Justin Brandt

Land subsidence and recovery in the Albuquerque Basin, New Mexico, 1993–2014

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Land subsidence in the southwestern Mojave Desert, California, 1992–2009

In cooperation with the Mojave Water Agency (MWA), the U.S. Geological Survey (USGS) has been monitoring land subsidence in the southwestern Mojave Desert of California using satellite Interferometric Synthetic Aperture Radar (InSAR) combined with ground-based techniques. Maps of land subsidence constructed from the InSAR data have proven to be an economical means to evaluate subsidence—with the goal of identifying small problems before they become large ones. The maps of subsidence over the considerably large (nearly 5,000 square miles [mi2]) MWA management area (fig. 1) enabled researchers to detect small magnitude, localized areas of subsidence near five lakebeds.

Land subsidence, groundwater levels, and geology in the Coachella Valley, California, 1993-2010

Land subsidence associated with groundwater-level declines has been investigated by the U.S. Geological Survey in the Coachella Valley, California, since 1996. Groundwater has been a major source of agricultural, municipal, and domestic supply in the valley since the early 1920s. Pumping of groundwater resulted in water-level declines as much as 15 meters (50 feet) through the late 1940s. In 1949, the importation of Colorado River water to the southern Coachella Valley began, resulting in a reduction in groundwater pumping and a recovery of water levels during the 1950s through the 1970s. Since the late 1970s, demand for water in the valley has exceeded deliveries of imported surface water, resulting in increased pumping and associated groundwater-level declines and, consequently, an increase in the potential for land subsidence caused by aquifer-system compaction. Global Positioning System (GPS) surveying and Interferometric Synthetic Aperture Radar (InSAR) methods were used to determine the location, extent, and magnitude of the vertical land-surface changes in the southern Coachella Valley during 1993–2010. The GPS measurements taken at 11 geodetic monuments in 1996 and in 2010 in the southern Coachella Valley indicated that the elevation of the land surface changed –136 to –23 millimeters (mm) ±54 mm (–0.45 to –0.08 feet (ft) ±0.18 ft) during the 14-year period. Changes at 6 of the 11 monuments exceeded the maximum expected uncertainty of ±54 mm (±0.18 ft) at the 95-percent confidence level, indicating that subsidence occurred at these monuments between June 1996 and August 2010. GPS measurements taken at 17 geodetic monuments in 2005 and 2010 indicated that the elevation of the land surface changed –256 to +16 mm ±28 mm (–0.84 to +0.05 ft ±0.09 ft) during the 5-year period. Changes at 5 of the 17 monuments exceeded the maximum expected uncertainty of ±28 mm (±0.09 ft) at the 95-percent confidence level, indicating that subsidence occurred at these monuments between August 2005 and August 2010. At each of these five monuments, subsidence rates were about the same between 2005 and 2010 as between 2000 and 2005. InSAR measurements taken between June 27, 1995, and September 19, 2010, indicated that the land surface subsided from about 220 to 600 mm (0.72 to 1.97 ft) in three areas of the Coachella Valley: near Palm Desert, Indian Wells, and La Quinta. In Palm Desert, the average subsidence rates increased from about 39 millimeters per year (mm/yr), or 0.13 foot per year (ft/yr), during 1995–2000 to about 45 mm/yr (0.15 ft/yr) during 2003–10. In Indian Wells, average subsidence rates for two subsidence maxima were fairly steady at about 34 and 26 mm/yr (0.11 and 0.09 ft/yr) during both periods; for the third maxima, average subsidence rates increased from about 14 to 19 mm/yr (0.05 to 0.06 ft/yr) from the first to the second period. In La Quinta, average subsidence rates for five selected locations ranged from about 17 to 37 mm/yr (0.06 to 0.12 ft/yr) during 1995–2000; three of the locations had similar rates during 2003–mid-2009, while the other two locations had increased subsidence rates. Decreased subsidence rates were calculated throughout the La Quinta subsidence area during mid-2009–10, however, and uplift was observed during 2010 near the southern extent of this area. Water-level measurements taken at wells near the subsiding monuments and in the three subsiding areas shown by InSAR generally indicated that the water levels fluctuated seasonally and declined annually from the early 1990s, or earlier, to 2010; some water levels in 2010 were at the lowest levels in their recorded histories. An exception to annually declining water levels in and near subsiding areas was observed beginning in mid-2009 in the La Quinta subsidence area, where recovering water levels coincided with increased recharge operations at the Thomas E. Levy Recharge Facility; decreased pumpage also could cause groundwater levels to recover. Subsidence concomitant with declining water levels and land-surface uplift concomitant with recovering water levels indicate that aquifer-system compaction could be causing subsidence. If the stresses imposed by the historically lowest water levels exceeded the preconsolidation stress, the aquifer-system compaction and associated land subsidence could be permanent. Land Subsidence, Groundwater Levels, and Geology in the Coachella Valley, California, 1993–2010 By Michelle Sneed, Justin T. Brandt, and Mike Solt 2 Land Subsidence, Groundwater Levels, and Geology in the Coachella Valley, California, 1993–2010 Introduction Groundwater has been a major source of agricultural, municipal, and domestic water supply in Coachella Valley, California (fig. 1), since the early 1920s. Pumping of groundwater resulted in water-level declines as much as 15 meters (m), or 50 feet (ft), between the early 1920s and late 1940s. In 1949, the importation of Colorado River water through the Coachella Canal, a branch of the All-American Canal, to the southern Coachella Valley began. As a result of the importation of surface water, pumping of groundwater decreased in the southern Coachella Valley during the 1950s through the 1970s, and water levels in some wells in the lower valley recovered as much as 15 m (50 ft). Since the late 1970s, however, the demand for water in the southern Coachella Valley has exceeded the deliveries of imported surface water, pumping has increased, and water levels have again declined. By 2010, water levels in many wells in the southern Coachella Valley had declined 15–30 m (50–100 ft), and water levels in some wells were at their lowest recorded levels. The Coachella Valley Water District (CVWD) is currently involved in several agreements and projects including the Quantification Settlement Agreement, tiered-rate structures, aquifer-recharge projects, and conversion from groundwater to surface water resources for (primarily) golf course irrigation through the Mid-Valley Pipeline Project, that could reduce reliance on the groundwater resource (Coachella Valley Water District, 2012). Continued monitoring could track the effect these agreements and projects have on groundwater levels. Declining water levels can contribute to or induce land subsidence in aquifer systems that consist of a substantial fraction of unconsolidated fine-grained sediments (silts and clays). Ikehara and others (1997) reported as much as 150 millimeters (mm) ±90 mm (0.5 ft ±0.3 ft) of subsidence in the southern parts of the Coachella Valley between 1930 and 1996. Land subsidence can disrupt surface drainage and watersupply or flood-control conveyances; cause earth fissures; and damage wells, buildings, roads, and utility infrastructure. A large earth fissure was discovered in 1948 about 3 kilometers (km), or 2 miles (mi), north of Lake Cahuilla in La Quinta (unpublished field notes, Coachella Valley Water District, 1948). Because subsidence had not been documented in the southern parts of the Coachella Valley prior to the report by Ikehara and others (1997), it is not known if this fissure formed in response to differential land subsidence during the earlier period (early 1920s–late 1940s) of groundwater-level declines. However, fissuring has recurred in this area (Clay Stevens, TerraPacific Consultants, Inc., written commun., 2006). Subsidence-related earth fissures and reactivated surface faults have been identified in many other groundwater basins in the western United States (Holzer, 1984). The CVWD works cooperatively with local stakeholders to manage the water supply for a large part of the Coachella Valley (fig. 1). Because of the potential for groundwater pumping to cause land subsidence, the CVWD entered into a cooperative agreement with the U.S. Geological Survey (USGS) to monitor vertical changes in land surface to determine if land was subsiding in the Coachella Valley. In 1996, the USGS established a geodetic network of monuments to monitor vertical changes in land surface in the southern Coachella Valley by using Global Positioning System (GPS) surveys and to establish baseline values for comparisons with results of future surveys. This geodetic network can be surveyed periodically to determine the distribution and amount of land subsidence. Interferometric Synthetic Aperture Radar (InSAR) data collected since 1993 were used to detect and quantify land subsidence in areas distant from the geodetic monuments. Purpose and Scope The objectives of this study were to detect and quantify land subsidence in the southern Coachella Valley from 1993 through 2010 by completing GPS surveys at the established geodetic network of monuments and by using InSAR data. For purposes of this report, the southern Coachella Valley represents the southern half of the Coachella Valley, which extends from the communities of Palm Desert, Indian Wells, Indio, and La Quinta on the north to the Salton Sea on the south (fig. 1). This report presents the results and interpretations of GPS data collected at the monuments in the monitoring network during surveys in 1996, 1998, 2000, 2005, and 2010 and also of spatially detailed maps of vertical land-surface changes generated by using InSAR data collected between 1993 and 2010. The InSAR-generated maps extend from near Palm Desert to near the Salton Sea (fig. 1). Data showing groundwater-level changes from the early to mid1990s to 2010 were examined and compared with the GPS measurements and the InSAR-generated maps to determine if the vertical changes in land surface could be related to the changes in groundwater levels.