Hydrogeology and Hydrologic Landscape Regions of Nevada
By Douglas K. Maurer, Thomas J. Lopes, Rose L. Medina, and J. LaRue Smith
Report availability: Portable Document Format (PDF).
In 1999, the U.S. Environmental Protection Agency initiated a rule to protect ground water in areas other than source-water protection areas. These other sensitive ground water areas (OSGWAs) are aquifers 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. If the injection well is in a ground-water protection area or an OSGWA, well owners must either close the well or apply for a permit. The Nevada Division of Environmental Protection will evaluate site-specific information and determine if the aquifer associated with a permit application is susceptible to contamination. A basic part of evaluating OSGWAs is characterizing the hydrogeology of aquifer systems including the lithology, hydrologic properties, soil permeability, and faulting, which partly control the susceptibility of ground water to contamination. Detailed studies that evaluate ground-water susceptibility are not practical in a largely unpopulated State like Nevada. However, existing and new information could be extrapolated to other areas of the State if there is an objective framework to transfer the information. The concept of hydrologic landscape regions, which identify areas with similar hydrologic characteristics, provides this framework. This report describes the hydrogeology and hydrologic landscape regions of Nevada.
Consolidated rocks that form mountain ranges and unconsolidated sediments that fill the basins between the ranges are grouped into hydrogeologic units having similar lithology and assumed to have similar hydrologic properties. Consolidated rocks and unconsolidated sediments are the two major hydrogeologic units and comprise 51 and 49 percent of the State, respectively. Consolidated rocks are subdivided into 8 hydrogeologic units. In approximate order of decreasing horizontal hydraulic conductivity, consolidated-rock hydrogeologic units consist of: (1) carbonate rocks, Quaternary to Tertiary age; (2) basaltic, (3) rhyolitic, and (4) andesitic volcanic flows; (5) volcanic breccias, tuffs, and volcanic rocks older than Tertiary age; (6) intrusive and metamorphic rocks; (7) consolidated and semi-consolidated tuffaceous rocks and sediments; and (8) clastic rocks consisting of sandstone and siltstone. Unconsolidated sediments are subdivided into four hydrogeologic units on the basis of flow regime, topographic slope, and mapped stream channels. The four units are (1) alluvial slopes, (2) valley floors, (3) fluvial deposits, and (4) playas.
Soil permeability was grouped into five descriptive categories ranging from very high to very low, which generally correspond to mapped geomorphic features such as playas and alluvial slopes. In general, soil permeability is low to moderate in northern, northeastern, and eastern Nevada and high to very high in western, southwestern, and southern Nevada. Within a particular basin, soil permeability decreases downslope from the bedrock contact. The type of parent rock, climate, and streamflow velocities are factors that likely cause these spatial patterns.
Faults in unconsolidated sediments usually are barriers to ground-water flow. In consolidated rocks, permeability and ground-water flow is reduced in directions normal to the fault zone and increased in directions parallel to the fault zone. With time, mineral precipitation may seal fractures in consolidated rocks, reducing the permeability. However, continued movement along the fault may form new fractures, resulting in a fault alternating from a zone of preferred flow to a flow barrier during geologic time. The effect of faults on ground-water flow at a particular location is difficult to determine without a site- specific investigation.
Hydrologic landscape regions were delineated by overlaying a grid of 100-foot (30-meter) cells over the State, estimating the value of five variables for each cell, and conducting cluster analysis to assign each cell to a region such that each region is fairly homogeneous and distinct from other regions. The five variables include mean annual precipitation, soil permeability, slope, aspect, and hydrogeologic unit. The number of clusters was increased until each region had only one category of hydrogeologic unit, which resulted in 16 regions. Most of Nevada has moderate (8 to 16 inches) precipitation (58 percent), low (less than 5 feet per day) soil permeability (50.1 percent), moderate (3 to 25 percent) slope (58.1 percent), non-northerly aspect (88.7 percent), and hydrogeologic units with high (greater than 40 feet per day) horizontal hydraulic conductivity (59.8 percent).
Regions with moderate to high precipitation (equal to or greater than 8 inches per year), moderate to high soil permeability (greater than 5 feet per day), low to moderate slope (equal to or less than 25 percent), and high hydraulic conductivity could have greater recharge rates and be more susceptible to contamination than other regions. These characteristics describe hydrologic landscape regions 9, 14 and 15, which comprises 27.1 percent of Nevada. These hydrologic landscape regions represent valley floors and alluvial slopes of most basins in eastern and central Nevada. In the most populated areas of Nevada, hydrologic landscape regions 9, 10, 14 and 15 comprise a large portion of Las Vegas, Reno, Carson City, Minden, Gardnerville, and Spanish Springs. These areas could be most vulnerable due to their hydrologic characteristics and contaminants associated with urban land-use practices.
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