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Active Management Area Hydrology - Groundwater: Tucson and Prescott AMAs

Tucson AMA

The Tucson AMA contains two parallel sub-basins: the Upper Santa Cruz Valley Sub-basin in the east half and the Avra Valley Sub-basin in the west half (Figure 8.5-6).  The sub-basins consist of relatively deep alluvial basins filled with layers of sediments and bordered by mountains.  The sediments contain substantial volumes of groundwater, but the composition and productivity of the sediment layers differ between the two sub-basins.

Groundwater enters the Tucson AMA from north from the Santa Cruz AMA and from bordering mountains and then flows to the north-northwest (Figure 8.5-6). Natural recharge also occurs along stream channels (primarily the Santa Cruz River). About 84% of the total net natural recharge in the basin is estimated to occur within the Upper Santa Cruz Valley Sub-basin.  Groundwater storage in the AMA during predevelopment times is estimated to have ranged from 68 maf to 76 maf to a depth of 1,000 feet (ADWR, 2006a).

mountains

Rincon Mountain Foothills, Tucson AMA.Groundwater enters the Tucson AMA from north from the Santa Cruz AMA and from bordering mountains and then flows to the north-northwest

The median well yield reported for 1,063 large diameter (>10-inch) wells is 520 gpm.  As shown in Figure 8.5-8, well yields in excess of 1,000 gpm are found in the vicinity of Sahuarita and Green Valley, near Marana and north of Three Points.  During the period from 1994-’95 to 2004-‘05 water level rises occurred in the northern half of the Avra Valley Sub-basin due to agricultural retirement, use of CAP water in lieu of groundwater pumping and groundwater recharge activities (see Figure 8.5-6).  Similar widespread water level rises have not been noted in the Upper Santa Cruz Sub-basin with the exception of an area north of Sahuarita where CAP water is being recharged at the Pima Mine Road USF. Elsewhere in the sub-basin, water levels have generally decreased.

Water quality in the Tucson AMA is suitable for most uses, although 26 groundwater contamination sites have been identified (Table 8.5-9).  Volatile organic compounds (VOCs) associated with industrial and transportation activities are common at the contamination sites.  In addition, elevated concentrations of certain natural constituents, including arsenic, fluoride and metals have been measured in wells. Elevated nitrate, sulfate and total dissolved solid concentrations have been detected in wells near mining and agricultural operations.

Upper Santa Cruz Sub-basin

The depth to bedrock in the center of the Upper Santa Cruz Sub-basin exceeds 11,000 feet. Sediments in this sub-basin have been divided into four hydrogeologic units that form the main regional aquifer and are hydrologically connected to varying degrees. In descending order these units are the recent alluvial deposits, Fort Lowell Formation, Tinaja Beds and Pantano Formation. A basement unit underlies the sediments and forms a relatively impermeable bedrock floor that extends to the surrounding mountains.

The recent alluvial deposits underlie streambed channels of the Santa Cruz River and its major tributaries and are generally less than 100 feet thick. The Fort Lowell Formation consists of unconsolidated to moderately consolidated sands and silts that are 300 to 400 feet thick throughout the sub-basin. The underlying Tinaja Beds are up to 5,000 feet thick in the center of the sub-basin and consist of sandstones, conglomerates, siltstones and mudstones. The Tinaja Beds have become the principal supply of groundwater in the Tucson AMA due to widespread dewatering of the overlying Fort Lowell Formation. Beneath the Tinaja Beds, the Pantano Formation, composed of consolidated sandstones, conglomerates and mudstones, is little used as a water supply because of its depth and relatively low well yields. Groundwater flow is from mountain fronts to the valley and from the south to the northwest (Figure 8.5-6). The pre-development groundwater in storage estimate for the sub-basin is 52 maf to a depth of 1,000 feet.

Well yields are generally between 100 to 1,000 gpm in the sub-basin with higher yields found in wells in the Sahuarita/Green Valley area and southwest of Marana. As mentioned previously and shown on Figure 8.5-6B, water levels in most measured wells in the sub-basin declined by more than 15 feet from 1994-’95 to 2004-’05. Locations of water quality exceedences are shown on Figure 8.5-10 and constituents exceeded are listed in Table 8.5-8.  Concentrations of arsenic, metals, nitrate and other constituents that exceed drinking water standards have been measured in wells throughout the sub-basin.

Avra Valley Sub-basin

Sediments in the Avra Valley Sub-basin have been divided into upper and lower alluvial units. The upper unit is the primary water producer. Composed of silt and gravel, it includes streambed deposits along Altar and Brawley washes and ranges in thickness from less than 100 feet to more than 1,000 feet. The lower alluvial unit consists of gravel and conglomerates near the edges of the valley, grading to silts and mudstones along the central axis of the sub-basin. Groundwater flow is from the south to north. The pre-development groundwater in storage estimate for the sub-basin ranges from 17 to 24 maf to a depth of 1,000 feet.

Well yields are generally higher in the Avra Valley Sub-basin than in the Upper Santa Cruz Sub-basin (Figure 8.5-8) with measured yields often exceeding 1,000 gpm. As mentioned previously and shown on Figure 8.5-6A, water levels rose in the northern part of the sub-basin, in some wells by 30 feet or more, from 1994-’95 to 2004-’05. Constituents exceeding drinking water standards in the sub-basin are similar to those found in the Upper Santa Cruz Sub-basin (Table 8.5-8)

avra valley

Avra Valley, Tucson AMA. 

Highlands Basins

Prescott AMA

The Prescott AMA consists of two sub-basins, the Little Chino in the north and the Upper Agua Fria in the south (Figure 8.3-6).  The sub-basins are separated by a surface drainage divide.  Prescott AMA aquifers are discontinuous, with the major aquifer found in a deep structural trough that extends 25 miles from near Dewey-Humboldt to near Del Rio Springs. The trough appears to have formed from basin-and-range faulting and warping and filled with alluvial, sedimentary, and volcanic rocks of Quarternary to upper Tertiary age.

Three hydrogeologic units have been identified in the AMA. In ascending order they are named the Basement Unit, the Lower Volcanic Unit, and the Upper Alluvial Unit.  The relatively impermeable Basement Unit consists of igneous and metamorphic rocks that form the floor and sides of the groundwater sub-basins and is exposed at land surface in the surrounding mountains.  The Basement Unit has limited groundwater storage and production capacity and is not regarded as an aquifer except for domestic purposes.

The Lower Volcanic Unit overlies the Basement Unit across most of the Little Chino Sub-basin.  It is composed of a relatively thick sequence of basaltic and andesitic lava flows interbedded with layers of pyroclastic and alluvial material.   The Lower Volcanic Unit forms a highly productive confined (artesian) aquifer with discharge points northwest of and at Del Rio Springs.   The most productive portion is estimated to range from less than 100 feet up to several hundred feet thick. Natural recharge occurs mainly through infiltration of runoff in ephemeral stream channels and along the mountain fronts of the Little Chino Sub-basin.

The Upper Alluvial Unit consists of relatively thick sedimentary and volcanic rocks that fill a structural trough that extends across both sub-basins. This unit constitutes the main, unconfined aquifer in the Prescott AMA.  Natural recharge occurs from streambed infiltration and mountain front recharge.  The thickness of the unit varies considerably. In the Upper Agua Fria Sub-basin it varies from 800-1,200 feet near Prescott Valley to 200-400 feet near Dewey-Humboldt.  In the Little Chino Sub-basin, its thickness is difficult to determine but is estimated to be about 700 feet thick near Del Rio Springs with a median thickness of about 450 feet (Blasch and others, 2006). The combined thickness of the Upper Alluvial Unit and Lower Volcanic Unit is greatest in the central and southeastern portions of the Little Chino Sub-basin.

Groundwater flows generally from the mountain fronts toward the valleys, then north beneath the Little Chino Sub-basin and south beneath the Upper Agua Fria Sub-basin. ADWR (2005) estimated that there was 3.0 maf of groundwater in storage in the AMA; 2.1 maf in the Little Chino Sub-basin and 0.9 maf in the Upper Agua Fria Sub-basin. The median reported well yield for 78 large diameter (>10-inch) wells is 763 gpm (Table 8.3-6). Well yields are generally between 500 gpm and 1,000 gpm in wells near  Chino Valley, and between 100 gpm to 500 gpm in the Upper Agua Fria Sub-basin.  Between 1993-‘94 and 2004, water levels declined in most measured wells (Figure 8.3-6).  Recent depths to groundwater in wells ranged from 16 feet bls near Del Rio Springs to almost 500 feet bls in the east-central part of the basin.

Water quality is generally good; however arsenic, and to a lesser extent other constituents have been measured at concentrations exceeding water standards, at several locations (Table 8.3-8). Sites contaminated with hydrocarbons, lead, cyanide and other contaminants are found near Prescott, Chino Valley and Dewey-Humboldt (see Figure 8.3-11)

 

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