Hydrogeologic study of the Soquel-Aptos area, Santa Cruz County, California

Abstract

The geologic framework of the Soquel-Aptos area includes partly metamorphosed sedimentary rocks of pre-Cretaceous age, igneous and sedimentary rocks of Cretaceous age, and sedimentary rocks of Tertiary and Quaternary age.The Cretaceous igneous rocks underlie the area and form the basement complex.The Tertiary rocks predominate and are chiefly a sequence of interbedded sandstone and siltstone.In this report they are subdivided into formally and informally named geologic formations including the Butano Sandstone (Eocene), unnamed sandstone and conglomerate (Eocene), the San Lorenzo Formation (Eocene and Oligocene), unnamed clay and shale (Miocene), Monterey Shale (Miocene), unnamed siliceous shale (Miocene), the Santa Margarita Formation (Miocene), and the Purisima Formation (Pliocene).In general, the older Tertiary rocks are more consolidated than the younger Tertiary rocks.The Quaternary rocks include the Aromas Red Sands of Alien (1946) (Pleistocene), terrace deposits (late Pleistocene), and alluvium (Recent).HYDROGEOLOGIC STUDY, SOQUEL-APTOS AREA, SANTA CRUZ COUNTY, CALIF.Two well defined large faults trend approximately northwest through the area, the San Andreas fault which cuts through the northernmost boundary and the Zayante fault, roughly parallel to the San Andreas and 3 to 4 miles to the southwest.The attitude and thickness of the Tertiary sedimentary rocks on either side of the Zayante fault are materially different.In the area between the Zayante and San Andreas faults, the, rocks are intensively folded and faulted and are as much as 10,000 feet thick; whereas in the area between the Zayante fault and Monterey Bay the rocks, predominantly the Purisima Formation, are only slightly folded and faulted, and probably are not more than 3,000 feet thick.The sedimentary rocks southwest of the Zayante fault rest upon the Cretaceous igneous rocks.The sedimentary rocks of Cretaceous age, of Tertiary age older than the Purisima Formation, and of Quaternary age are of minor significance to the present water-supply situation of the Soquel-Aptos area.The Butano Sandstone and the Santa Margarita (?) Formation of Tertiary age, and the Aromas Red Sands of Alien (1946) of Quaternary age may offer possibilities for future ground-water development.However, very few data are available to establish their potential.The sandstone beds in the Purisima Formation are the most important ground-water source in the area.The Purisima Formation extends from the Zayante fault southward into Monterey Bay, and in this report has been arbitrarily divided into three subunits.All three subunits contain water but only the upper two, B and C, which are mainly sandstone, yield usable quantities.In general wells in subunit B yield more water than wells in subunit C.Continuous long-term records of water levels from two wells in subunit B show no clear rising or falling trend, however, both wells are inland, distant from the areas of major withdrawals.Other wells in subunit B, nearer the coast, provide shorter, noncontinuous records that indicate that before present-day pumping rates were reached water levels were higher than now.As fresh water is pumped from the Purisima Formation and the water levels are lowered, salt water enters.As yet, salt water has not penetrated inland to reach wells, but continued and perhaps increased pumpage may eventually bring about salt-water contamination of some wells.A system of specifically designed and located monitoring wells could provide warning of imminent contamination before it actually affects the supply.The quality of water from subunit C is better than that from subunit B. Water from subunit B has a slight hydrogen-sulfide odor and its calcium, iron, manganese, sulfate, and total dissolved-solids concentrations generally exceed those of subunit C. Water from the Aromas Red Sands of Alien (1946) sampled at La Selva Beach has much better chemical characteristics than either subunit B or subunit C.The ground water from five locations was tested and found to be moderately corrosive to mild steel and not at all corrosive to stainless steel.This report was prepared under the overall supervision of R. Stanley Lord, California district chief for the Water Resources Division of the U.S. Geological Survey, and under the immediate