Delta-Mendota Canal: Evaluation of Groundwater Conditions and Land Subsidence

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In areas adjacent to the Delta-Mendota Canal (DMC), extensive groundwater withdrawal from the San Joaquin Valley aquifer system has caused areas of the ground to sink as much as 10 feet, a process known as land subsidence. This could result in serious operational and structural issues for the Delta-Mendota Canal (DMC). In response, the USGS is studying and providing information on groundwater conditions and land subsidence in the San Joaquin Valley.

Map of land subsidence in the San Joaquin Valley form 1926-1970, shaded by amount of subsidence in meters

The extensive withdrawal of groundwater from the unconsolidated deposits of the San Joaquin Valley has caused widespread land subsidence—locally exceeding 8.5 meters (m) between 1926 and 1970 (Poland and others, 1975; fig. 2), and reaching 9 m by 1981 (Ireland, 1986). Long-term groundwater-level declines can result in a vast one-time release of “water of compaction” from compacting silt and clay layers (aquitards), which causes land subsidence (Galloway and others, 1999). Land subsidence from groundwater pumping began in the mid-1920s (Poland and others, 1975; Bertoldi and others, 1991; Galloway and Riley, 1999), and by 1970, about half of the San Joaquin Valley, or about 13,500 km2 had land subsidence of more than 0.3 m (Poland and others, 1975; fig. 2). (Public domain.)

Regional Setting and Historical Context

Groundwater pumping caused widespread compaction and resultant land subsidence between 1926 and 1970, locally exceeding 26 feet. Surface-water imports in the early 1970s resulted in decreased pumping, reduced compaction rate, and a steady recovery of groundwater levels. However, lack of imported surface-water availability during 1976-77, 1986-92, and 2007-09 caused groundwater-pumping increases, renewed compaction, and declines in water-levels to near-historic lows. The resulting land subsidence has reduced the freeboard and flow capacity of the Delta-Mendota Canal, the California Aqueduct, and other canals that transport floodwater and deliver irrigation water , requiring expensive repairs.

One area of the Central Valley, southwest of Mendota, has experienced some of the highest levels of subsidence in California. From 1925 to 1977, this area suffered over 29 feet of subsidence.

The Delta-Mendota Canal

The canal begins at the C.W. Bill Jones Pumping Plant, which pumps water 197 feet from the Sacramento-San Joaquin Delta. The canal runs south along the western edge of the San Joaquin Valley, parallel to the California Aqueduct for most of its journey, but it diverges to the east after passing San Luis Reservoir, which receives some of its water. The water is pumped from the canal and into O'Neill Forebay. Then it is pumped into San Luis Reservoir by the Gianelli Pumping-Generating Plant. Occasionally, water from O'Neill Forebay is released into the canal. The Delta-Mendota Canal ends at Mendota Pool, on the San Joaquin River near the town of Mendota, 30 miles west of Fresno.

Study Objectives

  • improve the understanding of groundwater conditions and land subsidence and how groundwater resources have changed over time
  • determine the location, extent, and magnitude of changes in land-surface elevation along the DMC for 2003-10 using persistent scatterer Interferometric Synthetic Aperture Radar (InSAR) methods
  • develop and implement an approach to use persistent scatterer InSAR to monitor subsidence along the DMC
  • develop groundwater flow and land-subsidence simulations to provide stakeholders with information to help manage and limit future land subsidence along the DMC

Evaluation of Land Subsidence along the Delta-Mendota Canal

The U.S. Geological Survey, in cooperation with the U.S. Bureau of Reclamation and the San Luis and Delta-Mendota Water Authority, assessed land subsidence in the vicinity of the Delta-Mendota Canal as part of an effort to minimize future subsidence-related damages to the canal. The location, magnitude, and stress regime of land-surface deformation during 2003-10 were determined by using extensometer, Global Positioning System (GPS), Interferometric Synthetic Aperture Radar (InSAR), spirit leveling, and groundwater-level data. Comparison of continuous GPS, shallow extensometer, and groundwater-level data, combined with results from a one-dimensional model, indicated the vast majority of the compaction took place beneath the Corcoran Clay, the primary subsurface regional confining unit.

Interferometric Synthetic Aperture Radar (InSAR)

For the DMC study, scientists use persistent scatterer InSAR to better understand land surface movement. This technique requires 20 or more satellite images taken at different times. The images are then processed to reveal relative ground-elevation change over the time the images were taken. The processed images are displayed as maps, called interferograms, that scientists interpret to see land elevation change along the DMC. The InSAR data is then combined with available historical spirit leveling, extensometer, and GPS data to produce information that describes land subsidence and aquifer-system compaction over a period of time.

The interpreted persistent scatterer InSAR data will be used to create maps of changes in land-surface elevations along the DMC. The InSAR data will be combined with available historical leveling, extensometer, and GPS data to produce a time series of land subsidence and aquifer-system compaction.

Continuous Global Positioning System (CGPS) Stations

A CGPS station continuously measures the three-dimensional (3D) position of a point on, or more specifically, near the earth's surface. For this study, scientists are primarily interested in vertical movement (subsidence and uplift), but horizontal movement can be obtained with CGPS and can also be informative for subsidence studies.

This study will use data (courtesy of EarthScope Plate Boundary Observatory with funding from the National Science Foundation) from several CGPS sites near the Delta-Mendota Canal:

GPS stations generally collect point position information every 15 seconds. To make this data useful for analysis, they are processed to produce a daily point position, which is then concatenated to track the 3D position over time (months and years)(for example, P303).

Extensometers

An extensometer measures the compaction and expansion of the aquifer system to a specified depth. A number of extensometers were constructed in the 1950s, 1960s, and 1970s near the DMC and California Aqueduct, but some had fallen into disrepair. For this study, four were refurbished in 2012. USGS personnel have visited these sites to download data, to make manual dial gauge and water-level measurements for quality control, and to adjust equipment.

For the most updated compaction and groundwater-level data for the 4 refurbished extensometers mentioned above, please visit the USGS National Water Information System (NWIS):

12S/12E-16H2

14S/13E-11D6

18S/16E-33A1

20S/18E-6D1

Piezometers

A piezometer is a specialized well used to measure water levels at specific depths. When extensometer or CGPS data are paired with groundwater level data from a nearby well, some storage properties of the affected aquifer system can be calculated. During winter and spring of 2010, The USGS constructed 3 piezometers near CGPS station P259 and 3 piezometers near CGPS station Althea. All of these piezometers were instrumented with submersible pressure transducers to measure hourly groundwater levels. Additionally, 2 wells near CGPS station P304 were instrumented to measure hourly groundwater levels. The continuous GPS data and water-level data is used for stress-strain analysis. If water levels fluctuate in the elastic range of stress, the elastic skeletal storage coefficient will be computed. The elastic skeletal storage coefficient is a standard measure of aquifer storage owing to the compressibility of the aquifer-system skeleton and largely governs the recoverable (reversible) deformation of the aquifer system. If water levels continue to decline beyond historically low levels (the inelastic range of stress), it may be possible to compute the inelastic storage coefficient that governs the permanent compaction of the aquifer system. If water levels are fluctuating in both ranges of stress (fluctuating seasonally and declining annually), both the elastic and inelastic storage coefficients could be estimated.

For the most updated groundwater-level data for the 2 wells near Station P304, please visit the USGS National Water Information System (NWIS):

Althea (3 wells):

P259 (3 wells):

 

Map of the San Joaquin Valley displaying contours of land subsidence that occurred from 2008 to 2010.

Land subsidence contours showing vertical changes in land surface in the central San Joaquin Valley area, California, during January 8, 2008-January 13, 2010. The top graph illustrates elevation changes computed from repeat geodetic surveys along Highway 152 for 1972-2004. The bottom graph depicts elevation changes computed from repeat geodetic surveys along the Delta-Mendota Canal for 1935-2001. Subsidence data along Highway 152 were computed from published National Geodetic Survey elevations. Subsidence graph for the Delta-Mendota Canal was obtained from the San Luis and Delta-Mendota Water Authority and the Central California Irrigation District. (Public domain.)