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Active Management Area Hydrology - Groundwater Overview and Phoenix AMA

With the exception of the Prescott AMA, a large portion of the AMA planning area is located in what Anderson, and others (1992) categorized as the Central basins. Stream alluvial deposits and upper basin fill are the principal water bearing sediments in these basins (see Figure 8.0-4).  The Central basins are characterized by relatively small to moderate amounts of mountain-front recharge, streamflow infiltration and significant underflow in and out of the basins. Groundwater flows tend to move inward from the edges of the basin and higher elevations and then downstream towards the outflow portion of the basin.

The Prescott AMA is located in what Anderson, and others (1992) categorized as the Highland basins. Highland basins consist of basin fill and alluvium deposits, similar to the Central basins; however, due to their discontinuous nature, relatively little or no underflow occurs between basins. As shown in Figure 8.0-4, much of this basin is covered by sedimentary and volcanic rocks. Recharge occurs from surrounding consolidated rock and inflow from stream infiltration.

The central AMAs (Phoenix, Pinal and Tucson) contain relatively deep alluvial aquifers and significant volumes of water in storage. However, since aquifer recharge rates are relatively low and pumping volumes large, the aquifers have been in an overdraft condition. Within an AMA, overdraft is defined as a condition where groundwater is pumped in excess of safe-yield.

Click to view Figure 8.0-4

Figure 8.0-4 Surface Geology of the AMA Planning Area

The definition of safe-yield is, “to achieve and thereafter maintain a long-term balance between the annual amount of groundwater withdrawn in an active management area and the annual amount of natural and artificial groundwater recharge in an active management area.” A.R.S. § 45-561(12).   The Prescott AMA aquifers are more discontinuous and less extensive than the large basin-fill aquifers of the central AMAs.  As with the central AMAs, the Prescott AMA is in an overdraft condition. In the Santa Cruz AMA a close interrelationship exists between water levels in the stream alluvium along the Santa Cruz River, and precipitation and drought events. The Santa Cruz AMA is in a safe-yield condition. (Erwin, 2007)

Gilespie Dam

All of the AMAs, with the exception of the Santa Cruz AMA, contain sub-basins: two in the Prescott AMA, seven in the Phoenix AMA, five in the Pinal AMA, and two in the Tucson AMA.  Characteristics of each  basin and sub-basin are described individually below.

Central Basins

Phoenix AMA

The primary source of groundwater in the Phoenix AMA is basin-fill sediments. Three distinct water bearing units are identified in most of the sub-basins in the AMA:  an upper alluvial unit, a middle fine-grained unit, and a lower conglomerate unit.  Although conditions and circumstances vary across the AMA, most groundwater is pumped from the middle unit.  Bedrock, consisting of metamorphic and igneous rock, underlies the basin-fill sediments and is not considered an aquifer. Groundwater occurs under generally unconfined conditions throughout most of the AMA. Depth to water ranges from just below land surface (bls) to more than 800 feet bls.

There are seven groundwater sub-basins in the Phoenix AMA: East Salt River Valley (ESRV), West Salt River Valley (WSRV), Hassayampa, Rainbow Valley, Fountain Hills, Lake Pleasant, and Carefree. (Figure 8.1-6)  Each sub-basin has its own unique hydrogeologic characteristics, discussed below.

Groundwater flow directions are shown on Figure 8.1-6. In several areas, historic flow directions have been altered by well pumping.  Prior to extensive pumping, groundwater flowed primarily from the ESRV to the WSRV along or toward the Salt and Gila Rivers, exiting the AMA near Gillespie Dam. By 1964, a regional groundwater depression had formed in the WSRV sub-basin east of the White Tank Mountains, redirecting flow in the sub-basin to the depression (Rascona, 2005).  By 1983, agricultural pumping had produced localized groundwater depressions throughout the AMA (Reeter and Remick, 1986). A groundwater divide now exists in the southwest quarter of Township 1N, Range 4E that severs the hydraulic connection between the ESRV and WSRV sub-basins (Corkhill and others, 1993). Groundwater flow patterns are discussed further in the sub-basin sections. 

Groundwater recharge is from mountain front and stream channel recharge. Groundwater inflow into the AMA occurs as groundwater flows north from the Pinal AMA into the ESRV, and from the north and east.  Groundwater exits the basin at Gillespie Dam where the Gila River exits the AMA.  In general, between 1991-’92 and 2002-’03, water levels rose in the eastern part of the AMA, declined in the central part and were stable or rose or declined slightly in the western part of the AMA (Figure 8.1-6). Well yields throughout the AMA are generally high, with median values of over 1,400 gpm reported (Table 8.1-6).

Groundwater quality is generally suitable for most uses, but 68 groundwater contamination sites associated with industrial and other activities have been identified in the AMA (Table 8.1-9, Figure 8.1-11). Volatile Organic Compounds (VOCs) are the most common contaminant at these sites.  In addition, over 1,500 measurements have been made of parameter concentrations that have equaled or exceeded drinking water standards.  Of these, nitrate, fluoride, arsenic, and organics are the most common. All water providers in Arizona that serve more than 25 people or having 15 or more connections are regulated under the Safe Drinking Water Act and treat water supplies to meet drinking water standards. Detailed information on groundwater quality in the Phoenix AMA is found in the 1999 Third Management Plan.

East Salt River Valley Sub-basin

The ESRV Sub-basin encompasses the eastern part of the AMA and includes a portion of the City of Phoenix, the cities of Scottsdale, Tempe, Mesa, and Chandler, and the towns of Superior, Apache Junction, Gilbert and Queen Creek. The thickness of basin-fill sediments range from less than 100 feet near the basin margins to over 10,000 feet southeast of Gilbert.  The primary source of groundwater (49%) is from the lower basin fill, with another 40% withdrawn from the middle basin fill and only 11% withdrawn from the upper basin fill (Rascona, 2005).

Superior

Town of Superior, East Salt River Valley Sub-basin. The primary source of groundwater (49%) in this Sub-basin is from the lower basin fill, with another 40% withdrawn from the middle basin fill and only 11% withdrawn from the upper basin fill.

Groundwater flows into the ESRV Sub-basin from the Lake Pleasant Sub-basin, the Eloy Sub-basin in the Pinal AMA, and between the Santan and Sacaton mountains in the southern part of the sub-basin. Groundwater also flows toward a cone of depression caused by groundwater pumping east of Chandler (see Figure 8.1-6).  Natural groundwater recharge occurs along stream channels and from mountain front recharge. Other sources of recharge include infiltration of agricultural irrigation water, canal leakage and storage at underground storage facilities (USFs). From 1990 to 2002, groundwater recharge exceeded withdrawals by almost 2.7 million acre-feet (maf) (Rascona, 2005). Groundwater in storage to a depth of 1,000 feet bls is estimated at more than 68 maf in the ESRV and WSRV sub-basins (ADWR, 1998a).

Earth fissuring and subsidence have occurred in the ESRV sub-basin due to localized pumping. These occurrences are found near Apache Junction and in the vicinities of Queen Creek, North Scottsdale and Paradise Valley (Rascona, 2005).

Well yields commonly exceed 1,000 to 2,000 gpm (Figure 8.1-8). The median well yield reported for 2,397 large (10-inch) diameter wells is 1,280 gpm (Table 8.1-6). Substantial water level rises were measured between 1991-‘92 and 2002-‘03 in a number of wells in the sub-basin (see Figure 8.1-6A).  Increases of over 60 feet were reported in some areas due to a combination of cessation of farming and associated reduction in pumping, and direct use and recharge of CAP water.  Groundwater level depths measured during 2002-‘03 ranged from ten feet bls near Superior to over 800 feet bls south of Cave Creek. Locations of water quality exceedences are shown on Figure 8.1-10 and constituents exceeded are listed in Table 8.1-8.

Phoenix

Camelback Road, City of Phoenix; West Salt River Valley Sub-basin. Groundwater flow historically was toward and along the Salt and Gila Rivers.

West Salt River Valley Sub-basin

The WSRV Sub-basin includes the communities of Phoenix, Buckeye, Surprise, Glendale, Peoria, Goodyear, Tolleson and Avondale. It is a broad, gently-sloping alluvial plain bounded by hills and low-elevation mountains with a depth to bedrock of over 10,000 feet beneath the Luke Air Force Base area. A large salt body lies southeast of Luke Air Force Base at a depth of 880 feet to over 6,000 feet, which locally affects groundwater salinity. Groundwater in the sub-basin is obtained almost evenly between the upper, middle and lower basin fill (Rascona, 2005). The middle basin fill ranges in thickness from less than 100 feet to over 1,300 feet southwest of Glendale. Natural groundwater recharge occurs along stream channels and from mountain front recharge.  Groundwater also enters the sub-basin from the Lake Pleasant, northern Hassayampa and ESRV sub-basins, and from the Maricopa-Stanfield Sub-basin in the Pinal AMA.  Incidental recharge of agricultural irrigation water and effluent discharged from the City of Phoenix 23rd and 91st Avenue wastewater treatment plants also recharges the aquifer.

Groundwater flow historically was toward and along the Salt and Gila Rivers. As mentioned previously, a regional groundwater depression has formed east of the White Tank Mountains in the vicinity of Sun City and Litchfield Park. Associated water level declines of more than 300 feet in the area of Luke Air Force Base resulted in surface subsidence of more than 18 feet by 1991 (see Figure 8.1-6) (Hipke and others, 1996). While groundwater levels rose in that part of the sub-basin between 1991-‘92 and 2002-‘03, they declined in the Glendale/Goodyear/Phoenix area.  Depths to groundwater vary widely in the sub-basin with shallower levels present south of I-10 along the Salt and Gila River drainage (Figure 8.1-6D). Well yields commonly exceed 1,000 to 2,000 gpm (Figure 8.1-8). Locations of water quality exceedences in the sub-basin are shown on Figure 8.1-10 and constituents exceeded are listed in Table 8.1-8.

Hassayampa Sub-basin

The Hassayampa Sub-basin is bounded by hills and mountains and drained by the ephemeral Hassayampa River. The sub-basin consists of the largely undeveloped Hassayampa Plain in the north and the Lower Hassayampa Area in the south. Groundwater occurs in the basin-fill deposits primarily under unconfined conditions (Rascona, 2005).  There are, however  local occurrences of confined (artesian) or perched aquifer conditions in the Lower Hassayampa Area (Long, 1983).

Little groundwater development has occurred in the Hassayampa Plain so the basin-fill sequence is not well understood in that part of the sub-basin.  Depths to bedrock beneath the Hassayampa Plain range from a few tens of feet near the basin margins to over 1,200 feet near the sub-basin center. In the Lower Hassayampa Area depths to bedrock exceed 1,200 feet in the central part of the Tonopah Desert and Centennial Wash area (Long, 1983). 

Groundwater enters the Hassayampa Plain from the northeast and flows south toward the Gila River. Groundwater historically flowed into the sub-basin from the WSRV Sub-basin, but this no longer occurs due to groundwater pumping in that sub-basin.  Sources of groundwater recharge include streambed (Gila and Hassayampa rivers) infiltration and mountain front recharge. Groundwater in storage is estimated at more than 12 maf for the area north of I-10 (ADWR, 2003).

Well yield data are available primarily in the Lower Hassayampa Area where yields may exceed 2,000 gpm (Figure 8.1-8). Groundwater pumpage has declined across the sub-basin compared to pumpage in the 1970s and 1980s, resulting in groundwater level rises in several areas. Groundwater depressions still exist in Tonopah and south of Tonopah in the Centennial Wash area (Rascona, 2005) (see Figure 8.1-6).  Depths to groundwater ranges from about 20 feet bls in the southwest to over 600 feet bls in the northern part of the sub-basin (Figure 8.1-6B). Locations of water quality exceedences are shown on Figure 8.1-10 and constituents exceeded are listed in Table 8.1-8.

Rainbow Valley Sub-basin

The Rainbow Valley Sub-basin is a relatively undeveloped alluvial plain located in the southern part of the AMA and drained by Waterman Wash, an ephemeral stream that joins the Gila River near Buckeye.  Depths to bedrock may reach nearly 10,000 feet in the center of the sub-basin.  The basin-fill sediments consist of poorly sorted gravel, sand, silt and clay. Sources of groundwater recharge include streambed infiltration along Waterman Wash and mountain front recharge. Groundwater flow is from south to north and may have historically entered the sub-basin from the Maricopa-Stanfield Sub-basin in the Pinal AMA.  Groundwater storage data are not available for the sub-basin.

Agricultural well pumpage in the sub-basin began in the 1940s and by 1952 a groundwater depression had developed in the northwest portion of the sub-basin. This depression is still evident (Rascona, 2005).

Well yield data are available primarily for the northern part of the sub-basin where yields may exceed 2,000 gpm (Figure 8.1-8). Groundwater levels generally declined between 1991-‘92 and 2002-‘03. Depths to groundwater measured in 2002-‘03 ranged from 140 feet bls to almost 500 feet bls (Figure 8.1-6C).  Fluoride is the water quality constituent most commonly exceeded in measured wells in the sub-basin (Figure 8.1-10,Table 8.1-8).

Agriculture

Agricultural well pumpage in the sub-basin began in the 1940s and by 1952 a groundwater depression had developed in the northwest portion of the sub-basin.

Fountain Hills Sub-basin

The Fountain Hills Sub-basin is a dissected alluvial plain bounded by mountains.  It is drained by the lower Verde River, which is perennial along the axis of the sub-basin, and by the Salt River in the southern part of the sub-basin. The two rivers converge in the southern portion of the sub-basin.

The regional aquifer consists of older basin-fill sediments and more recent unconsolidated alluvium deposited by and hydraulically connected to the Verde River. The regional aquifer in the Fountain Hills Sub-basin may not be connected to adjacent sub-basins.  The depth to bedrock may exceed 4,800 feet. A geologic cross-section through the Town of Fountain Hills indicates a lower confined aquifer system and more shallow alluvial aquifers along streams and washes around the Town and along the Verde River (HydroSystems, 1999).

The general direction of groundwater flow is from north to south, parallel to the sub-basin axis.  A clay sequence forms a barrier to groundwater flow between the shallow alluvial aquifer along the Verde River and decomposed and fractured granites that exist north and east of the McDowell Mountains (ADWR, 2001). Groundwater recharge occurs through streambed (Verde and Salt rivers) infiltration and from mountain front recharge.  Groundwater storage data are not available for the sub-basin.

Reported well yields are greatest in the southern part of the sub-basin where they may exceed 2,000 gpm (Figure 8.1-8). Groundwater levels rose in several wells in the sub-basin between 1991-‘92 and 2002-‘03 with depths to groundwater ranging from about 50 feet bls to over 500 feet bls (see Figure 8.1-6A). Arsenic and fluoride concentrations exceeded drinking water standards in several wells measured in the sub-basin (Figure 8.1-10, Table 8.1-8).

Lake Pleasant

Lake Pleasant, created by the Agua Fria River in the Agua Fria Basin. The Lake Pleasant Sub-basin is drained by the lower Agua Fria River, the New River and by Skunk Creek.

Lake Pleasant Sub-basin

The Lake Pleasant Sub-basin is a relatively small, gently sloping alluvial plain surrounded by hills and mountains in the northern part of the AMA.  It is drained by the lower Agua Fria River, the New River and by Skunk Creek. Basin fill, interbedded with volcanics, intrusives and conglomerate make up the main water-producing aquifer (Clear Creek & Associates, 2003). Depth to bedrock exceeds 800 feet near the center of the sub-basin where reported well yields are generally between 100 and 500 gpm.  In the New River area, the local aquifer consists of fractured schist and gneiss and the groundwater supply is drought-sensitive. Well yields in this area are relatively low.

Sources of groundwater recharge include streambed infiltration and mountain front recharge.  Groundwater flow is generally from north to south and into the WSRV and ESRV sub-basins. Groundwater storage data are not available for the sub-basin.  Groundwater levels were stable or rose in most measured wells between 1991-‘92 and 2002-‘03.  Depth to water ranged from 17 feet bls to almost 300 feet bls in 2002-’03 (see Figure 8.1-6D). Fluoride was the most commonly measured constituent exceeding drinking water standards in wells in the sub-basin (Figure 8.1-10, Table 8.1-8).

Carefree Sub-basin

The Carefree Sub-basin, located in the northeastern part of the AMA, is drained by Cave Creek, a relatively small ephemeral stream. A  northwest-trending alluvial plain in the southern part of the sub-basin contains aquifers consisting of streambed alluvium and members of the Carefree Formation, the major water-producing unit (HydroSystems, 2000).  The basin fill is up to 2,000 feet thick and composed of older, partially-consolidated to consolidated sedimentary rocks.  The Carefree Formation consists of alluvial fan and playa deposits and is underlain by volcanic rocks. The Grapevine Member is the only significant source of groundwater in this formation and reaches a maximum thickness of 1,300 feet.

Historic groundwater pumping caused cones of depression to form near the Carefree Airport in the south-central part of the basin and in the northern part of the Town of Cave Creek. The cone near the Town is still well defined and draws in groundwater from the northwest and southeast (Rascona, 2005).  Natural groundwater recharge is from mountain front recharge and infiltration of streamflow along Cave Creek. ADWR (1994) estimated that the volume of groundwater in storage in the Carefree Sub-basin was 570,000 acre-feet to a depth of 1,200 feet bls.

Well yields vary across the sub-basin, with the highest (>1,000 gpm) yields east of Carefree (Figure 8.1-8). Groundwater levels began declining in the early 1960s, but rose in several wells between 1991-‘92 to 2002-‘03 as many local golf courses converted from solely groundwater to a combination of CAP water, groundwater and effluent. Depth to water in wells measured in 2002-‘03 ranged from 27 feet bls to 330 feet bls (Figure 8.1-6).  Fluoride, arsenic and radionuclides were the parameters most commonly exceeding drinking water standards in wells in the sub-basin (Figure 8.1-10, Table 8.1-8).

 

water drop  Continue to 8.0.2 - Hydrology: Groundwater: Pinal and Santa Cruz AMAs

 

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