NAWQA and AZ Water Quality

Has the Clean Water Act resulted in improvements in surface water and groundwater quality in Arizona? What is the fate of nitrates, pesticides, and other contaminants in the hydrologic system? What happens to nutrients in Arizona's effluent-dominated streams? These are just some of the questions that scientists at the Water Resources Division of the U.S. Geological Survey, Arizona District, want to answer using data collected for the National Water-Quality Assessment (NAWQA) program.

The long-term goals of the NAWQA program are to describe the current status and trends in the quality of a large, representative part of the nation's surface- and groundwater resources and to provide a sound, scientific understanding of the primary natural and human factors affecting the quality of these resources.

The Central Arizona Basins (CAZB) study area (see map) is one of 15 NAWQA study areas throughout the United States where 3-year intensive data-collection phases have begun; intensive data collection has been completed in 20 areas, and the intensive phase will begin in more areas in 1999. In addition to contributing to the current knowledge of local water-quality, the surface-water, groundwater, and ecological data collected during these studies will be combined to assess water quality at regional and national scales.

Surface Water Studied

The quality of surface water is evaluated in the NAWQA program using three interrelated components: (1) water-column studies, (2) bed-sediment and tissue studies, and (3) ecological studies. The occurrence and distribution of a variety of constituents will be determined with the combination of data collected during these studies.

The water-column studies provide an assessment of physical and chemical characteristics including suspended sediment, dissolved solids, major ions, trace elements, nutrients, organic carbon, and dissolved pesticides. Trace elements and hydrophobic organic contaminants are initially assessed in NAWQA by the bed sediment and tissue (fish and mollusks) studies. The ecological studies, which consist of collecting samples of fish, benthic invertebrate, and algal aquatic communities and characterizations of riparian and instream habitat conditions, provide an evaluation of the effects of the physical and chemical characteristics of water and the hydrologic environment on aquatic biota.

A basic fixed-site network (see map) of 11 sites has been established on rivers in the CAZB study area. Of the 11 sites, five are integrator sites representing water quality of large heterogeneous basins that are affected by complex combinations of land use settings, point sources, and natural influences. Six sites are indicators of water quality for basins with relatively homogeneous land use and physiographic conditions such as urban and agricultural land uses, effluent-dominated streams, and reference (minimally impacted) sites.

During the first year (1996) of data collection at these sites, the water column will be sampled monthly and during periods of high flow, bed sediment and tissue will be sampled, and an ecological assessment of a stream reach associated with each site will be completed. During the second year, the water column will be intensively sampled (at least twice per month) at two sites at the downstream end of the study area (intensive fixed sites) to determine the occurrence and seasonal variations in pesticide concentrations in addition to regular monthly sampling at the remaining nine sites. Intensive ecological assessments also will be completed on the San Pedro River at Charleston and on West Clear Creek near the Verde River. Intensive data collection during the third year will include additional bed-sediment and tissue sampling and special synoptic studies (short-term investigations to answer specific ecological or water-quality questions).

Groundwater Evaluated

The focus for the groundwater study is on assessing water-quality of major aquifers with emphasis on the quality of recently recharged groundwater associated with present and recent human activities. The three primary study components are: 1) study-unit surveys (or sub-unit surveys) to assess current water quality by sampling existing wells; 2) land-use studies using observation wells and existing wells to assess the quality of recently recharged groundwater associated with regionally extensive land uses (urban, agricultural, rangeland) and hydrogeologic conditions; and 3) flowpath studies using observation wells to determine specific relations among land-use practices, groundwater flow, contaminant occurrence and transport, and surface water and groundwater interaction.

The CAZB groundwater studies include two major areas of water use and population—west Salt River Valley (Phoenix) and the upper Santa Cruz basin (Tucson). For comparison, the Sierra Vista basin also will be studied as a relatively less impacted groundwater basin. In the west Salt River Valley, the results of three land-use studies—urban, agricultural, and the transition zone between the urban and agricultural areas will be combined into a sub-unit survey. Ninety shallow wells, primarily small-yield domestic wells, nine wells that tap deeper parts of the aquifer, and five wells that are in natural desert scrub areas, will be sampled. In the upper Santa Cruz basin, small yield domestic wells and observation wells will be the focus of sampling efforts for the sub-unit surveys and land-use studies.

The U.S.G.S will sample 15 wells and the Arizona Department of Environmental Quality will sample an additional 15 wells in a cooperative effort for the sub-unit survey in the Sierra Vista basin. No flowpath studies are planned in the CAZB study area for the current intensive data-collection phase. For groundwater studies, major ions, nutrients, pesticides, and volatile organic compounds will be analyzed in addition to trace elements, radon, and stable isotopes of hydrogen and oxygen.

Analysis of Results

After quality assurance reviews, analytical data for surface water, groundwater, and aquatic ecology will be included in the Water Resources Data book for Arizona published annually by the U.S.G.S. Future plans include providing access to all field and laboratory data and digital map coverages over the Internet. Results and conclusions as well as retrospective data will be published on an on-going basis in journals, newspapers, conference proceedings, and U.S.G.S. publications.

More information about the NAWQA program can be obtained on the National NAWQA home page at http://wwwrvares.er.usgs.gov/nawqa/nawqa_home.html. The U.S.G.S. fact sheet for the CAZB NAWQA is on the Arizona District home page at http://wwwdaztcn.wr.usgs.gov.