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| Scientific Investigations Report 2006-5100 |
Version 1.0
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In 1999, the U.S. Environmental Protection Agency began a program to protect the quality of ground water in areas other than ground-water protection areas. These other sensitive ground water areas (OSGWA) are areas that are not currently, but could eventually be, used as a source of drinking water. The OSGWA program specifically addresses existing wells that are used for underground injection of motor-vehicle waste. To help determine whether a well is in an OSGWA, the Nevada Division of Environmental Protection needs statewide information on depth to water and the water table, which partly control the susceptibility of ground water to contamination and contaminant transport. This report describes a study that used available maps and data to create statewide maps of water-table and depth-to-water contours and surfaces, assessed temporal changes in water-table levels, and characterized water-table gradients in selected areas of Nevada. |
A literature search of published water-table and depth-to-water contours produced maps of varying detail and scope in 104 reports published from 1948 to 2004. Where multiple maps covered the same area, criteria were used to select the most recent, detailed maps that covered the largest area and had plotted control points. These selection criteria resulted in water-table and depth-to-water contours that are based on data collected from 1947 to 2004 being selected from 39 reports. If not already available digitally, contours and control points were digitized from selected maps, entered into a geographic information system, and combined to make a statewide map of water-table contours. Water-table surfaces were made by using inverse distance weighting to estimate the water table between contours and then gridding the estimates. Depth-to-water surfaces were made by subtracting the water-table altitude from the land-surface altitude.
Water-table and depth-to-water surfaces were made for only 21 percent of Nevada because of a lack of information for 49 of 232 basins and for most consolidated-rock hydrogeologic units. Depth to water is commonly less than 50 feet beneath valley floors, 50 to 500 feet beneath alluvial fans, and more than 500 feet in some areas such as north-central and southern Nevada. In areas without water-table information, greasewood and mapped ground-water discharge areas are good indicators of depth to water less than 100 feet. The average difference between measured depth to water and depth to water estimated from surfaces was 90 feet. More recent and detailed information may be needed than that presented in this report to evaluate a specific site.
Temporal changes in water-table levels were evaluated for 1,981 wells with 10 or more years between the first depth-to-water measurement and last measurement made since 1990. The greatest increases in depth to water occurred where the first measurement was less than 200 feet, where the time between first and last measurements was 40 years or less, and for wells between 100 and 600 feet deep. These characteristics describe production wells where ground water is fairly shallow in recently developing areas such as the Las Vegas and Reno metropolitan areas. In basins with little pumping, 90 percent of the changes during the past 100 years are within ±20 feet, which is about the natural variation in the water table due to changes in the climate and recharge.
Gradients in unconsolidated sediments of the Great Basin are generally steep near mountain fronts, shallow beneath valley floors, and depend on variables such as the horizontal hydraulic conductivity of adjacent consolidated rocks and recharge. Gradients beneath alluvial fans and valley floors at 58 sites were correlated with selected variables to identify those variables that are statistically related. Water-table measurements at three sites were used to characterize the water table between the valley floor and consolidated rock.
Water-table gradients beneath alluvial fans had a median of 0.02, a mean of 0.04, and a standard deviation of 0.05. Gradients beneath valley floors had a median of 0.005, a mean of 0.03, and a standard deviation of 0.07. Information from this and other reports suggest that the average linear velocity of ground water is roughly 10 times faster beneath alluvial fans than beneath valley floors. Contaminants may travel about 10 times faster beneath alluvial fans than beneath valley floors, depending on the physical and chemical properties of the aquifer material and contaminant.
Abstract
Introduction
Purpose and Scope
Methods
Water-Table and Depth-to-Water Contours
Water-Table and Depth-to-Water Surfaces
Water-Table Gradients
Water-Table Levels
Water-Table Gradients
Summary
References Cited
Plates
| Figure 1. | Hydrographic areas and locations of wells used to evaluate the accuracy of depth-to-water surfaces in Nevada. |
| Figure 2. | Depth to water in spring 2001 in the Diamond Valley hydrographic area (153), Nevada. |
| Figure 3. | Differences in depth to water estimated from depth-to-water surface and water-table surfaces versus depth to water estimated from depth-to-water surfaces. |
| Figure 4. | Sites where water-table gradients were characterized. |
| Figure 5. | Variables measured at water-table-gradient sites. |
| Figure 6. | Depth to water estimated from depth-to-water surface versus measured depth to water. |
| Figure 7. | Depth-to-water surfaces and areas of predominantly greasewood. |
| Figure 8. | Ground-water discharge areas, areas of predominantly greasewood, and depth-to-water surfaces. |
| Figure 9. | Differences between last and first depth-to-water measurements versus A, well depth, B, years between measurements, and C, first depth-to-water measurement. |
| Figure 10. | Differences between last and first depth-to-water measurements grouped by hydrographic-area number. |
| Figure 11 | Ranges in water-table gradients A, beneath alluvial fans and valley floors and B, grouped by adjacent consolidated-rock hydrogeologic unit. |
| Figure 12. | Water-table gradients in unconsolidated sediments versus A, horizontal hydraulic conductivity of adjacent consolidated hydrogeologic unit, B, upgradient precipitation, and C, distance to consolidated-rock contact. |
| Figure 13. | Water-table and land-surface altitude versus distance from the well at the lowest land-surface altitude in Kyle Canyon, Pine Nut Creek, and Vicee Canyon. |
| Table 1. | Scoring system for selecting published water-table and depth-to-water contours used to determine water-table levels in Nevada. |
| Table 2. | Information used to characterize water-table gradients in Nevada. |
| Table 3. | Water-table measurements for Vicee Canyon, Pine Nut Creek, and Kyle Canyon, Nevada. |
Appendix 1. Reports with water-table contours for Nevada.
Plate 1. Map showing water-table contours in Nevada, 1947–2004.
Plate 2. Map showing water-table surfaces in Nevada, 1947–2004
Plate 3. Map showing depth-to-water surfaces in Nevada, 1947–2004.
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| U.S. Department of the Interior, U.S. Geological Survey
Persistent URL: http://pubs.water.usgs.gov/sir20065100 Page Contact Information: Publishing Service Center Last modified: Monday, September 18 2006, 02:00:54 PM |
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