Ambient Groundwater Quality of tlie Douglas Basin: An ADEQ 1995-1996 Baseline Study I. Introduction »Ragstaff The Douglas Groundwater Basin (DGB) is located in southeastern Arizona (Figure 1). It is a picturesque broad alluvial valley surrounded by rugged mountain ranges. This factsheet is based on a study conducted in 1995- 1996 by the Arizona Department of Enviroimiental Quality (ADEQ) and Douglas summarizes a comprehensive regional Groundwater groundwater quality report (1). Basin The DGB was chosen for study for the following reasons: • Residents predominantly rely upon groundwater for their water needs. • There is a history of management decrees designed to increase groundwater sustainability (2). • The basin extends into Mexico, making groundwater issues an international concern. II. Background The DGB consists of the southern portion of the Sulphur Springs Valley, a northwest-southeast trending trough that extends through southeastern Arizona into Mexico. Covering 950 square miles, the DGB is roughly 15 miles wide and 35 miles long. The boundaries of the DGB include the Swisshelm (Figure 2), Pedregosa, and Perilla Mountains to the east, the Mule and Dragoon Mountains to the west, and a series of small ridges and buttes to * AguaPrieta the north (Figure 1). Although the Mexico DGB extends south hydrologically into Figure 1. Infrared satellite image of the Douglas Groundwater Basin (DGB) taken in June, 1993. Mexico, the international border serves Irrigated farmland is shown in bright red in the central parts of the basin, grasslands and mountain as the southern groundwater divide for areas appear in both blue and brown. The inset map shows the location of the DGB within Arizona. this report. The principal landowners in the DGB DGB's principal water-bearing unit and are private entities and the state of consists of valley basin-fill deposits. Arizona. Bisbee, Douglas, Elfrida, and The upper layer of these deposits McNeal are the major communities contains unconsolidated to poorly within the DGB. Bisbee and Douglas consolidated gravel, sand, and silt. formerly served as copper mining and These alluvial lenses are largely ore processing centers, respectively and intercoimected to form a single aquifer; currently are government, retail, and however, there are considerable spatial service centers. Elfrida and McNeal are differences in water transmissivity, agriculturally-oriented small towns storage, and hydraulic conductivity (3). located near the center of the basin. III. Hydrology "Study results suggest that most groundwater in the DGB is This study examined the water quality suitable for domestic purposes," Figure 2. The Swisshelm Mountains stand of two aquifers: the alluvial and the out across the DGB's broad alluvial valley. hardrock. The alluvial aquifer is the The hardrock aquifer is found in mountainous areas and includes significant expanses of sedimentary Site Exceedances rock with lesser amounts of volcanic, + Primary & Secondary MCL granitic, and metamorphic rock (Figure • Secondary MCL 3). Limited amounts of groundwater are contained in the hardrock aquifer, which O None is most productive in fractured sedimentary and granitic rock (3). Rocl< Types • Recent Alluvium Groundwater generally flows toward the • Sedimentary center of the valley and then south toward Mexico (3). The main drainage • Granitic is Whitewater Draw, an ephemeral • Volcanic watercourse that flows from the • Metamorphic Swisshelm Mountains through the center of the valley before exiting the basin near the city of Douglas. The majority of groundwater pumped in the DGB is used for irrigation; lesser amounts are withdrawn for municipal, domestic, stock, and mining purposes. The sustainable use of groundwater resources for irrigation has historically been a concern in the DGB. In response to annual pumping increases from 5,000 acre-feet (af) in 1938 to 110,000 af m 1964, the State designated most alluvial portions of the basm a Critical Groundwater Area (CGA) in 1965. This prohibited drilling of new irrigation wells within the CGA (2). The CGA evolved into an Irrigation Non-Expansion Area (INA) in 1980 A Agua Prieta with the passage of the Arizona Groundwater Management Act. The Figure 3. Locations of 51 sample sites, including 3 sites exceeding health-based water quality INA limited the acreage that could be standards and 16 sites exceeding aesthetics-based water quality guidelines, are shown in this map. irrigated. During the 1980s, pumping was reduced because of rising energy acquired through unpurities in filters cosmetic or laundry effects, but is not costs and by 1990, annual groundwater during sample processing. considered a health concern. Sixteen of pumpage was only 43,000 af (2). the 51 sites sampled had parameters V. Water Quality Sampling Results exceeding a Secondary MCL (Figure IV. Methods of Investigation 3). Exceedances included fluoride and The collected groundwater quality data total dissolved solids (TDS) (eight sites This study was conducted by ADEQ were compared with U.S. each), pH and sulfate (two sites each), Ambient Groundwater Monitoring staff. Environmental Protection Agency and chloride, iron, and manganese (one This program is based on the legislative (USEPA) Safe Drinking Water quality site each). mandate in Arizona Revised Statutes standards. Primary Maximum §49-225. To characterize regional Contaminant Levels (MCLs) are None of the 152 pesticides or related groundwater quality, 51 sites were enforceable, health-based water quality degradation products on the ADEQ Groundwater Protection List were sampled: 29 grid-based random sites standards that public systems must meet detected at the two sites sampled. One and 22 targeted sites. Samples were when supplying water to their site had a VOC detection of chloroform, collected at all sites for inorganic customers. Primary MCLs are based on a common byproduct of chlorination. constituents (physical characteristics, a lifetime daily consumption of two liters of water. Three of the 51 sites general mineral parameters, nutrients, sampled had parameter levels exceeding These results suggest that groundwater and trace elements). At selected sites, a Primary MCL: arsenic, beryllium, and in die DGB generally supports drinking- samples were also collected for Volatile nitrate each exceeded their respective water uses and seems largely suitable Organic Compounds (V0Cs)(12 sites). Primary MCLs at one site each (Figure for domestic purposes. Groundwater Protection List (GWPL) 3). pesticides (7 sites), and radiochemistry (6 sites) analysis. Sampling protocol USEPA Secondary MCLs are followed the ADEQ Quality Assurance ''Fluoride and TDS each exceeded unenforceable, aesthetics-based water Project Plan (QAPP). The effects of their respective aesthetics-based quality guidelines for public water sampling equipment and procedures on water quality guidelines at 16 systems. Water with parameters data results were not considered percent of the sample sites." exceeding guidelines may be unpleasant significant according to quality control to drink and/or create unwanted data, except for antimony contamination VI. Groundwater Composition control of fluoride through precipitation of the mineral fluorite. Since fluorite In general, the DGB has slightly, solubility is not often attained in alkaline, fresh groundwater. Sample groundwater, hydroxyl ion exchange is sites in both the hardrock and alluvial also an important fluoride control. The aquifers typically exhibited a calcium- exchange of fluoride and hydroxyl ions bicarbonate chemistry. In the alluvial typically increases downgradient as pH aquifer, sodium-bicarbonate, sodium- values rise. sulfate, and calcium-sulfate sites were also present. Groundwater was VII. Groundwater Quality Patterns predominantly moderately hard and Levels of bicarbonate, calcium, hard, though some sites had soft and hardness (Figure 5), magnesium, very hard water. Soft water was found sulfate, and turbidity were significantly in the extreme northeast and south- higher in the hardrock aquifer than the central basin areas. Very hard and hard alluvial aquifer. The opposite trend groundwater was found in hardrock occurred with temperature and pH 15 20 25 30 areas and in the center of the basin near (Kruskal-Wallis test, p < 0.05). Temperature (Celsius) the town of McNeal. Nitrate (as nitrogen) was found at 41 Figure 6. Temperature generally increases percent of sample sites at levels over 3 with increasing groundwater depth below milligrams per liter (mg/1), which may land surface (regression analysis, p< 0.01). indicate impacts from human activities. Areas widi the highest nitrate levels levels that were all, nonetiieless, below include the intensively-farmed areas the 10 mg/1 Primary MCL. near Elfrida and in the foothills of both the Dragoon and Mule Mountains. Mine tailings appear to be contributing to elevated groundwater sulfate levels Most trace elements such as aluminum, found along Mule Gulch, downgradient beryllium, cadmium, chromium, copper, of the town of Bisbee (Figure 7). iron, lead, manganese, mercury, Sulfides in these tailings are oxidized to selenium, silver, and thallium were yield sulfate that is soluble in water. A rarely detected. Arsenic, barium, site near where Mule Gulch enters the Alluvial Hardrock fluoride, and zinc were the only trace Aquifer alluvium of Sulphur Springs Valley had elements detected at more than ten a sulfate level of 1330 mg/1, exceeding Figure 5. Hardness levels are higher in percent of sites at levels above Arizona the