Aquifers of the Tucson Active Management Area

John Barker
Applied Hydrogeology
Emporia State University, 2009

Table of Contents


Introduction to the AMA

The Tucson Active Management Area, Figure 1, is located in southeastern Arizona and covers approximately 4,000 square miles. The Tucson AMA is one of five AMAs established following the 1980 Groundwater Management Act and is governed by the Arizona Department of Water Resources. The area is located within the basin and range physiographic region, which is characterized by broad, gently sloping alluvial basins separated by northwest trending fault block mountains. The AMA encompasses parts of Pima, Pinal and Santa Cruz Counties, which includes the cities of Tucson, South Tucson, Oro Valley, Marana and Sahuarita. The Pasqua Yaqui tribal lands, part of the Schuk Toak District, and the entire San Xavier District of the Tohono O'odham Nation also lie within the Tucson AMA boundary. As of 2008 the population of Tucson was 1,023,320. The goal of the Tucson AMA is to recharge the aquifers in the region by 2025. To reach this goal implementing mandatory conservation requirements for agricultural, industrial, and municipal water users (ADWR, 2009).

Figure 1, The Tucson Active Management Area. Map courtesy of University of Arizona Water Resources Research Center, 2008


Hydrologic Setting

The Tucson AMA consists of three ground water sub-basins, the Avra Valley Sub-basin, the Upper Santa Cruz Sub Basin snd the Altar Valley sub-basin. Our focus will be on the Avra Valley and Upper Santa Cruz basins, the Altar Valley basin in the southern section of the Tucson AMA is home to a wildlife refuge and is largely undeveloped (Anderson, Carruth & Pool, 2007). The Upper Santa Cruz sub-basin consists of the Tucson metropolitan area, the heaviest populated area of the Tucson AMA. The Avra Valley sub-basin contains the Altar and Avra Valleys, which mainly consist of large agricultural areas. The climate within the basins is semi-arid, characterized by high summer temperatures and large rates of evaporation and transpiration. Precipatation on the basin floor averages from eleven to sixteen inches a year and only about five percent of rainfall will recharge the basin-fill aquifers (USGS, 2008). The mountains bordering the Tucson AMA will receive up to thirty inches of precipitation a year which is the aquifers main supply source. Water not lost to evapotranspiration in the mountains will supply streams running from the mountains or infiltrate the fractured bedrock and eventually discharge into the basin, this is referred to as mountain front recharge. Beginning in mid summer and lasting until late September the Tucson AMA will receive the majority of its rainfall, this is referred to as the monsoon season. It is estimated that just two to three percent of the monsoon rain will reach the water table (Poole, 2007). The rainfall during the monsoon falls to quickly for the ground to soak up, and in the city of Tucson's case, the water quickly moves out of town and out of the area via the Santa Cruz river. The Santa Cruz River is the main surface water drainage for the Tucson AMA and only flows briefly responding to local rainfall events.

Figure 2, The extent of the Avra Valley Sub-basin (left) and Upper Santa Cruz Sub-basin (right). Highlighted in purple, Bota & Mason, 2006.


Hydrologic and Geologic Features

The sedimentary deposits that fill both the Upper Santa Cruz (Figure 3) and Avra (Figure 4) sub-basins are Tertiary to Quaternary age. The deposits are composed of volcanic deposits and unconsolidated to consolidated sediments consisting of gravel, sand, silt and clay with a small percentage of gypsiferous and anhydrous sediments. The sediments along the margins of the basins are usually coarse grained and grade into finer sediments towards the center of the basin. The sediments of the basin have been divided into lower and upper basin-fill units based on their hydrogeologic characteristics. The basin-fill unit has been subdivided into stratigraphic units based on lithological descriptions, structure, and depositional history. The upper and lower basin-fill sediments are saturated at depth and form the regional aquifer system. To depths of 1,000 ft the groundwater can be found in unconfined or partially confined conditions. (Bota & Mason, 2006).

Upper Basin-Fill Unit

In both sub-basins the upper basin-fill unit ranges from several hundred to 1000 ft.in thickness. The unit consists of mostly semi-consolidated to unconsolidated gravel, sands, and clayey silt. The upper basin-fill unit is divided into the upper Tinaja beds, the Fort Lowell Formation, and the surficial alluvium deposits, also known as the Younger Alluvium.

The Younger Alluvium which refers to the stream channel deposits is extremely permeable and ranges from 40 to 100 ft thick. The Younger Alluvium serves as an important hydrological component to the area because it is the channel between flood waters and the underlying aquifer. Due to its limited extent and water level declines the Younger Alluvium is not considered a significant aquifer (Bota & Mason, 2006).

Beneath the Younger Alluvium is the Fort Lowell Formation. Ranging from 300 to 400 ft. thick the formation is mostly unconsolidated composed of gravel, sands and clayey silt. Underlying the Fort Lowell Formation and making up the bottom of the upper basin-fill unit is the upper Tinja beds. The Tinja beds reach depths of several hundred feet. Both the Fort Lowell Formation and the upper Tinja beds contain similar hydrologic and geologic characteristics, the boundary between the two is based on color and mineral content. The sediments of the Fort Lowell Formation in the northern parts of the Upper Santa Cruz sub-basin are coarser-grained than in the central and southern areas of the basin. In the Avra Valley sub-basin the formation is coarse grained in the south and fine-grained in central and northern parts of the basin. The Fort Lowell Formation along with the upper Tinaja beds make up the most productive unit of the Tucson AMA regional aquifer. The Fort Lowell Formation is considered the main regional aquifer due to its vast saturated thickness throughout the Upper Santa Cruz sub-basin and the northern parts of the Avra Valley sub-basin. Groundwater in the formation usually occurs under unconfined conditions. The hydraulic conductivity and the storativity values for the Fort Lowell Formation vary greatly and are dependent upon sediment size, distribution, and degree of cementation. The hydraulic conductivity ranges from less than 5 to over 700 ft per day and transmissivity values range from 1,500 to 40,000 ft2 per day. Specific yields for the upper basin-fill unit range from 0.05 to 0.25 (Bota & Mason, 2006).

Figure 3, Hydrogeologic cross section of the Upper Santa Cruz Sub-basin, Tucson AMA, (Bota & Mason,2006).

Lower Basin-Fill Unit

The lower basin-fill unit of the Tucson Active Management Area is several thousand feet thick consisting of conglomerates, gravels, sands, silts, anhydritic clayey silts, and mudstones. In the Avra Valley sub-basin the lower basin-fill unit grades from a composition of sands, gravels, and conglomerates in the south to finer anhydritic clayey silts and mudstones to the north. In the northern part of the Upper Santa Cruz sub-basin the sediments are coarser grained and grade to finer deposits including finer evoprite deposits. The lower basin-fill unit is divided into the middle and lower Tinaja beds, the Pantano Formation and bedrock consisting of pre-Oligocene Igneous, Sedimentary, and Metamorphic rocks.

The middle and lower Tinaja beds are stretch to depths of several hundred to several thousand feet and are composed of gravels, conglomerates, to gypsiferous and anhydritic clayey silts. The underlying Pantano Formation consists of semi-consolidated to consolidated conglomerates sandstones, mudstones, and gypsiferous mudstones. It is estimated to be several thousand of feet thick. The middle an lower Tinja beds produce little amounts of water. The consolidation and amounts of fine grained sediment in the Upper Santa Cruz sub-basin result in little to moderate yield. Transmissivity and storage values vary greatly depending on location and composition. Hydraulic conductivity is estimated to range from 1 to over 200 ft per day and transmissivity values in the middle and lower Tinaja beds range from 1,000 to 40,000 ft2 per day. Specific yield values between 0.03 and 0.10 and storage coefficients for the lower basin-fill are estimated to be 0.0001 (Bota & Mason, 2006). The Pantano Formation is capeable of producing moderate amounts of water and posses transmissivity and storage values similar to the lower Tinaja beds.

Figure 4, Hydrogeologic cross section of the Avra Valley Sub-basin, Tucson AMA, (Bota & Mason, 2006).


Water Resources

In the Tucson AMA groundwater is a depleting resource. Groundwater in the area has been constantly pumped for over a century. To limit withdrawal the Groundwater Management Act was established in 1980. It regulates the amount of water that may be taken from the sub-basins allowing the aquifer to recharge, sustaining the groundwater resource.

The population of the Tucson AMA has increased from 518,438 in 1980 to 853,423 in 2000 increasing the demand of a water resource. The industrial sector of the Tucson Management Area has and continues to be the most demanding consumer of groundwater. The metal mines are the major users and in 2003 metal mines accounted for approximately 57% of the Tucson AMA's total industrial water demand (Arizona Water Atlas, pg. 398). Municipal water demand is the major demand sector of the area, but this demand is met by the Central Arizona Project (CAP). The CAP is an aqueduct that delivers Colorado River water to central and southern Arizona, Tucson, the largest metropolitan area in the Tucson AMA, along with the other cities in the area benefit from the CAP. Since 2001 the non-groundwater average annual use was the result of CAP use through recharge and recovery. The agriculture sector is also a beneficiary of the CAP, agriculture demand accounts for 30% percent of the water demand in the Tucson AMA (Arizona Water Atlas). In the Avra Valley sub-basin about 95% of groundwater has been used for agricultural irrigation and the remaining 5 percent used by the municipal and industrial sectors. Many years of withdrawl from the aquifer has not only depleted water but also caused ground subsidence in the area. Most subsidence is unnoticable but when subsidence occurs above uneven bedrock there is potential to create a fissure. These fissures have caused million of dollars in property and infrastructure damage.

Efforts to resupply the aquifer have on going since 1980. The CAP water diverted from the Colorado has been put towards recharge of the Tucson AMA since 2001 and effluent releases during the winter have created a relatively consistent surface water outflow component of the Tucson AMA that provides potential recharge to the aquifer. In urbanized areas, like Tucson, where rainwater infiltration is limited, projects have been established to encourage recharge beneath the concrete jungles, Figure 5.

Figure 5, in Tucson, a city without storm sewers, these "bump outs" are used to catch the monsoon rainfall and recharge the aquifer,(John Barker, 2009)


Groundwater Quality and Contamination

The water quality boundary varies within the Upper Santa Cruz sub-basin, from depths of 450 ft to over 1000 ft. Past these depths the water contains more than 500 milligrams per liter of dissolved solids (Bota & Mason, 2006). This boundary, in most areas, marks the seperation between the upper basin-fill unit and lower basin-fill unit. Both Upper Santa Cruz and Avra Valley sub-basins are largely unconfined so any contaminant that reaches the soil may eventually reach the aquifer. Parameters found in well samples exceeding or equal to drinking water standards include Arsenic, Beryllium, Cadimium, Chromium, Copper, Dissolved oxygen, Fluoride, mercury, manganese, nitrogen, phosphorus, lead, and nitrate/nitrite (Arizona Water Atlas, 2008). Effluent generation is also practiced in the area, most is discharged into the Santa Cruz River, stored at permitted recharge projects or used for golf course irrigation. Most of the contaminantion seen in Figure 6 represent leaking underground storage tanks (LUST). In the early 1980s a superfund site near the Tucson International Airport was identified after the public water supply near the area had been contaminated. Found in the water was trichloroethylene, dichloroethylene, tetrachlorothene, chloroform, and chromium. As of 1994 the EPA has concluded that no private drinking well users are drinking contaminated water (EPA, 2009). The site is still subject of remediation, plumes of dioxane and TCE are still present and in 2007 the EPA completed two extraction wells in the area.

When contaminants are found one of the first agencies to be called is the Arizona Department of Environmental Quality, which is the executive aqency mandated to enforce and administer the State's environmental laws and regulations. Depending on if the site is operated by the federal government, for example on an Air Force base the Air Force's environmental division head clean-up activties. Publicly the EPA will be governing the remediation in conjunction with regional agencies like the Arizona Environmental Contamination Prevention and Remediation Program, Arizona Department of Water Resources, Arizona Department of Health and Services and Pima County Regional Wastewater Reclamation Department.

Figure 6, contaminated sites of the Upper Santa Cruz and Avra Valley Sub-basins, (Arizona Water Atlas).


Conclusions

The Upper Santa Cruz and Avra Valley sub-basins have supplied the Tucson Active Management Area for over a century and this has led to serious cosequences for the semi-arid region. Following the regulations of the Groundwater Management Act the areas in Tucson have seen groundwater levels increase, but areas all around are still experiencing decline. With monumental population increases over the past couple of decades the demand for water is evergrowing and this resource is bound to reach the point of being unsustainable. With organizations, like the Nature Conservancy, starting projects to harvest rainwater by recharging the aquifer and also contributing to the municipal needs, may be the begining of a new generation for groundwater in the Tucson Active Management Area.


References

ADWR, 2009. Tucson AMA: overview. Arizona Department of Water Resources. http://www.azwater.gov/dwr/WaterManagement/Content/AMAs/TucsonAMA/TAMA_overview.htm [retrieved April 20, 2009].

Anderson, C.E., R.L. Carruth, and D. R. Pool, 2007. Land subsidence and aquifer-system compaction in the Tucson Active Management Area, South-Central Arizona, 1987-2005. U.S. Department of the Interior and the United States Geological Society, Scientific Investigations Report 2007-5190. http://pubs.usgs.gov/sir/2007/5190/sir5190.pdf [retrieved April 20, 2009].

Arizona Water Atlas, 2006. Section 8.5 Tucson AMA. http://www.azwater.gov/dwr/Content/Find_by_Program/Rural_Programs/content/water_atlas/v8/TUC_draft.pdf [retrieved April 20, 2009].

Bota, L. and D. A. Mason, 2006. Regional groundwater flow model of the Tucson Active Management Area Tucson, Arizona: Simulation and application. Arizona Department of Water Resources. Hydrology Division, Phoenix Arizona Modeling report No. 13. http://www.azwater.gov/dwr/content/find_by_program/hydrology/modeling_section/documents/Modeling_Report_13.pdf [retrieved April 20, 2009].

EPA, 2009. Tucson international airport area. Region 9: Superfund. U.S. Environmental Protection Agency. http://yosemite.epa.gov/r9/sfund/r9sfdocw.nsf/vwsoalphabetic/Tucson+International+Airport+Area?OpenDocument [retrieved April 20, 2009].

Poole, B., 2007. Monsoon little help in recharging aquifer. The Tucson Citizen, August 14, 2007. http://www.tucsoncitizen.com/ss/local/60163.php [retrieved April 20, 2009].

USGS, 2008. Ground water atlas of the United States Arizona, Colorado, New Mexico, Utah, HA 730-C. http://pubs.usgs.gov/ha/ha730/ch_c/C-text3.html [retrieved April 20, 2009].