N., and Bentley, H.W., 1982, Dating Groundwater, a short review, in Currie, L.A., ed., Nuclear and chemical dating techniques--Interpreting the environmental record: American Chemical Society Symposium Series, v. 176, p, 187-222.

Hanson, R.T., 2003, Geohydrologic framework of recharge and seawater intrusion in the Pajaro Valley, Santa Cruz and Monterey Counties, California: U.S. Geological Survey Water-Resources Investigation Report WRIR 03-4096, 88 p. (https://pubs.water.usgs.gov/wrir034096/)

Luhdorff & Scalmanini, 1987, Pajaro Valley ground-water investigations-- Phase I: Consultants report to Pajaro Valley Water Management Agency, January, 1987, v. p.

Dupre, W.R., 1975, Quaternary history of the Watsonville lowlands north-central Monterey Bay region, California: unpublished Ph.D. dissertation, Stanford University, 145 p.

TECHNICAL CONTACT

Randall T. Hanson

U.S. Geological Survey

5735 Kearny Villa Road, Suite O

San Diego, CA 92123-1135

e-mail: rthanson@usgs.gov

Voice: (858) 637-6839

Fax: (858) 637-9201

California District activities web site
http://ca.water.usgs.gov/