It is important to understand how natural processes affect ground-water quality in order to identify the effects of urban and agricultural development under similar hydrogeologic conditions. In the CAZB, the majority of ground-water basins do not have significant urban or agricultural development. The ground-water quality in these basins is primarily a product of natural processes such as the interaction of ground water with rocks and sediment in the basins (Robertson, 1991).
Natural sources of dissolved-solids and nitrate can control ground-water quality in basins with minimal urban development. Specific-conductance values (an indirect measure of the dissolved-solids concentration) and nitrate concentrations for ground water in basins with minimal urban development increase northwestward from southeastern Arizona toward the central part of the State (figs. 16 and 17). The increasing specific-conductance values can be attributed to a corresponding increase in evaporite deposits in basin sediments from southeast to northwest (Gellenbeck and Coes, 1999). Evaporite deposits in the basins contain minerals such as halite (salt) and gypsum that can be easily dissolved in ground water. (Robertson, 1991). The increasing nitrate concentrations can be largely attributed to natural sources; however, human activities such as agriculture can be a source in some basins. In some locations in the CAZB, high nitrate concentrations in ground water reported prior to any agricultural or urban development indicate that natural sources of nitrate are present in some basins (Hem, 1985; Robertson, 1991; Gellenbeck, 1994; Gellenbeck and Coes, 1999). Dissolution of evaporite deposits, decay of buried organic matter, precipitation, weathering of rocks and soils, and fixation by microorganisms are just a few of the possible sources of naturally occurring nitrate in ground water.
Concentrations of arsenic, fluoride, and molybdenum exceeded drinking-water standards in samples from major aquifers. The median arsenic concentration in ground water for the three CAZB basins sampled was 4 µg/L. One sample from the Upper Santa Cruz Basin and one sample from the West Salt River Valley exceeded the current MCL for arsenic of 50 µg/L; however, a new, lower standard of 5 µg/L has been proposed by the USEPA because of the cancer risk posed by arsenic in drinking water (U.S. Environmental Protection Agency, rev. August 25, 2000). When arsenic concentrations in ground water sampled in the CAZB are compared to the proposed standard, more than 50 percent of samples from aquifers in West Salt River Valley that are used for drinking water exceed 5 µg/L. Seventeen percent of sampels in the Upper Santa Cruz Basin and 10.5 percent of samples in the Sierra Vista subbasin exceed 5 µg/L. The USEPA may not settle on 5 µg/L, but the new standard is likely to be significantly lower than the current MCL.
The median concentration of fluoride was 0.5 µg/L; about 2 percent of the smaples exceeded the current MCL for fluoride of 4 µg/L. The median concentration of molybdenum was 3 µg/L; about 1 percent to the samples exceeded the current lifetime health advisory for molybdenum of 40 µg/L established by the USEPA.
Radon and uranium are detected in most ground-water samples. Radon is a colorless and odorless radioactive gas that is carried in the water pumped from wells (fig.19) and released to indoor air by activities such as cooking and showering. Breathing radon increases the risk of lung cancer (U.S. Environmental Protection Agency, rev. October 18, 1999). Radon is naturally formed in rocks and soils from the radioactive decay of radium, an intermediate product in the uranium decay process. In the CAZB Study Unit, radon was present in 100 percent of the samples, and uranium was detected in 90 percent of the samples. The median concentrations for radon and uranium were 584 picocuries per liter and 3 micrograms per liter, respectively. Currently (2000), there are no USEPA MCLs for radon and uranium; however, proposed MCLs could result in increased costs for water suppliers to treat drinking water for these constituents or find alternate supplies. Additional costs would probably be passed on to the water user (see information on proposed standards for arsenic, radon, and uranium on p. 20).
The contamination of major aquifers is largely controlled by hydrology and land use (U.S. Geological Survey, 1999). In the CAZB Study Unit, deep ground water that was recharged prior to 1953 typically has not been affected by human activities (see p. 18). In areas with recent recharge (after 1953), ground water is more likely to be contaminated by nutrients and man-made chemicals associated with urban and agricultural land uses.
Ground-water quality deteriorates in irrigated areas. Irrigation water that seeps downward is a principal source of ground-water recharge in irrigated areas of the CAZB. Dissolved-solids concentrations in seepage can be as much as five times those in the original irrigation water (Bouwer, 1990) because of concentration by evaporation and plant use (see p. 9). The greater the dissolved-solids concentration in the applied irrigation water, the greater the concentration in the seepage moving downward to the ground water.
To determine the effects of irrigated agriculture on shallow ground-water quality, nine monitoring wells were drilled and sampled in the southwestern part of the West Salt River Valley (see “Study Unit Design,” p. 26). Because the average depth to ground water in the nine wells is 32 feet (table 1) compared to 230 feet for wells sampled basinwide, irrigation seepage does not have to travel far to reach the shallow ground water in the agricultural area. Sources of irrigation water in this area include treated sewage effluent, water from the Salt River and CAP canal, irrigation return flows, and ground water. Dissolved-solids concentrations of these sources range from about 900 mg/L for treated sewage effluent (Tadayon and others, 1998) to 650 mg/L for CAP water and 470 mg/L for Salt River water (Salt River Project, 1997).
The median dissolved-solids concentration in water from the nine shallow wells exceeded 3,000 mg/L (table 1). In addition, the effects of nitrate from fertilizer applications and reuse of irrigation return flows were evident from the median nitrate concentration that was nearly twice the MCL of 10 mg/L (table 1).
The highest concentrations of nitrate and dissolved solids were in shallow ground water beneath an irrigated agricultural area. Shallow ground water from the agricultural land-use study area in the West Salt River Valley had median concentrations of nitrate (19 mg/L) and dissolved solids (3,050 mg/L) that exceeded the USEPA MCL and SMCL, respectively (table 1). Nitrate and dissolved solids from irrigation and agricultural practices are accumulating in shallow ground water (see p. 9 and 11). The shallow ground water in this area is not used for drinking water, and clay beds reduce the likelihood of contamination of the aquifers below that are used for drinking water (see p. 22).
Deeper ground water from urban, rangeland, and agricultural areas in other parts of the West Salt River Valley had a median nitrate concentration that was less than the MCL of 10 mg/L; however, the median concentration of dissolved solids exceeded the SMCL of 500 mg/L (table 1). Median concentrations of nitrate from the Upper Santa Cruz Basin and the Sierra Vista subbasin also were less than the MCL, and median concentrations of dissolved solids were less than the SMCL (table 1).
Occurrence and distribution of pesticides in ground water in the CAZB reflect both agricultural and urban land uses. Ten pesticides were detected in shallow ground water from the agricultural land-use study area in the West Salt River Valley, west of Phoenix (fig. 22). In other parts of the West Salt River Valley, consisting of agricultural, urban, and rangeland areas, eight pesticides were detected in ground water. Five pesticides were detected in ground water from the Upper Santa Cruz Basin, where there is a mixture of land-use types, but 60 percent of the basin is undeveloped rangeland (Coes and others, 2000). In the Sierra Vista subbasin, where urban and agricultural land uses are minimal (3.3 percent of basin; Coes and others, 1999) and have been minimal in the past, no pesticides were detected in ground-water samples. During 1996–98, the largest quantities of pesticides used among the three basins were for agriculture in the West Salt River Valley (Ken Agnew, University of Arizona, Pesticide Information and Training Office, written commun., 1999).
Most of the pesticides detected in ground water in the CAZB were herbicides used to control unwanted plants in urban and agricultural areas (fig. 23). Herbicide use in urban areas is indicated by detections of simazine and prometon in the West Salt River Valley and prometon and 2,4-D in the Upper Santa Cruz Basin. These herbicides are used primarily in nonagricultural areas (U.S. Geological Survey, 1999). Detections of atrazine and deethylatrazine (a breakdown product of atrazine) in the West Salt River Valley and the Upper Santa Cruz Basin are an indication that herbicides used in areas of present and historical agriculture are affecting ground-water quality. Atrazine is one of the most heavily used herbicides in agricultural areas in the United States (U.S. Geological Survey, 1999).
Concentrations of pesticides in ground water did not exceed drinking-water standards or guidelines. Although deethylatrazine, simazine, prometon, DDE, atrazine, and diuron were detected in more than 30 percent of the ground-water samples from the agricultural land-use study area of the West Salt River Valley, none of the concentrations exceeded drinking-water standards or guidelines. Similarly, pesticides detected in ground water from the basinwide sampling in the West Salt River Valley during 1996–98 did not exceed drinking-water standards or guidelines.
DDE was detected in 10 (56 percent) of the shallow ground-water samples from the agricultural land-use study area in the West Salt River Valley. Detections of DDE in this area are the result of the persistence of this insecticide breakdown product in the environment and the physical characteristics of the ground-water system in the area. In particular, the shallow depth to ground water in the agricultural land-use study area means that irrigation seepage and recharge, containing pesticides and their breakdown products, do not have to travel far to contaminate the ground water. Clay layers impede the movement of pesticides into the deeper aquifers in the area. The soils in the agricultural area have been identified as a source of DDE for the ground water (Brown, 1993). The only detection of DDE in the West Salt River Valley outside of the agricultural area was in a sample from the northern part of the Phoenix metropolitan area. DDE was not detected in samples from the Upper Santa Cruz Basin or the Sierra Vista subbasin.The large depths to ground water and small amounts of DDT used in most of the West Salt River Valley, the Upper Santa Cruz Basin, and the Sierra Vista subbasin limit the potential for introduction of DDE to the ground water.
Detections of multiple pesticides indicate the complexity of contamination from land-surface activities. No standards or guidelines currently exist for mixtures of pesticides in drinking water because their effect on human health is not known (U.S. Geological Survey, 1999). All 9 wells in the agricultural land-use study area had 3 or more pesticides detected, whereas only 3 of the 35 wells sampled basinwide in the West Salt River Valley had 3 or more pesticides detected, and none of the wells in the Upper Santa Cruz Basin had 3 or more pesticides detected (fig. 24). No pesticides were detected in the Sierra Vista subbasin.
Volatile organic compounds (VOCs), including gasoline compounds, solvents, and refrigerants, have been identified as a major concern for ground-water contamination in Arizona (Marsh, 1994). Leaking underground storage tanks and disposal of solvents have been linked to most of the documented cases of ground-water contamination by VOCs. Electronic- and aerospace-manufacturing facilities use solvents for degreasing and are known to be sources of some of the largest VOC contamination problems in Arizona. Disposal of solvents from these types of facilities has occurred since the 1950s (Marsh, 1994). Dry-cleaning facilities also have been identified as sources of recent ground-water contamination by VOCs. Some municipal supply wells in the urban areas of Phoenix and Tucson are no longer used because of contamination by VOCs (Marsh, 1994).
VOCs were detected in ground water from all three basins sampled during 1996–98 (fig. 25). Of the 96 samples collected, 33 (34 percent) contained trichloromethane, 24 (24 percent) contained 1,2,4-trimethylbenzene, and 20 (21 percent) contained tetrachloroethene (otherwise known as perchloroethylene, PCE, a solvent commonly used in dry cleaning). Only two VOC detections exceeded drinking-water regulations—PCE (5.48 µg/L) in the Upper Santa Cruz Basin and 1,2-dibromoethane (0.080 µg/L) in shallow ground water in the agricultural area of the West Salt River Valley.
Shallow ground water from the nine wells in the agricultural land-use study area had the largest number of VOC detections (35). Ground water from the other 35 wells in the West Salt River Valley had 32 detections. The Upper Santa Cruz Basin (18) and the Sierra Vista subbasin (13) had fewer detections. The larger area of urban land use in the West Salt River Valley appears to be the reason for the greater number of detections there than in the other basins sampled.
Three wells that had five or more VOCs detected in ground water were located in the metropolitan area of Phoenix in the West Salt River Valley. The VOCs detected in these wells were either refrigerants, solvents and chemicals used to make solvents, or gasoline additives. These detections are typical of detections found in small-capacity wells in the metropolitan Phoenix area (Marsh, 1994). Combinations of solvents and gasoline additives are often detected in ground water because their use is widespread, not necessarily because they are from the same source (Squillace and others, 1999).
Detections of VOCs in ground water in the relatively undeveloped Sierra Vista subbasin indicate that ground water in localized areas of the subbasin may be affected by human activities. These detections are not widespread; therefore, the effects of human activity on present-day ground-water quality are not considered significant for the entire subbasin. These detections are an “early warning” of what could occur in the future in a basin that is presently considered minimally affected by urban activities.
|AccessibilityFOIAPrivacyPolicies and Notices|
|U.S. Department of the Interior, U.S. Geological Survey
Page Contact Information: Contact USGS
Last modified: Wednesday, November 23 2016, 12:20:49 PM