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Active Management Area Hydrology - Groundwater: Pinal and Santa Cruz AMAs

Pinal AMA

The Pinal AMA consists of five sub-basins with unique groundwater recharge and storage characteristics.  These sub-basins include the Maricopa-Stanfield, Eloy, Vekol Valley, Santa Rosa Valley, and Aguirre Valley (Figure 8.2-8).  Sub-basin boundaries follow surface water topographic divides, and in the case of the Eloy and Maricopa-Stanfield sub-basins, a groundwater divide.  Groundwater underflow between these two sub-basins is limited. Most groundwater development has occurred within the Maricopa-Stanfield and Eloy sub-basins while relatively little development and hydrologic information is available for the Vekol Valley, Santa Rosa Valley and Aguirre Valley sub-basins, which are primarily tribal lands.

The most productive groundwater-bearing units in the Maricopa-Stanfield and Eloy sub-basins consist of unconsolidated sands, gravels, silts, and clays that were deposited by the ancestral Gila and Santa Cruz rivers.  Demand for water by irrigated agriculture has drained much of this upper alluvial unit in both sub-basins and changed the direction of groundwater flow between them.

Gila River Bed

Gila River Bed, Pinal AMA. Natural recharge is primarily from underflow into the basin and from streambed infiltration along the Gila and Santa Cruz rivers, which produce relatively large volumes of runoff from upstream basins outside the AMA following heavy rains.

Natural recharge is primarily from underflow into the basin and from streambed infiltration along the Gila and Santa Cruz rivers, which produce relatively large volumes of runoff from upstream basins outside the AMA following heavy rains. Lesser amounts of natural recharge occur from mountain fronts. The estimated groundwater in storage for the Maricopa-Stanfield, Eloy and Vekol Valley sub-basins is 35.2 maf to a depth of 1,000 feet bls. Median well yield in the AMA, reported from 1,582 large diameter (> 10-in.) wells, is 1,000 gpm (see Table 8.2-6).  Water levels rose between 1993-’94 and 2003-’04 in many wells as shown on Figure 8.2-6, although areas of historic decline are found near Florence, Coolidge, southwest of Picacho and in the vicinity of Casa Grande.

Water quality in the Pinal AMA generally meets state and federal drinking water standards, however exceedences of nitrate, fluoride, arsenic and to a lesser extent, other constituents have been measured at some locations (see Table 8.2-8). Pesticide, jet-fuel and hydraulic fluid contamination has been reported at several contamination sites in the AMA (Table 8.2-9 and Figure 8.2-11).

Maricopa-Stanfield Sub-basin

Groundwater in storage is estimated at 8.6 maf in the Maricopa-Stanfield Sub-basin.  Groundwater flow is north toward the Gila River and toward cones of depression that have formed west of the towns of Maricopa and Stanfield (see Figure 8.2-6).  Groundwater levels have been recovering and rising in much of the sub-basin due to use of CAP water in lieu of groundwater pumping. Water level rises of more than 60 feet were observed in many wells between 1993-‘94 and 2003-‘04 (Figure 8.2-6). Recent depths to groundwater range from 51 feet bls near the Gila River in the north to more than 600 feet bls in the vicinity of Stanfield (Figure 8.2-6A). Well yields in excess of 1,000 gpm are common. Fluoride and arsenic were the most common constituents exceeding drinking water standards in wells measured in the sub-basin, with elevated TDS concentrations and nitrate exceedences also detected (Figure 8.2-10, Table 8.2-8).

Eloy Sub-basin

An estimated 22.6 maf of groundwater is in storage to a depth of 1,000 feet bls in the Eloy Sub-basin. Groundwater flow is generally to the north toward the Gila River and Phoenix AMA. Well yields in excess of 500 gpm to more than 2,000 gpm are common (Figure 8.2-8). Reductions in groundwater pumping and use of CAP water have contributed to recent rising water levels in several wells in this sub-basin.  However, groundwater levels are also declining in the north due to dissipation of a groundwater mound formed after Gila River flooding; and in the south central sub-basin, probably from deep well pumping (see Figure 8.2-6).  Recent depths to groundwater range from 53 feet bls in the northeast to over 400 feet bls near Picacho (Figure 8.2-6B). Concentrations of fluoride, arsenic, nitrates and other constituents have exceeded drinking water standards in wells throughout the sub-basin (Figure 8.2-10, Table 8.2-8)

Santa Cruz AMA

Basin-fill sediments along the Santa Cruz River from east and north of the City of Nogales to Amado form three named aquifer units.  Listed in ascending order they are the Nogales Formation, Older Alluvium, and Younger Alluvium (also referred to as the stream alluvium). The alluvial units are generally unconfined and hydraulically connected, although the Older Alluvium aquifer exhibits semi-confined to confined conditions in some places, most notably in Potrero Creek.  The Nogales Formation is not generally considered an important aquifer, although exceptions occur.  The Older Alluvium varies in thickness from a few feet along the mountains to more than 1,000 feet in the north-central part of the basin. Well yields are often low in wells drilled in this aquifer. The Younger Alluvium forms the most productive and widely utilized aquifer in the AMA with well yields commonly in excess of 1,000 gpm.  The Younger Alluvium ranges from about 40 to 150 feet thick, becoming thicker and wider to the north along the Santa Cruz River. 

Santa Cruz AMA

Santa Cruz AMA.  Basin-fill sediments along the Santa Cruz River from east and north of the City of Nogales to Amado form three named aquifer units. Listed in ascending order they are the Nogales Formation, Older Alluvium, and Younger Alluvium (also referred to as the stream alluvium).

Groundwater enters the basin along the Santa Cruz River and west of Nogales. Groundwater flow is then generally from south to north. Natural groundwater recharge occurs from infiltration of Santa Cruz River channel flow and mountain front recharge.  Groundwater storage in the Younger Alluvium has been estimated at about 160,000 acre-feet. The median well yield reported for 115 large (>10-inch) diameter wells is 800 gpm, with the highest yields located between Rio Rico and Tubac (Figure 8.4-8). Water levels have generally declined in wells measured between 1995 and 2004-’05 throughout the AMA, with most declines totaling from 1 to 15 feet (see Figure 8.4-6). However, a characteristic of the Younger Alluvium in the Santa Cruz AMA is the potential for rapid water level fluctuations resulting from river charge.

Groundwater quality is generally good, although arsenic concentrations exceeding the drinking water standard have been measured at some wells in the basin (Table 8.4-7).  In addition, there are two sites near Nogales with VOC and chromium contamination (Table 8.4-8 and Figure 8.4-10)

 

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