U.S. Geological Survey
National Water-Quality Assessment Program

USGS Internal Access Only

ENVIRONMENTAL FRAMEWORK

Three main factors comprise the environmental framework for studying water quality in the area covered by the Basin and Range regional aquifer system: hydrogeologic setting, water use, and land use. The primary hydrogeologic setting and the one most common among the GRSL, NVBR, and CAZB study units in the Basin and Range regional aquifer system is the setting of unconsolidated basin-fill aquifers. This setting has been and will continue to be the focus of study for these NAWQA study units.

The Basin and Range regional aquifer system occurs within the Basin and Range physiographic province of Fenneman and Johnson (1946). The province covers about 200,000 square miles characterized by generally parallel, north- to northeast-trending mountain ranges separated by broad alluvial desert basins that are a result of Cenozoic extensional faulting. Topographic relief between the mountains and the adjacent basins is generally 1,000 to 5,000 feet. The Great Basin is in the northern Basin and Range, largely in Nevada and Utah . Streams within the Great Basin drain internally to terminal lakes, except for an area of southeastern Nevada that is tributary to the Colorado River .

The basin-fill aquifers are the principal source of ground water for domestic and municipal supply and for irrigated agriculture in the Basin and Range region. These uses of water account for most of the water use in this region. Most basins are topographically and hydraulically closed by low permeability bedrock or are only partially drained by perennial streams or by flow through basin-fill deposits or permeable bedrock. Contaminants reaching these aquifers are more likely to be retained and concentrated.

The source of recharge water to the basin-fill aquifers is an important attribute of the hydrogeologic framework that needs to be considered in the study design. Basin-fill deposits usually range in thickness from zero at margins of valleys to several thousand feet in valley lowlands. Recharge primarily occurs along the mountain front in the NVBR and GRSL study units whereas the primary source of recharge in the CAZB study unit is streambed recharge within the basins.

Carbonate rocks form a regional aquifer system in the eastern part of the Great Basin that includes the eastern part of Nevada and the western part of Utah . The carbonate-rock aquifers are comprised of thick sections of carbonate and clastic sedimentary rocks that constitute an extensive hydrogeologic unit. Ground water is transmitted mostly through faults and fractures. This aquifer system consists of several major flow systems (Prudic and others, 1995, p. D49) that are bounded by intruded or faulted impermeable rocks and by ground-water divides. Ground water generally flows from the mountains and basin-fill deposits to the underlying and adjacent carbonate rock, resulting in large regional flow systems that encompass numerous topographic basins and circulation depths of several thousand feet.

The carbonate-rock aquifers are not currently utilized for public supply because of their remoteness and depth to water. As population growth continues in arid areas with limited water supplies, these aquifers are likely to be drawn upon to meet future water needs in the region. Characterization of the water quantity and quality of the carbonate-rock aquifers would be advisable. NAWQA should make every effort to make available the resources to undertake this characterization. To encourage this effort, a proposal to do a Major Aquifer Study of the carbonate-rock aquifers in the Basin and Range has been forwarded to the NAWQA National Leadership Team. The Nevada and Utah Districts should also pursue studies of the carbonate-rock aquifers with state or other federal agencies in order to more fully characterize water-quality conditions within the Great Basin part of the Basin and Range region.

The other hydrogeologic settings in the Basin and Range regional aquifer system (basalt and volcanic-rock aquifers and sandstone aquifers) are not aerially extensive, do not supply much of the water used, with the exception of the Fallon area in Nevada , and are not common among the GRSL, NVBR, and CAZB study units.

Besides hydrogeology, land use and the corresponding water use are the other main factors in determining the environmental framework for the Basin and Range region.

Urban and agricultural land uses are the most important because of their effects on water quality. The parts of the Basin and Range region covered by NAWQA study units are rapidly urbanizing (greater than 21% increase in population in Phoenix , the fastest growing city in the Nation during the last 10 years) with major land areas changing from rural and agricultural use to urban. About half of the population in the Basin and Range area study units gets their water from wells, except the Las Vegas area, which receives most of its water from Lake Mead .

Most of the ground and surface water used in the Basin and Range region is applied as irrigation to crops or is used for public supply. Crops in the region generally must be irrigated with either surface or ground water or a combination of both. Ground water, surface water imported from the Colorado River by the Central Arizona Project, and treated sewage effluent are the main sources of irrigation water in the CAZB. Cotton is the main crop grown in the CAZB (365,000 acres in 1995). Multiple applications of fertilizers and pesticides used on cotton annually resulted in more than 67 million pounds of fertilizer and 2.9 billion pounds of pesticides and other chemicals being applied to cotton fields in 1995. Alfalfa and hay, the main crops grown in the Nevada and Utah parts of the Basin and Range, typically do not require fertilizer or large quantities of pesticides. The main impacts of agricultural land use on water quality in the region are from the runoff of dissolved salts from irrigated land to streams and lakes, and the accumulation of salts and chemicals in ground water. Mining also impacts ground-water quality but the impact is not as intensive or extensive as urban and agricultural land uses.