Water is the most basic natural resource. Its availability controls not only the distribution and health of plant and animal communities but also the extent and ultimate longevity of human cultures and economies in a given area. Throughout much of the western United States and particularly in the arid regions of Nevada and Utah, knowledgeable use of water is required for long-term occupation and economic growth as well as the health and productivity of the natural environment. The primary water resource in most arid areas lies below Earth’s surface in the form of groundwater, and in the Great Basin significant quantities of groundwater lie beneath most basins. Sustainable development of these resources depends on an accurate understanding of the groundwater system.
Major recent population growth in Las Vegas and elsewhere in the Great Basin will require water in excess of current local supplies. This has caused water suppliers and developers to consider large-scale groundwater extraction and transport schemes to supply future demand. Foremost among these schemes is the now well-known plan by the Southern Nevada Water Authority to pipe significant amounts of groundwater from east-central Nevada southward to the Las Vegas area. In response to this plan and at the request of the Utah State Legislature and water managers, the Utah Geological Survey (UGS) has installed a series of new monitoring wells. A campaign of groundwater sampling for environmental tracers from the new wells and springs is providing insight to the groundwater resource in west-central Utah and its connection with groundwater to the west in Nevada. Analyses ranging from standard dissolved-ion chemistry to measurement of various isotopes and dissolved gases provide a range of environmental-tracer data that place basic constraints on the groundwater flow paths and processes in the Snake Valley area of west-central Utah.
Environmental tracers are any natural or anthropogenic chemical compound or isotope in groundwater that can be measured and used to interpret sources of recharge and discharge, rates of groundwater movement, and groundwater age. Groundwater age may be used to estimate the rates and distribution of recharge along flow paths, and can provide constraints on the potential availability of groundwater for human use. Groundwater age is a relative concept that assumes groundwater begins as recharge and steadily aquires “age” as it moves along a flow path. Under this assumption, groundwater is youngest near areas of recharge, and its relative age increases with distance from the recharge area.
Environmental tracers that include radiogenic isotopes of hydrogen and carbon (tritium and carbon-14, respectively) can provide important information concerning the age of groundwater. These isotopes are created naturally in the upper atmosphere, or in much higher concentrations during above-ground nuclear testing, and exist in predictable concentrations throughout the atmosphere. As precipitation seeps into the ground, it incorporates these isotopes into the groundwater system. Following recharge, concentrations of tritium and carbon-14 in groundwater decrease at rates dictated by their half-lives (12.3 and 5730 years, respectively) and their concentrations may be used to estimate time since recharge. Tritium is directly incorporated into the water molecule and is chemically inert, so its concentration in a sample primarily records the initial concentration and subsequent radioactive decay. The process is more complicated for carbon-14 because this isotope readily undergoes geochemical exchange with various minerals in the aquifer system that can alter its concentration, and calculating groundwater age based on carbon-14 concentration requires a variety of complicated numerical techniques. Fortunately, these tracers may also be used in a qualitative sense to yield more general age ranges that can indicate important trends in a groundwater system.
Previous studies have shown that tritium concentrations greater than 0.5 Tritium Units (TU) and carbon-14 concentrations greater than 50 percent modern carbon (pmc) indicate a water sample is modern, recharged since approximately the 1950s. Tritium concentrations less than 0.5 TU and pmc greater than 50 indicate a sample is premodern and may consist of water recharged prior to the 1950s and up to hundreds of years ago. Samples having tritium less than 0.5 TU and pmc less than 50 are considered old and consist of water recharged more than several hundred years ago and possibly up to tens of thousands of years ago. Samplesare considered to be a mix of young and old water when their tritium is greater than 0.5 TU and pmc is less than 50.
Results for Snake Valley
Environmental-tracer data indicate that more than half of the groundwater sampled in the Snake Valley area is old, and modern recharge comprises less than a fifth of all the samples. Samples classified as premodern or mixed comprise the remaining 25 percent of the dataset. Modern water is limited to parts of southern Snake Valley and other isolated areas likely supplied by uplands having relatively high precipitation and recharge rates. Apart from these areas, most of the groundwater likely has significant age, implying that low recharge rates and/or long flow paths are typical of much of the Snake Valley groundwater system. The environmental-tracer results suggest that away from localized major sources of recharge in mountain ranges, groundwater is recharged very slowly if at all. This implies that significant groundwater extraction from this system could easily exceed long-term recharge rates and produce permanent declines in groundwater levels and spring flow.
Environmental-tracer data and measured groundwater levels in wells and springs, collected by the UGS, provide the basic data that constrain the Snake Valley groundwater flow system. All other refinements of our understanding of groundwater in Snake Valley, including numeric models and hydrogeologic framework studies, must also explain the distribution of environmental tracers such as carbon-14 and tritium. These data, therefore, provide the fundamental information that allows water managers to make informed decisions concerning water allocation and use.