Scientific Investigations Report 2007–5179
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2007–5179
The NAWQA program focuses sampling efforts into geographically defined areas called “study units.” A study unit covers about 3,850 mi2 or more and is based on its ground- and surface-water features (Gilliom and others, 1995). Data collected as part of nationally designed NAWQA land-use studies from seven study units (fig. 1) were used in this investigation: Central Arizona Basins (CAZB), Great Salt Lake Basins (GRSL), Nevada Basin and Range (NVBR), Rio Grande Valley (RIOG), Sacramento River Basin (SACR), San Joaquin-Tulare Basins (SANJ), and Southern California Basins (SOCA). These areas generally are classified as arid to semiarid, with average annual temperatures ranging from 23 to 86°F, precipitation ranging from 4.5 to 17.9 in/yr, and estimated evaporation rates ranging from 50 to 110 in/yr (table 1). Although there is some variability, ground water within the regional area typically is a calcium-bicarbonate type likely derived from the aquifer materials through which the waters flow (table 2). Median alkalinity among the study units ranges from 171 to 495 mg/L as calcium carbonate. Median specific-conductance values within the study units ranged from 503 to 5,140 µS/cm. The range in median pH is relatively small and circumneutral (7.0 to 7.5). The number of wells sampled varied among study units, ranging from 9 to 110, with median well depths ranging from 20 to 129 ft below land surface (table 2). The general location of wells sampled in each study unit is shown in figure 2.
Water was sampled from nine wells in agricultural land-use areas in the CAZB that were located in the western part of the West Salt River Valley (fig. 2). The principle crops are citrus, cotton, alfalfa, and grains. In 1995, the population of the West Salt River Valley was estimated to be about 1.4 million (Hitt, 1994), most of which was concentrated in Phoenix and its eastern suburbs. In 1995, about 252 mi2 of land in the valley was agricultural and 285 mi2 was urban, which corresponded to 19.0 and 21.5 percent of the total land area, respectively (Edmonds and Gellenbeck, 2002).
The West Salt River Valley is a sediment-filled basin of about 1,300 mi2, bordered by desert mountain ranges with altitudes ranging from about 800 to 4,500 ft along the Gila River in the western part of the valley (Edmonds and Gellenbeck, 2002). Sediments tend to be coarse grained near the mountain fronts and fine grained toward the center of the basin. The basin fill consists of alluvial deposits with lenticular beds of clay, silt, sand, and gravel that cannot be traced laterally for long distances (Edmonds and Gellenbeck, 2002). Ground water occurs under unconfined and semiconfined conditions in the basin-fill aquifer. Semiconfined conditions occur locally where clay and silt beds form a confining layer. The agricultural wells from which samples were collected are perforated above these confining beds. Stream deposits overlie the basin fill along and beneath present river channels, forming the most productive and important aquifer in the West Salt River Valley.
The West Salt River Valley aquifer is recharged by the infiltration of runoff along the mountain fronts, major streams flowing through the valley, urban runoff, and irrigation drainage. Precipitation is not a significant source of recharge to the aquifer system (Anderson and others, 1992). Treated effluent originating from Phoenix has been used in agricultural irrigation since the early 1960s. Since 1985, Colorado River water has been used to supplement public and irrigation supplies and can be stored in the basin-fill aquifer (Edmonds and Gellenbeck, 2002). Ground-water discharge is primarily from pumpage, seepage to the Gila River at the southwestern edge of the study area, transpiration by phyreatophytes growing on flood plains, and underflow out of the valley (Anderson and others, 1992).
The 30 wells included from the GRSL urban study unit are in northern Utah (fig. 2) within Salt Lake Valley, southeast of Great Salt Lake. This urbanized valley covers about 400 mi2 and includes the Salt Lake City metropolitan area. In 2000, the population in Salt Lake County was estimated at 898,000 and was growing rapidly, almost doubling between 1963 and 1994 and projected to be about 1,233,000 in 2020 (Thiros, 2003a). Because the natural boundaries of the valley restrict expansion of residential areas, population growth will occur primarily through increased population density. During 1960–94, agricultural land in the valley decreased from 145 to 67 mi2 and the amount of urban land increased from 89 to 198 mi2 (Thiros, 2003a). In 1994, urban land use in the valley was approximately 63 percent residential, 6 percent commercial, 14 percent industrial, and 17 percent other (includes open and idle spaces, transportation, and utilities (Thiros, 2003a)). Approximately 33 percent of the public-water supply in Salt Lake Valley is ground water. Shallow ground-water resources are used sparingly as stock and domestic supplies (Thiros, 2003a).
Topographic relief between the Wasatch Range and Salt Lake Valley is as much as 7,000 ft. The basin-fill deposits in the valley consist of unconsolidated to semiconsolidated sand, gravel, silt, and clay that were deposited in alluvial fans and as volcanic ash (Hely and others, 1971; Thiros and Manning, 2004). The basin-fill ground-water system in Salt Lake Valley includes a shallow aquifer and a deeper aquifer that are separated by layers of fine-grained sediment. In some areas in the valley, shallow ground water is perched on fine-grained deposits, whereas in other areas it is laterally continuous and forms an aquifer (Thiros, 2003b). The shallow aquifer in the valley generally is unconfined and present within the upper 50 ft of the basin-fill deposits (Thiros, 2003b). Nearly all wells in the valley are screened within sandy deposits that occur throughout the valley, except near the mountain fronts where coarser-grained deposits exist.
Sources of water to the aquifer system within Salt Lake Valley include mountain-front recharge, infiltration of precipitation, and recharge of irrigation water and seepage from streams (Thiros, 2003a; Thiros, 2003b). The shallow aquifer also receives inflow from the deeper aquifer in areas of the valley where this system discharges (Thiros, 2003b). Generally, ground water is discharged from areas in the center of the valley along the Jordan River, and by evapotranspiration (Thiros, 2003a; 2003b). In these discharge areas, water moves upward from the principal aquifer; thus, there is little potential for contamination of the deep aquifer from the shallow aquifer unless pumpage is great enough to reverse the vertical gradient or a contaminant has a greater density than water.
The 61 agricultural and 81 urban land-use wells from the NVBR study unit are within the watersheds of the Carson and Truckee Rivers and within Las Vegas Valley (fig. 2). The Carson and Truckee River basins are in northwestern Nevada and northeastern California and cover approximately 3,970 and 3,230 mi2, respectively. Las Vegas Valley, located in southern Nevada, encompasses approximately 1,640 mi2 (Covay and others, 1996; Kilroy and others, 1997). In 2000, the Truckee and Carson River basins contained populations of approximately 341,000 and 153,000 people, respectively (Nevada State Demographer’s Office, 2006). In the Truckee River basin, the estimated population in the Reno/Sparks metropolitan area increased 55 percent from 1990 to 2005 (Nevada State Demographer’s Office, 2006). Carson City showed a population increase of approximately 39 percent during this same period. The Las Vegas area is the most urbanized in the NVBR. In 2000, the population of Las Vegas was approximately 480,000 people. The estimated growth rate of Las Vegas was 112 percent from 1990 to 2005 (Nevada State Demographer’s Office, 2006). Most irrigated agriculture in the NVBR study unit is located within the Carson River basin, which covers approximately 180 mi2 (Covay and others, 1996; Welch, 1994). The primary crops in the Truckee and Carson River basins are alfalfa, hay, grains, and onions (Nevada Department of Agriculture, 2007). Agricultural and rangeland areas in the Truckee and Carson River basins have become increasingly urbanized over time (Covay and others, 1996). Irrigation is the dominant water use in the Truckee (59 percent) and Carson River (95 percent) basins. Most irrigation water is obtained from the Truckee and Carson Rivers. Ground-water resources are used to supplement surface-water supplies in the Truckee and Carson River basins (Covay and others, 1996). Prior to 1971, ground water was the main source of water supply for the Las Vegas Valley area. In 1990, it was estimated that approximately 80 percent of the water in Las Vegas Valley originates from Lake Mead (Covay and others, 1996).
The Truckee and Carson River basins are located on the eastern side of the Sierra Nevada, where maximum altitudes reach about 10,900 ft (Covay and others, 1996), and extend eastward to Pyramid Lake and Carson Sink, respectively. The Carson and Truckee River basins each contain several ground-water basins that are primarily composed of unconsolidated deposits (Covay and others, 1996) of coarse sand and gravel intermixed with layers of clay. The headwaters of the Las Vegas Valley Hydrographic Area are located in the Spring Mountains to the west, where altitudes reach about 11,900 ft. Lake Mead lies to the southeast at an altitude of about 1,200 ft (Covay and others, 1996). Las Vegas Valley consists of a single ground-water basin composed of unconsolidated to semiconsolidated gravel, sand, and clay deposits underlain by consolidated-carbonate aquifers.
The Truckee and Carson River ground-water basins are hydraulically connected to the Truckee and Carson Rivers, respectively, which are sources of water to shallow aquifers. Aquifer recharge occurs primarily in mountain-front areas. However, shallow aquifers within agricultural areas also can be recharged by infiltration of irrigation water. Irrigation water in Carson Valley originates from surface-water diversions from the Carson and Truckee Rivers and from ground-water pumpage (Welch, 1994). Additionally, constructed wetlands in the Carson River basin store treated-sewage effluent from the Lake Tahoe Basin for use in irrigation (Lico, 1998). The shallow perched aquifers within Las Vegas Valley are influenced largely by irrigation return flow from urban landscapes, whereas the deeper aquifer is mostly confined beneath a thick layer of clay (Covay and others, 1996). As part of an artificial-recharge program, almost 30,000 acre-ft of surface water imported from the Colorado River was injected into the principal aquifer beneath Las Vegas Valley (Lopes and Evetts, 2004). Ground-water discharge is primarily by pumpage; however, a considerable amount of discharge occurs through evapotranspiration in many areas.
Both agricultural and urban wells from Rio Grande Valley in the Colorado, New Mexico, and Texas (RIOG) study unit were included in this investigation. These wells are located within the urban area of the Albuquerque Basin (24 monitoring wells) and the agricultural areas of San Luis Valley in south-central Colorado (35 monitoring wells) and Rincon Valley, south-central New Mexico (30 wells) (Anderholm, 1996; 1997; 2002) (figs. 1 and 2). The Albuquerque metropolitan area is the largest population center in Rio Grande Valley with an estimated population of 500,000 in 1990 (Anderholm, 1997). Land use in Albuquerque is approximately 50 percent urban (residential, commercial, and recreational) with intermixed agriculture (Anderholm, 1997). Agricultural lands in the Albuquerque area have been steadily becoming more urbanized (Thorn and others, 1993). San Luis Valley is dominated by the cultivation of alfalfa, native hay, barley, wheat, potatoes, and other vegetable crops (Anderholm, 1996). Rincon Valley is primarily agricultural but also includes residential, urban, vacant, roadways and open-water areas. The main agriculture crops in Rincon Valley include alfalfa, peppers, onions, wheat, cotton, and pecan orchards (Anderholm, 2002). Ground water is used as a potable water source in the Albuquerque area (Anderholm, 1997). Both ground- and surface-water resources are used for irrigation in agricultural areas in the RIOG (Anderholm, 1996; 2002).
The Albuquerque Basin contains basin-fill deposits as thick as 14,000 ft and is bounded by mountainous areas along the north, east, and south margins and by an area of low topographic relief along the west. The aquifer materials are composed of gravel, sand, silt, and clay and have large variations in hydraulic conductivity as a result of grain size and sorting (Anderholm, 1997). San Luis Valley covers approximately 7,500 mi2, has an average altitude of 7,700 ft, and is bounded by mountains with several peaks over 14,000 ft (Anderholm, 1996). San Luis Valley consists of thick basin-fill deposits interbedded with clay, silt, gravel, and volcanic rocks (Burroughs, 1981; Emery and others, 1973). The two main aquifers in this valley are the confined and unconfined, separated by a confining zone approximately 60–100 ft below land surface that underlies a large part of the center of the valley (Emery and others, 1973; Anderholm, 1996). Rincon Valley in south-central New Mexico covers approximately 76 mi2, is relatively flat, and is bordered by steep bluffs 50–100 ft high (Anderholm, 2002). The shallow aquifer beneath Rincon Valley is within the upper basin-fill deposits. These upper deposits generally are less than 80 ft thick and consist of gravel, sand, silt, and clay-sized sediment (King and others, 1971; Anderholm, 2002).
The main sources of recharge to the shallow aquifer in the Albuquerque Basin are infiltration of surface water, sewage effluent, and precipitation. The main sources of discharge are ground-water outflow to adjacent areas, ground-water withdrawals, and evapotranspiration (Anderholm, 1997). Ground-water movement within the Albuquerque Basin is dominated by cones of depression in the water table resulting from ground-water withdrawals. Ground-water levels have declined as much as 160 ft east of Albuquerque, resulting in reversals in ground-water flow direction (Anderholm, 1997). Within San Luis Valley, recharge to the unconfined aquifer comes from infiltration of precipitation, surface-water infiltration, irrigation seepage, and inflow of ground water from adjacent areas and deeper aquifers (Powell, 1958). Since 1990, irrigation water from the Rio Grande is diverted into irrigation canals. The primary mechanism of discharge from the unconfined aquifer is evapotranspiration; however, upward leakage to streams and ground-water flow to the south also are involved (Anderholm, 1996). Since the 1880s, changes in agricultural practices within San Luis Valley such as sources of irrigation water, location of irrigated areas, crop rotation, and the application of irrigation water to crops have affected the ground-water flow system and likely ground-water quality (Hearne and Dewey, 1988; Anderholm, 1996). Fertilizers and pesticides are applied to fields in irrigation water and by aerial and land-based spraying (Anderholm, 1996). In Rincon Valley, areas are flood irrigated with surface water diverted from the Rio Grande River. Fertilizer is applied to crops several times during the year either directly to the soil or by application in irrigation water. The amounts of specific pesticides used in Rincon Valley are unknown (Anderholm, 2002). Ground-water recharge within Rincon Valley occurs primarily through the infiltration of irrigation water, precipitation, water from the Rio Grande, and inflow of water from adjacent areas (King and others, 1971; Anderholm, 2002). Discharge of ground water from Rincon Valley primarily is through discharge to the Rio Grande River and agricultural drains, ground-water outflow, pumpage, and evapotranspiration (Anderholm, 2002).
The 28 agricultural and 19 urban wells included from the SACR study unit are within Sacramento Valley in north-central California (fig. 2). The Sacramento River basin covers approximately 27,000 mi2 and has been farmed and irrigated since the mid-1800s (Domagalski, Knifong, and others, 2000). Sacramento Valley is in the northern third of California’s Central Valley, and approximately 3,000 mi2 of the valley is irrigated (Hull, 1984; Page, 1986). Until as recently as 1975, agriculture was the dominant land use in SACR, with cropland and pasture occupying the largest amount of acreage (Shelton, 2005). Major crops grown in the valley are rice, orchards, grains, and tomatoes (Domagalski and others, 1998). Most of the population in Sacramento Valley resides in the Sacramento area. The population of Sacramento County increased by 41 percent from 1980 to 1995 (Shelton, 2005). Both surface and ground water are used as a potable water supply in the Sacramento Valley (Dawson, 2001; Solley and others, 1998a). The use of ground-water resources has been increasing in the Sacramento Valley, and in 1995 approximately 40 percent of irrigation and 44 percent of the potable-water supplies were obtained from ground water (Shelton, 2005).
Sacramento Valley is a northwest-trending, asymmetric structural trough, filled with marine and continental rocks and sediments (Hull, 1984; Page 1986; Domagalski, Dileanis, and others, 2000). Fine-grained sediments compose greater than 50 percent of aquifer materials in most areas of the valley, with approximately 200 ft of the top unconfined (Page, 1986; Bertoldi and others, 1991). The alluvial deposits are a heterogeneous mix of gravel, sand, silt, and clay (Helley and Harwood, 1985; Dawson, 2001; Shelton, 2005).
During the last century, changes in the use of surface- and ground-water resources in the Sacramento Valley have greatly influenced the hydrologic system. As a result of pumping for irrigation and potable water, cones of depression have developed in the northern and southern areas of the valley and depth to water in shallow aquifers has declined (Bertoldi and others, 1991; Shelton, 2005). Aquifer recharge occurs primarily through infiltration of precipitation, water from the American and Sacramento Rivers, and irrigation water (Dawson, 2001). Recharge to the Sacramento Valley aquifer occurs primarily in the upper reaches of river channels and in irrigated areas, and ground-water discharge primarily occurs as pumpage in the southern part of the valley and loss to streams (Bertoldi and others, 1991).
The 110 agricultural wells sampled within the SANJ study unit are located on the eastern side of San Joaquin Valley in central California (fig. 2). San Joaquin Valley can be divided into two primary basins: the San Joaquin and Tulare. Together these two basins occupy nearly 31,200 mi2 of the eastern alluvial fan of the Coast Ranges, San Joaquin Valley, and the western alluvial fan of the Sierra Nevada (Burow, Stork, and Dubrovsky, 1998). San Joaquin Valley has a long history of intense farming and irrigation. In 1987, approximately 10.5 million acres were dedicated to farmland where approximately one-half of the total value of agricultural production in California was generated (Gronberg and others, 1998). Major products include livestock and livestock products, fruit and nuts, cotton, corn, vegetables, hay, grains, and other crops. Together, vineyards, almonds, and a rotational crop grouping of corn, alfalfa, and vegetables cover 47 percent of the agricultural land in the eastern alluvial fan region (Burow, Shelton, and Dubrovsky, 1998). Although SANJ is still a very important agricultural area, urban land use has become increasingly important as the cities grow to accommodate the rise in population. The major urban areas include Bakersfield, Fresno, Modesto, and Stockton (Burow, Stork, and Dubrovsky, 1998). Large urban centers with populations in excess of 450,000 people rely predominantly on ground-water resources for potable water (Burow and others, 1999). Both surface water and ground water resources are used for irrigation in the SANJ agricultural areas (Burow and others, 1999)
Alluvial-fan deposits of San Joaquin Valley are composed predominantly of coarse-grained sediments near the head of each fan that become finer toward the axis of the valley (Bertoldi and others, 1991; Burow, Stork, and Dubrovsky, 1998). The aquifer system is made up of continental deposits overlying marine sediments. The upper parts of the aquifer are unconfined to semi confined, whereas the deeper aquifer is confined (Poland and Lofgren, 1984; Bertoldi and others, 1991; Burow, Stork, and Dubrovsky, 1998). Historically, lacustrine diatomaceous clay deposited within San Joaquin Valley confined deeper parts of the aquifer, primarily along the western areas of the valley, thinning toward the eastern side of the valley (Page, 1986; Bertoldi and others, 1991; Planert and Williams, 2000). However, the effectiveness of this clay confining layer is questionable (Bertoldi, 1991; U.S. Geological Survey, 2000). The sedimentary deposits in San Joaquin Valley are composed of gravel, sand, silt, and clay.
The hydrologic system of San Joaquin Valley is complex, in part because of the heterogeneous nature of the deposits. Ground water generally flows in a southwestern direction toward the center of the valley (Bertoldi and others, 1991). Dominant sources of recharge to the aquifer system are infiltration of water from rivers and streams, precipitation, and irrigation seepage (Burow and others, 1999); however, infiltration of irrigation water has replaced infiltration of intermittent stream water and precipitation as the primary mechanism of recharge (Williamson and others, 1989). In the hydrologically closed Tulare Basin, most ground water flows to the Tulare Lake area and is discharged by evapotranspiration. Toward the center of San Joaquin Valley, the hydraulic gradient has been reversed from an upward to downward direction as a result of heavy ground-water pumpage for purposes of irrigation and public supply (Williamson and others, 1989; Bertoldi and others, 1991). Depth to water ranges from approximately 20 ft below land surface along the eastern edges of San Joaquin Valley to about 400 ft below land surface in the southern areas of the valley (Burow, Stork, and Dubrovsky, 1998).
Twenty-six urban land-use wells were sampled in the SOCA study unit (fig. 2). This urbanized area is located on the Pacific coastline between Los Angeles and San Diego, California. Nearly 4 million people live within the Santa Ana Basin, and by the year 2020, the population is estimated to increase by more than 50 percent (Hamlin and others, 2002). Estimates of land use within the Santa Ana watershed are 35 percent urban, 10 percent agriculture, and 55 percent open space. Population density within developed areas in the SOCA study unit is about 3,000 people per square mile (Belitz and others, 2004). The 2,700 mi2 of the Santa Ana watershed can be subdivided into three primary ground-water basins: the San Jacinto, the Inland, and the Coastal Basins (Hamlin and others, 2005). The 26 wells included in this study are located within the 100-mi2 Coastal Basin, where land use is almost entirely urban (Scott Hamlin, U.S. Geological Survey, written commun., 2005). Ground water is used as the primary source of fresh water in the SOCA study unit (Hamlin and others, 2005). The Coastal Basin has been subdivided into the Main and Irvine subbasins, which are hydrologically connected (Herndon and others, 1997).
The Main and Irvine subbasins are alluvium-filled valleys. The Main subbasin has a relatively small area of mountain-front recharge, approximately 50 mi2 along the Santa Ana River, consisting of unconsolidated sands and gravels with occasional lenses of clay and silt (Herndon and others, 1997). The Irvine subbasin contains clay and silt deposits that are typically 200–1,000 ft thick. The Irvine subbasin is composed of thin aquifers that consist primarily of silty sand (Herndon and others, 1997). Water quality in the Irvine subbasin is suitable for irrigation, but generally not for municipal supply (Hamlin and others, 2002). Within the Coastal Basin, the freshwater-bearing deposits are as much as 4,000 ft thick (Herndon and others, 1997). The shallow wells sampled during the land-use investigation are located in an area where the deeper aquifer is confined (Hamlin and others, 2005).
The hydrologic cycle within the Coastal Basin is greatly affected by human activities: ground-water pumping, engineered-recharge operations, and discharge of treated wastewater to local streams. Streams and rivers draining the mountains are diverted to ground-water recharge facilities. Eventually this diverted water is discharged back into the Santa Ana River upstream of the Coastal Basin as treated wastewater. The Coastal Basin aquifer system is recharged with water from the Santa Ana River, storm runoff, and water imported from the Colorado River and northern California through spreading basins (Hamlin and others, 2002; Belitz and others, 2004). The Orange County Water District manages approximately 200,000–250,000 acre-ft of engineered recharge to the aquifer system annually (Hamlin and others, 2002). Pumpage is the major component of ground-water discharge in the Coastal Basin (Herndon and others, 1997).
U.S. Department of the Interior | U.S. Geological Survey
URL: https://pubs.usgs.gov/sir/2007/5179
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