Keywords:coastal aquifers, Colorado River, conjunctive use, drought, Groundwater Exchange, groundwater recharge, monitoring, planning and management, seawater intrusion, water supply
This study describes the complex geology of the northern Sacramento Valley, focusing on the Late Cenozoic geologic formations and structures that compose or...
This study describes the complex geology of the northern Sacramento Valley, focusing on the Late Cenozoic geologic formations and structures that compose or influence the valley’s fresh groundwater aquifer formations. The California Department of Water Resources (DWR) acquired geologic data from groundwater observation well drilling operations that were conducted in the valley over the last 15 years. Using the observation well drilling data, DWR evaluated and classified the lithology of the subsurface sediments, implemented petrographic sand provenance analyses on lithologic sediment samples, and reviewed associated geophysical logs from each bore hole. In addition, DWR conducted an extensive literature review of published and unpublished data and then integrated the data to produce this geologic report, map, and cross sections that describe the geology of the northern Sacramento Valley.
Results from the lithologic logging, petrographic analyses, and data review show that the heterogeneous sediments of the northern Sacramento Valley’s most productive groundwater-bearing geologic formations, the Tehama Formation and the Tuscan Formation, intermix in the subsurface in various areas near the center of the valley. The results also show that toward the westward and eastward extents of the valley, the sediments of the formations become more unified in composition due to the proximity of their respective sediment source areas. However, because of the depositional environment of the geologic formations, sediment sizes within the formations can be discontinuous and intermittent in places, resulting in variable groundwater aquifer zones within the geologic formations.
Additional data are needed to further define the northern Sacramento Valley aquifer system. Drilling and installing groundwater observation wells in areas of little or no data can provide the information needed to determine the extent and variability of the valley’s groundwater aquifers.
Groundwater level data supplied by the observation wells can provide valuable information for monitoring aquifer conditions, for determining the change in groundwater levels over time, and for assessing the ability of groundwater to move through the geologic aquifer sediments. In addition, a textural analysis of formational sediments using lithologic cuttings and/or driller’s well logs could be performed to better identify aquifer production zones.
In summary, the geology of the northern Sacramento Valley is diverse and has a widely varied historical sequence of earth-shaping events. It includes periods of time when much of the area was below sea level, multiple and distinct periods of volcanic activity, several periods of mountain building, and intermingled periods of massive erosion and deposition. Analyses of the data illustrate the heterogeneity of the groundwater-bearing geologic formations in the subsurface, and the intermixing of formational sediments toward the center of the northern Sacramento Valley, resulting in a region with great geologic and hydrogeologic complexity.
Senate Bill X7 6 (SBX7 6) (Chapter 1, Statutes 2009) added provisions for Groundwater Monitoring to Division 6 of the Water Code (Water...
Senate Bill X7 6 (SBX7 6) (Chapter 1, Statutes 2009) added provisions for Groundwater Monitoring to Division 6 of the Water Code (Water Code § 10920 et seq.), and authorizes the Department of Water Resources (DWR) to establish permanent, locally managed, groundwater-elevation monitoring and reporting in all of California's 515 alluvial groundwater basins.
To implement SBX7 6, DWR developed the California Statewide Groundwater Elevation Monitoring (CASGEM) program. SBX7 6 requires DWR to report to the Governor and the Legislature by January 1, 2012, and thereafter in years ending in "5" or "0" regarding the findings of this program.
The purpose of CASGEM is to establish a program of regular and systematic monitoring of groundwater elevations and to track seasonal and long-term trends in groundwater elevations statewide. The law directs DWR to rely and build upon the many established, local, long-term groundwater monitoring and management programs conducted by local entities throughout the state. DWR's role is to coordinate the CASGEM program, to work cooperatively with local entities, and to maintain the submitted groundwater elevation data in a manner that is readily and widely available to the public. Collection and evaluation of groundwater elevation data throughout the state is an important fundamental step toward improving management of California's groundwater resources.
Within the first two years of program development, DWR has met the requirements specified in SBX7 6 to establish a statewide groundwater elevation monitoring and reporting program by January 1, 2012.
The Sacramento River Hydrologic Region (see Figure SR-1 includes the entire California drainage area of the Sacramento River (the state’s largest river) and...
The Sacramento River Hydrologic Region (see Figure SR-1 includes the entire California drainage area of the Sacramento River (the state’s largest river) and its tributaries. The region extends from Chipps Island in Solano County north to Goose Lake in Modoc County. It is bounded by the Sierra Nevada on the east, the Coast Ranges on the west, the Cascade and Trinity mountains on the north, and the Sacramento-San Joaquin River Delta (Delta) on the south. The Sacramento River Basin actually begins in Oregon, north of Goose Lake, a near-sink that intercepts the Pit River drainage at the California-Oregon border.
Some key issues for this region are summarized here and discussed further later in this report.
Agriculture. Between 2005 and 2010, the region supported about 1.95 million acres of irrigated agriculture on average. Approximately 1.58 million acres is irrigated on the valley floor. The surrounding mountain valleys add about 370,000 irrigated acres to the region’s total — primarily as pasture and alfalfa. The gross value of agricultural production in the Sacramento Valley for 2011 was about $4.1 billion (California Department of Food and Agriculture 2013). Rice and walnuts are the highest grossing crops in the region followed by almonds and tomatoes. The direct, indirect, and induced effects of the agricultural industry to the regional economy are discussed in this report.
Groundwater. With a 2005-2010 average annual extraction volume of 2.7 million acre-feet (maf), groundwater pumping in the Sacramento River Hydrologic Region accounts for 17 percent of all the groundwater extraction in California — the third highest among the 10 hydrologic regions in California, behind Tulare Lake Hydrologic Region with 38 percent and San Joaquin River Hydrologic Region with 19 percent of the total. Overall, groundwater contributes to about 31 percent of the total water supply. Most groundwater extraction in the region occurs for agricultural water use (2.4 maf), meeting about one-third of agricultural water demands. Groundwater extraction for urban water use is significantly less (465 thousand acre-feet [taf]), which meets about half of the urban water needs. Groundwater levels for much of the region have declined from 2005 to 2010. Groundwater level declines ranging from 20 to 30 feet are seen in the northwestern portion of the Sacramento Valley Groundwater Basin. Declines ranging from to 10 to 20 feet are seen in the northern, the mid- to south-western, and the southeastern portions of the valley. For the rest of the Sacramento Valley Groundwater Basin and the Redding Area Groundwater Basin, groundwater level declines have
ranged from zero to 10 feet.
Flood. Exposure to a 500-year flood event in the region threatens approximately one in three residents, almost $65 billion in assets (crops, buildings, and public infrastructure), 1.2 million acres of agricultural land, and over 340 sensitive species. Almost 95 percent of Sutter County residents, more than 55 percent of Yuba County and Yolo County residents, and more than 50 percent of agricultural land region-wide are exposed to the 500-year flood event.
Climate Change. Several different climate regions overlie portions of the Sacramento River Hydrologic Region. Air temperature data collected for the past century has been summarized by the Western Regional Climate Center (WRCC) for the different regions which are outlined below.
The region also is currently experiencing impacts from climate change through changes in statewide precipitation and surface runoff volumes, which in turn affect availability of local and imported water supplies. During the last century, the average early snowpack in the Sierra Nevada decreased by about 10 percent, which equates to a loss of 1.5 maf of snowpack storage (California Department of Water Resources 2008). Projections and impacts based on modeling of climate change are included in this report.
Section 229 of the Water Code directs that the California Department of Public Works, acting by and through the State Engineer, shall "investigate...
Section 229 of the Water Code directs that the California Department of Public Works, acting by and through the State Engineer, shall "investigate conditions of the quality of all the waters within the State, including saline waters, coastal and inland, as related to all sources of pollution of whatever nature and shall report thereon to the Legislature and to the appropriate regional water pollution control board annually, and may recommend any steps which might be taken to improve or protect the quality of such waters."
In order to carry out the intent of Section 228 of the Water Code with respect to investigations of quality of ground waters within the State, it has been necessary first to compile available geologic data in order to locale and define the approximate boundaries of the more important ground water basins.
A base index map showing the principal areas of groundwater storage in the State of California has not been previously prepared. Such a map has been complied for this report in order to establish a uniform name and numbering system for groundwater basins, which can be expanded as new areas of ground water storage are identified, or as it is found necessary to divide the larger areas into subbasins. It will serve as a basis for the planning of future investigations of the groundwater resources of California.
This report identifies alluvial or valley fill areas which contain the principal groundwater resources in California. However, the report is necessarily not complete because of lack of information for many areas of the state.
In general, the areas of groundwater storage indentified include: (a) the major alluvium-filled areas of known groundwater storage and extraction; (b) the extensive areas of alluvial-fill in the Colorado, Mojave, and Basin and Range desert areas which may contain usable groundwater, though little is known of their storage capacity or recharge; and (c) some of the smaller alluvium-filled areas which may furnish a portion of local domestic, irrigation, municipal, and industrial water supplies.