Scientific Investigations Report 2007–5009
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2007–5009
The U.S. Geological Survey (USGS) National Water-Quality Assessment (NAWQA) Program is studying the transport and fate of agricultural chemicals in a variety of hydrologic and agricultural settings across the nation. The agricultural chemicals studied include those associated with applied fertilizers, herbicides, and pesticides. One of these settings is in the northeastern San Joaquin Valley near Modesto, California (fig. 1). The regional-scale study area straddled the valley floor and the local-scale study area was focused on a small area along the Merced River south of Turlock (fig. 1). These study areas are hereinafter referred to as “regional” and “local.”
The purpose of this report is to summarize the hydrologic settings of the regional and local study areas and document the development and calibration of steady-state regional and local ground-water flow models. These models support future simulations and analyses of chemical mass balance, transport, and fate. Additional information on the regional hydrologic setting can be found in Burow and others (2004) and Gronberg and Kratzer (2006). The model described in this report is an updated version of the model described by Phillips and others (in press).
The San Joaquin Valley is more than 400 km long, 40 to 90 km wide, and occupies the southern two-thirds of the Central Valley of California (fig. 1). The valley floor is underlain by a structural trough filled with marine and continental sediments up to 10 km thick (Gronberg and others, 1998). The Sierra Nevada rise east of the valley to an altitude of more than 4,200 m; the Coast Ranges, having moderate altitude, form the western edge of the valley. The San Joaquin River drains the northern part of the San Joaquin Valley through San Francisco Bay; the southern part of the valley is a hydrologically closed basin.
The regional study area is about 2,700 km2 in the northeastern San Joaquin Valley, bounded on the west by the San Joaquin River, on the north near the Stanislaus River, on the south near the Merced River, and on the east by the Sierra Nevada foothills (fig. 1). Within the regional bounds is the local study area, about 17 km2, located south of Turlock along a reach of the Merced River (fig. 1).
The land surface in the regional study area slopes westward from the Sierra Nevada foothills to the San Joaquin River; gradients range from less than 1 m/km near the river to more than 5 m/km in places near the foothills and adjacent to streams and rivers, including the local study area (fig. 1). The climate is semi-arid, characterized by hot summers and mild winters; the rainfall averaged 315 mm annually from 1931 to 1997 during late fall through early spring (National Oceanic and Atmospheric Administration, 2005).
The San Joaquin River is the central drainage for the northern San Joaquin Valley and is the only major surface-water outlet from the valley. The Stanislaus, Tuolumne, and Merced Rivers drain the Sierra Nevada and are tributaries to the San Joaquin River. All rivers in the study area have been significantly modified from their natural state. Each has multiple reservoirs for irrigation and power generation, which effectively delay discharge of large amounts of snowmelt runoff. An extensive network of canals is used to deliver water for irrigation.
Agriculture is the primary land use, covering about 65 percent of the regional study area and about 80 percent of the local study area. The primary crops in the regional study area are almonds, walnuts, peaches, grapes, grain, corn, pasture, and alfalfa. Modesto, Turlock, and a number of smaller urban areas covered about 6 percent of the regional study area in 2000. The remaining area was predominantly natural vegetation near the foothills and in riparian areas (California Department of Water Resources, 2001a,b).
Agricultural irrigation supplied by surface water and ground water currently accounts for about 95 percent of the total water use in the region (Burow and others, 2004). Surface-water supplies originate primarily from a series of reservoirs in the Sierra Nevada foothills. These supplies are managed by irrigation districts and delivered to agricultural users through hundreds of kilometers of lined canals.
Most irrigation districts and private agricultural users also pump ground water for irrigation; in addition, some districts pump ground water to lower the water table in areas where it has risen too close to the land surface to support agriculture without active management. Private agricultural ground-water pumpage is not measured in the regional study area but was estimated in a water-budget analysis (Burow and others, 2004) to be about 32 percent of total agricultural water use in water year 2000 (October 1, 1999, through September 30, 2000). Ground-water use is insignificant in the local study area, where most irrigation water is delivered by canal or pumped from the Merced River.
Urban water demand is met by surface-water and ground-water supplies. Before 1995, the city of Modesto, the largest urban area in the regional study area, used ground water exclusively for public supply. A surface-water treatment plant was completed in 1994, which now provides about one-half of Modesto’s municipal and industrial water supplies. About 55 percent of urban water requirement was met by ground water in water year 2000 (Burow and others, 2004).
On the basis of information from local drillers’ logs, about 60 percent of wells in the study area are for domestic use, followed by 27 percent for irrigation, 4 percent for public supply, and 7 percent for test, stock, industrial, and other uses (Burow and others, 2004). Well depths range from 7 to 368 m below land surface; the median depth is 59 m. The median depths below land surface to the midpoints of perforated intervals in domestic, irrigation, and municipal wells are 47, 63, and 68 m, respectively. Fewer wells exist in the older sediments and terraces east of Modesto and Turlock and along the San Joaquin River. The deepest wells are in the older sediments in the eastern part of the regional study area, and the shallowest wells are in the western part and along the rivers. Additional clusters of deep wells are in the urban areas.
Clusters of monitoring wells were installed by the USGS at two primary locations in the regional study area: urban Modesto and the local study area. The wells in urban Modesto were installed as part of a USGS national assessment of the vulnerability of public-supply wells to contamination from urban, agricultural, and natural sources (Eberts and others, 2005). The Modesto-area wells used in this study for model calibration and the associated land use are shown in figure 2. There are four clusters of monitoring wells consisting of 3 or 4 wells each; mid-screen depths ranged from about 10 m near the water table to about 100 m. All well screens in the Modesto area were 1.5 m. Twelve of these wells were instrumented using pressure transducers that recorded hourly water-level measurements for periods ranging from 1 to 2.5 years.
Clusters of monitoring wells installed in the local study area as part of this study were located at three sites along a transect (fig. 3) that is roughly oriented in the direction of ground-water flow toward the Merced River. The site at the upper end of the transect (northern, up-gradient end) was in an almond orchard. The middle site was between an upslope almond orchard and a corn field (feed corn), and the site at the lower end was in native vegetation at the edge of the riparian zone (fig. 3) of the Merced River. Four to six wells at each site were used for this study; mid-screen depths ranged from about 6 m near the water table to about 27 m for those screened above the Corcoran Clay. Screen lengths for this group of wells were 0.6 m except at the water table (1.5 m). One well at the lower end of the transect was screened below the Corcoran Clay at a depth of 53 m; the screen length was 3 m. Fourteen of these transect wells were instrumented using pressure transducers that recorded hourly water-level measurements for periods ranging from 1 to 1.6 years.
U.S.
Department of the Interior | U.S. Geological Survey
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