2001 ENGINEERING GEOLOGY PRACTICE IN NORTHERN CALIFORNIA 19 GROUNDWATER RESOURCES OF NORTHERN CALIFORNIA: AN OVERVIEW HORACIO FERRIZ1 INTRODUCTION California hydrogeology is vast. Hopefully, this over- view will familiarize the new generation of hydroge- California has substantial water resources; how- ologists and engineering geologists with some of the ever, as more and more people compete for these basic, “classic” references that shaped much of our resources, coordinated utilization and planning on current understanding of California hydrogeology. a regional scale is becoming increasingly critical. In keeping with the theme of the volume, I have Groundwater is a particularly important component tried to include pertinent information on practical in the water supply/demand equation, and continues approaches to exploration and groundwater basin to be an attractive source of water for individual management. The interested reader will also want farmers, agro-businesses, rural homeowners, land to refer to the excellent summaries prepared by planners, and water purveyors. We geologists com- Thomas and Phoenix (1983), and Planert and Wil- pete with the dowsers and experienced drilling out- liams (1995). fi ts in the tasks of exploration and design of suitable wells for groundwater extraction; however, we take In preparing this summary I have drawn heavily on almost the full burden of solving the problems on the work of the U.S. Geological Survey (USGS), generated by groundwater development, such as the California Department of Water Resources overdraft and progressive depletion of storage, (DWR) and the California Department of Conserva- declining water tables, deterioration of quality in tion’s Division of Mines and Geology (DMG), which, freshwater aquifers due to seawater encroachment, in my opinion, are three of the foremost geologic and and land subsidence due to the compaction of the hydrogeologic agencies in the world. Those of us that underlying water-bearing sediments. In addition, work in California are lucky to be able to stand on we overlap considerably with civil and geotechnical the shoulders of giants! engineers, land planners, and business managers insofar as groundwater basin management and pro- THE CENTRAL VALLEY tection are concerned. The Central Valley is the largest groundwater This paper is intended to be a point of introduc- basin in the state, not only in terms of total storage tion to the hydrogeology of the major groundwater capacity, but also in terms of its high utilization basins of Northern California (Figure 1), organized rate. A conservative estimate for the year 1995 puts loosely by geologic province. The summaries pre- the annual extraction rate at 9,000,000 acre-feet sented are necessarily selective, as the database of (~11 km3), largely to support California’s foremost industry—agriculture (DWR, 1998). The basin is recharged by direct precipitation and infi ltration along the beds of the San Joaquin and Sacramento river systems (which in turn receive 1 HF Geologic Engineering most of their discharge from rainfall and snowmelt 1416 Oakdale-Waterford Hwy. Waterford, CA 95386 in the Sierra Nevada). Major withdrawals are through evapotranspiration, subfl ow into the Sac- Department of Physics and Geology ramento delta, and pumping. With regard to man- California State University, Stanislaus agement of the groundwater resource, Bertoldi Turlock, CA 95382 [email protected] et al. (1991) have noticed that development of 20 DIVISION OF MINES AND GEOLOGY BULLETIN 210 the resource has greatly modifi ed the total fl ow through the basin, increasing from an estimated 2 million acre-feet in the pre- 7 GROUNDWATER development period (early 1900s) BASINS to nearly 12 million acre-feet in the late 1980s. In the mid 1980s. 1. Central Valley the total volume of fresh water 6 S - Sacramento Valley storage in the upper 1,000 feet D - Delta SJ - San Joaquin Valley of the aquifer system was esti- T - Tulare sub-basin mated at about 800 million acre- 2. Salinas Valley feet, with an estimated annual 1 3. Santa Clara Valley net-depletion rate of 800,000 acre- 4. Hollister basin 5. Sonoma-Napa basins feet. S 6. Eureka basin 7. Modoc Plateau Stratigraphy and structure. 8. Owens Valley The stratigraphic and structural Lake Sacramento setting of the fresh groundwater Tahoe basin has been conveniently sum- 5 marized by Page (1986). The val- D Mono ley is a synclinal trough that has SIERRA NEVADALake a surface area of about 20,000 San Francisco 1 square miles (Figure 1). It is bound to the west by the Coast 3 Ranges, and to the east by the Sierra Nevada, and is often SJ divided into four areas: (1) A N 4 8 northern basin drained by the Fresno Sacramento River and its tribu- 100 miles taries. (2) The delta region, where T the Sacramento, American, and 100 kilometers San Joaquin rivers join to empty 2 into Suisun Bay (and from there 1 into the bays of San Pablo and San Francisco). (3) A merid- Basin-fill aquifers Bakersfield ional basin drained by the San Joaquin River. (4) The southern- Volcanic rock aquifers most Tulare basin, which under natural conditions had internal drainage into the now vanished Tulare Lake. A good percentage of the fl uvial infl ow into the Tulare Figure 1. Main groundwater basins in Northern California (modifi ed from basin is diverted toward agricul- Planert and Williams, 1995). tural irrigation, and the balance empties into the Central Valley aqueduct network. against the metamorphic foothills of the Sierra Nevada or against the fault-bound Franciscan base- The trough is fi lled with a thick sequence of late ment of the Coast Ranges. Fresh groundwater aqui- Cretaceous to Holocene sediments, which at the axis fers are most often found in post-Eocene units, of the syncline vary in thickness from 50,000 feet and the highest yields are obtained mainly from in the north to 30,000 feet in the south. The sedi- aquifers hosted by Miocene to Holocene units such mentary section decreases in thickness toward the as the Miocene Mehrten Formation; the Pliocene/ margins of the valley and, eventually, pinches out Pleistocene Tuscan, Tehama, Laguna, Kern River, 2001 ENGINEERING GEOLOGY PRACTICE IN NORTHERN CALIFORNIA 21 and Tulare formations; and the Pleistocene/Holocene the degree of aquifer confi nement. Consequently, the Victor, Turlock Lake, Riverbank, and Modesto for- exploration hydrogeologist should not be surprised mations. The Appendix contains summary descrip- to fi nd only trivial head differences across the Corco- tions and key bibliographic references to these for- ran Clay in west Fresno County, but a couple hun- mations. dred feet difference across some of the minor clay lenses in Kings County. At depth, the freshwater Hydrogeology. When describing the aquifers of aquifer boundary is “defi ned” by salinity contents the Central Valley, it has been traditional to regard higher than 2,000 milligrams per liter (obviously an the Sacramento Valley basin as having a single arbitrary boundary, but a convenient one for defi n- unconfi ned aquifer, and the San Joaquin Valley ing the groundwater resource; Page, 1986, Plate 3). basin as having an upper unconfi ned aquifer, an The base of the fresh water aquifer lies at an aver- intervening aquitard (the Corcoran Clay), and a age depth of 3,000 feet in the southern San Joaquin lower confi ned aquifer. This simplifi ed conception is Valley, 1,000 feet in the northern San Joaquin Val- adequate for general description purposes, but Wil- ley, 200 to 2,000 feet in the Delta area, and 1,500 to liamson et al. (1989) have convincingly argued that 3,500 feet in the Sacramento Valley. the continental deposits of the Central Valley form, in fact, a single heterogeneous aquifer system, Williamson et al. (1989) reconstructed the likely in which lateral and vertical differences in confi guration of the uppermost equipotential surface hydraulic conductivity lead to local variations in of the aquifer at the turn of the century, based 400 400 RB RB RB - Red Bluff 300 200 200 300 S - Sacramento 150 500 500 St - Stockton COAST 100 COAST 80 Mo - Modesto Me - Merced F - Fresno 150 200 V - Visalia SIERRA NEVADA SIERRA NEVADA 50 B - Bakersfield S S 0 -50 0 St 50 80 St Mo Mo Me Me 50 150 100 50 50 RANGES RANGES F N 200 F 50 V 100 V 200 200 100 300 200 250 400 50 miles 300 300 B 300 B 50 kilometers 200 a b Figure 2. Elevation of the water table throughout the Central Valley. (a) Estimated confi guration of the water table at the turn of the century (modifi ed from Williamson et al., 1989). (b) Confi guration of the water table in 1997. The contours for the San Joaquin Valley are based on data from DWR (1997a); the contours on the Sacramento Valley are loosely controlled from the data of DWR (1993, 1994, and 1997b). 22 DIVISION OF MINES AND GEOLOGY BULLETIN 210 on the data and observations of Hall (1889), Bryan the Pliocene.) The basin is also subject to seawater (1915), and Mendenhall et al. (1916) (Figure 2a). intrusion in the Sacramento delta area. At that time, groundwater generally moved from recharge areas in the higher ground at the edges of Engineering geology. Shallow water table levels the basin toward its topographically lower axis, and are of concern to civil works, particularly in the from there to its discharge point at the Sacramento Sacramento delta area. For example, some portions delta (or to the secondary discharge area of Tulare of the delta are susceptible to liquefaction under Lake). Prior to the development of large-scale agri- seismic loading, largely due to the low consolidation culture and irrigation pumping, evapotranspiration of the sediments and shallow depths to the zone of along the many tules (marshes) that covered the saturation.