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Lower Colorado River Hydrology - Groundwater (West Basins - Continued)

Colorado River, Lower Gila Basin

Colorado River in the Lower Gila Basin

West Basins

West Basins include Butler Valley, Gila Bend, Harquahala, Lower Gila, McMullen Valley, Ranegras Plain, San Simon Wash, Tiger Wash and Western Mexican Drainage basins.  Groundwater inflows and outflows are relatively small in these basins and there are no perennial streams.  Groundwater inflows consist of minor amounts of mountain front recharge and stream infiltration.  The basins are contain a relatively thin, heterogeneous layer of upper basin fill underlain by lower basin fill.  The lower basin fill consists of a unit of primarily fine-grained material underlain by a medium to coarse grained unit.  Pre-Basin and Range sediments underlie the basin fill.  Stream alluvium deposits occur along the Gila River and elsewhere and may be locally productive water-bearing sediments (Anderson and others, 1992).

McMullen Valley Basin

The principal aquifer in the McMullen Valley basin is alluvial-fan deposits in the basin fill.  These deposits underlie most of the valley floor, varying in thickness from 230 feet in the Wenden-Salome area to 3,100 feet north of Aguila.  Most large irrigation wells tap into this unit.  Fine grained lake-bed deposits of low permeability overlie the alluvial fan deposits in the central and lower parts of the valley. These deposits range in thickness from 150 feet southwest of Wenden to about 1,100 feet northeast of Wenden.  Because of their relatively low permeability, the lake-bed deposits may impede downward percolation of water, creating perched aquifers. Stream alluvium has been deposited by Centennial Wash and its tributaries and is composed of silt, sand and clay.  This unit ranges from 50 feet thick in the lower end of the basin, 100 feet thick in the Wenden-Salome area, and over 450 feet thick north of Aguila.  There has been some groundwater development in the stream alluvium for domestic and stock use, but irrigation pumpage has dewatered the unit in the Aguila area (Remick, 1981).  The basal unit of the basin fill is a conglomerate present at a depth of about 850 to 1,600 feet bls and is largely unexplored.

An estimated 1,000 acre-feet of groundwater recharge occurs annually. Groundwater storage estimates range from 14 to 15.1 maf.  The median well yield reported for 167 large diameter (>10 in.) wells is 1,500 gpm (Table 7.5-5).

Water levels in measured wells are generally more than 300 feet bls. As shown in Figure 7.5-5, water levels declined in all wells measured between 1990-’91 and 2003-’04, with significant declines (>30 feet) in a well east of Aguila and in five wells in the western half of the basin.

Fluoride and arsenic concentrations exceeding drinking water standards are found at wells throughout the basin with elevated nitrate concentrations measured in a number of wells near Salome (see Table 7.5-6).

McMullen Valley

McMullen Valley

Ranegras Plain Basin

Groundwater in the Ranegras Plain Basin occurs primarily in older (Tertiary) basin-fill deposits composed of clay, volcanics, conglomerate and smaller amounts of sand and gravel.  The thickness of the basin-fill deposit is not well known but is at least 1,500 feet northwest of Vicksburg.  The younger (Quaternary) alluvium, which includes stream alluvium, overlies the basin fill and is composed primarily of sand and gravel with a thickness of less than a few hundred feet.  Perched groundwater occurs in the central part of T6N, R16W and in Sections 9 and 10 of T5N, R16W where water levels are 10 to 60 feet higher than the surrounding area. (Johnson, 1990) 

Groundwater flow is generally to the northwest toward the community of Bouse but irrigation wells groundwater withdrawals have created a cone of depression southwest of Vicksburg (see Figure 7.7-5).

Groundwater recharge is from infiltration of runoff in Bouse Wash, Cunningham Wash and along mountain fronts.  About 32 miles of the CAP canal runs through the northeastern portion of the basin and may contribute 2,000 to 3,000 acre-feet of recharge a year. (Johnson, 1990)  Annual recharge estimates range from less than 1,000 acre-feet to more than 6,000 acre-feet. Groundwater storage estimates range from 9.0 to 27 maf. Although yields in some wells are relatively low due to the presence of clays, yields reported for 68 large (>10 in.) diameter wells reach 4,000 gpm with a median yield of 1,150 gpm (Table 7.7-3)

New Water Mountains, Ranegras Plain

New Water Mountains, Ranegras Plain Basin

As shown in Figure 7.7-5, water levels declined in almost all wells measured between 1990-’91 and 2003-’04, with significant declines (>30 feet) east of Vicksburg Road.

Groundwater quality is generally poor with elevated TDS concentrations measured in a number of wells.  Of 48 wells measured between 1984 and 1989, only five wells had TDS levels below the secondary maximum contaminant level of 500 milligrams per liter recommended by the Environmental Protection Agency (EPA).  The highest TDS concentrations were measured in the north-central part of the basin (Johnson, 1990).2  Water quality measurements taken between 1979 and 2000 also show a number of wells with elevated fluoride and arsenic concentrations (Table 7.7-4).

San Simon Wash Basin

Basin fill comprises the principal aquifer in the San Simon Wash Basin.  The thickness of the basin fill ranges from near zero at the mountain fronts to over 8,000 feet near the international boundary.  Four basin-fill units have been identified. Alluvial-fan deposits occur on the basin perimeter and vary in depth and well yield.  Streambed alluvium consisting of sand, gravel and boulders occurs along stream channels and may yield significant volumes to wells.  Deltaic deposits consisting of a sequence of clay, silt, sand and gravel are found near Papago Farms (T19S, R1E) where deposits may be 800 feet thick and well yields are relatively high.  Lakebed deposits consisting of thick sequences of fine-bedded silts and clays extend to depths of more than 1,000 feet.  Groundwater occurs under unconfined conditions in the basin. Groundwater flow is generally toward the southwest, then south into Mexico. (Hollett, 1985)

There is relatively little grounwater data available for the basin, which is almost entirely within the Tohono O’odham Nation. Natural recharge is estimated at 11,000 AFA and groundwater storage estimates range widely from 6.7 to 45 maf. Well yield estimates range from less than 50 to 3,000 gpm (Table 7.8-5).  Hollett (1985) reported that wells drilled into the lakebed deposits in the center of the basin generally yield less than 50 gpm and well yields appear to be highest at depths of 400 to 700 feet. Depth to water averaged about 300 feet bls (Hollett, 1985).

Elevated arsenic concentrations are found across the basin and fluoride concentrations that equal or exceed drinking water standards occur in the area around Papago Farms and the international boundary (Table 7.8-6).

Tiger Wash Basin

Tiger Wash Basin is a relatively small, shallow basin composed of heterogeneous deposits of clay, silt, sand and gravel that are likely less than 1,000 feet thick. There appears to be a groundwater divide near the center of the basin from which groundwater flows to the southwest and to the northeast (Hedley, 1990) (Figure 7.9-6).

Natural recharge is estimated to be less than 1,000 AFA.  Groundwater in storage estimates range from 700,000 acre-feet to 2.0 maf.  Measured well yield data are not available for the basin. Anning and Duet (1994) estimated a maximum yield of 500 gpm. Two wells measured in 2003-’04 had water levels of 29 feet and 219 feet bls (Figure 7.9-6).

Two water quality exceedences have been reported in basin wells, with concentrations of arsenic and nitrate that equal or exceed the drinking water standard (Table 7.9-4).

Tiger Wash

Tiger Wash

Western Mexican Drainage Basin

The Western Mexican Drainage Basin contains broad alluvial-filled valleys containing unconsolidated gravel, sand, silt and clay deposits that make up the main water-bearing unit.  Groundwater flow is toward Mexico.

Natural recharge is estimated to be 1,000 AFA.  Groundwater in storage estimates range from 3.0 to 4.1 maf. The median well yield reported for three large (>10 in.) diameter wells was 50 gpm (Table 7.10-4).

Water levels varied from 27 to 237 feet bls at wells measured in 2003-‘04 and levels appear to be declining near Lukeville, likely due to development in the Sonoyta area of Sonora, Mexico (Figure 7.10-6). Water quality data collected between 1976 and 1988 along the international boundary west of Lukeville show concentrations of fluoride, arsenic and lead that equal or exceed the drinking water standard (Table 7.10-5)


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