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<br />Background
<br />
<br />Both natural and human activities have had an effect on ground-water flow and evapotranspiration
<br />in the vicinity of the Whitney area, Naturally occurring salts in the shallow aquifer system. coupled with
<br />rapid population growth and the resulting increases in water use, have caused the need for salt-reduction
<br />strategies, These strategies are aimed at decreasing the dissolved-solids content of ground water entering
<br />Las Vegas Wash (fig, IB). which flows in turn to Lake Mead, The demand for water in Las Vegas Valley
<br />has increased from that obtained from a few domestic wells in the early part of the century to an annual
<br />use of more than 200,000 acre-ft (about two-thirds of which is imported from Lake Mead), Pumping of
<br />ground water has led to significant water-level declines in the central part of the valley (T~rry Katzer, !.As
<br />Vegas Valley Water District, oral commun.. 1986). r
<br />
<br />In contrast to the continued ground-water level declines in the central part of the valley, levels have
<br />been rising slightly in the southeast part (Harrill, 1976. p. 23; Morgan and Dettinger. in press). Increased
<br />flow through the salt-laden shallow alluvial deposits, coupled with industrial discharge and large volumes
<br />of treated effluent, is responsible for the increased dissolved-solids load entering Las Vegas Wash. Intense
<br />irrigation of lawns and golf courses, urban runoff. and discharge from sewage-treatment facilities (Harrill,
<br />1976, p. 26) have turned Las Vegas Wash from an ephemeral stream to a perennial stream with dis9harge
<br />averaging 110 fi3/s (Frisbie and others, 1985, p. 58). During stonns, flows in the wash can exceed 1,000
<br />fI3/S; a peak discharge of about 6,500 ft3/s has been estimated just easI of the Whitney area (Patrick A.
<br />Glancy, U.S, Geological Survey, oral commun., 1987). Figure 2 shows the relation between population
<br />growth in Las Vegas Valley and the total outflow to Las Vegas Wash. In this report, Las Vegas Wash
<br />refers specifically to the wasteway channel excavated in 1983 that constrained the effluent discharge from
<br />sewage-treatment facilities. Prior to 1983, treated effluent flowed in small channels throughout the densely
<br />vegetated flood plain that historically has also been referred to as Las Vegas Wash,
<br />
<br />Erosion and headcutting due to increased discharges to the wash, as well as to runoff from major
<br />stonns. have lowered the level of the channel by about 15 fi in the eastern part of the Whitney area.
<br />Ground-water levels have declined near the wash where the channel has been lowered. Vegetation types
<br />have changed in response to the declining water levels. Hydrophytic vegetation, {swamp and marsh types,
<br />such as reeds and cattails) dominates areas where the water levels are within 2 ft of land surface (fig, 3),
<br />whereas phreatophytes (mainly saltgrass and salt cedar) generally dominate elsewhere. As headcutting
<br />continues to lower water levels in the vicinity of the wash, hydrophyte-dominated vegetation tends to die
<br />off and re-establish itself in areas farther upgradient where water levels remain shallow.
<br />
<br />The increased dissolved-solids load entering Las Vegas Wash as a result of increased discharge has
<br />led to the development, by the U.S. Bureau of Reclamation, of several alternative strategies for reducing
<br />the dissolved-solids load that seeps into the wash by way of the shallow ground-water inflow. The salinity-
<br />control effort is part of the project authorized by the Colorado River Basin Salinity Control Act of 1974
<br />(Public Law 93-320). Under Title IT of the Act, a provision for a program to control the salinity of the
<br />Colorado River upstream from Imperial Dam was implemented in response to the Federal Water Pollution
<br />Control Act (public Law 92-500). One such strategy involves constructing a series of detention basins
<br />adjacent to Las Vegas Wash. These basins would consist of a surface impoundment. or dike, overlying a
<br />vertical slurry wall that would penetrate the entire thickness of the aquifer. The detention basins were
<br />intended to reduce the dissolved-solids content of groundwater seeping into the wash by impounding
<br />deeper, more saline water, As ground water from upgradient areas flowed into the detention basin,
<br />theoretically, it would become increasingly fresher because it would no longer be in contact with the Muddy
<br />Creek Fonnation at the base of the aquifer. The Muddy Creek Fonnation is known to contain significanI
<br />amounts of gypsum and other soluble salts in some areas (Bohannon, 1984. p. 56), Thus, over time. the
<br />saline, more dense ground water near the bottom of the aquifer theoretically would become a stagnant pool,
<br />while fresher, less dense water would flow in the upper part of the aquifer above the more saline water.
<br />The fresher water would be allowed to leave the detention-basin area through a lined channel near the top
<br />of the slurry wall into an adjacent, down-gradient detention basin. where this process would be repeated,
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