Eastern Snake River Plain Aquifer
By: Doug Geller
Emporia State University GO 571
Hydrogeology, Spring 2006
This paper examines the regional aquifer system in the Eastern Snake River Plain of southeastern Idaho. The groundwater system underlying the Eastern Snake River Plain is among the most important in Idaho and is believed to be the largest aquifer system west of the Continental Divide in the U.S. The aquifer is hosted primarily in flood basalts and related, interbedded sediments and occupies an arc-shaped area in southeastern Idaho (see map, below). The upgradient boundary of the aquifer system is near Yellowstone National Park and the downgradient boundary is near the Snake River at Thousand Springs (where, literally, the aquifer pours out of the rocks and into the river system). The climate of the region is semi-arid, with cold winters and hot dry summers, with little snowfall. The aquifer system provides drinking water to over 300,000 people and was designated as a Sole Source Aquifer by USEPA in 1991.
Map source: Idaho Department of Water Resources
Approximate aquifer area: 10,000 square miles
PHYSICAL AND HYDROLOGIC SETTING
The ESRP lies within a downwarped basin that has subsided nearly 3,300 feet (1 kilometer) relative to surrounding mountains (Smith 2004). To the north and south lie mountain ranges that are part of the extensive basin and range province in the western U.S. To the northwest are the high mountains of the Idaho Batholith (e.g. the Sawtooth Mountains). To the east and northeast are the Teton mountain range along the Idaho-Wyoming border and the Yellowstone Plateau.
The basin owes its origin to the passage of the Yellowstone volcanic 'hot spot' beneath the plain some 4 million years ago, and subsidence due to the extensive emplacement of thick lava flows across the land surface. Active volanism over the past several million years continuing through the Holocene has produced extensive lava flows, some of them very young (for example the Holocene Craters of the Moon lava in the western part of the plain near Arco, Idaho).
One of a handful of legendary rivers that epitomizes the grandeur and scale of the American west, the Snake River begins its journey toward the Pacific Ocean near Jackson Hole, Wyoming. From there, it cuts south and then northwest near the Wyoming - Idaho border, passing through canyons until it reaches the Snake River plain, turning southwest near Heise, Idaho. The Snake then cuts a large arc across the southern part of Idaho, passes through 6,000 foot deep Hells Canyon along the Idaho-Oregon border, and eventually joins the Columbia River near Pasco, Washington.
The southern Idaho climate is semi-arid, with some areas receiving as little as eight inches of precipitation per year, ranging up to 10 inches in higher elevations (IDWR 1999). For this reason, agriculture in the region has long relied on irrigation.
Despite the semi-arid climate, there is a relative abundance of water in the region owing to the origins of the Snake River in mountainous areas to the east (IDWR 1999).
GEOLOGY, HYDROSTRATIGRAPHY AND AQUIFER PROPERTIES
The Eastern Snake River Aquifer (ESRA) is composed primarily of Quaternary age basalt flows of the Snake River Group. The total thickness of the basalt reaches 5,000 feet, but the most significant groundwater resources are found in the upper 500 feet of the section (IDWR 1999). Aquifers form at the so-called 'interflow' zones comprised of highly porous vesicular basalt flow tops, and sometimes, highly fractured flow lobes and pillow lavas. There are also discontinuous sedimentary interbeds found in the basalt and the coarser grained sedimentary units also form aquifers. Throughout most of its extent the ESRA is an unconfined aquifer; however, there are localized perched groundwater conditions as well as confined groundwater conditions. Aquifer testing and associated groundwater flow modeling assessments have indicated widely varying but generally high to very high transmissivity values in the range of 10,000 to 1,000,000 ft2/day.
Groundwater flows from northeast to southwest across the plain. The potentiometric surface drops 2,000 feet along a 190 mile flowpath (Wood and Low, 1986). However, the hydraulic gradient is not uniform. There are three distinct zones where the gradient is relatively low and three distinct zones where the gradient is relatively high (see figure). The high gradient zones correspond to areas of lower hydraulic conductivity, such as the Great Rift Fault Zone (IDWR 1999). This zone and other features can be thought of as subsurface 'dams' that slow the westward regional movement of groundwater from recharge areas toward discharge areas.
The IDWR website provides links to several ongoing studies related to management of the ESRA.
A groundwater budget showing the components of recharge to and discharge from the ESRA shows the overwhelming importance of irrigation to the basin hydrology. Based on 1980 data Garabedian (1992) reported that surface water irrigation accounted for 60 percent of the recharge to the aquifer system, whereas precipitation, river and stream losses accounted for only about 22 percent, with the remaining recharge from tributary basin underflows. Garabedian estimated that 86 percent of the groundwater exits the system as Snake River gains, with groundwater withdrawals at 14 percent (see graph below). Groundwater storage in the upper 500 feet of the ESRA ranges from approximately 200 to 300 million acre-feet (Lindholm 1996).
The importance of irrigation and groundwater pumping to the basin water budget has long been recognized. By about 1960, groundwater levels had risen by as much as 60 to 70 feet in some parts of the basin due to decades of flood irrigation at rates far in excess of crop consumption (IDWR 1999). Since the 1960s, various water efficiency programs and increased groundwater development have caused groundwater levels to decline, but they still remain above historic levels.
GROUNDWATER USE AND DEVELOPMENT
The Idaho Department of Water Resources (IDWR) published a report in 1997 titled "Upper Snake River Basin Study" which examined in detail the effects of groundwater development on the ESRP aquifer system (IDWR 1999). Some of the early work on groundwater resource development on the plain is summarized by Mundorff et. al (1964). As noted above, a major source of recharge to the ESRA in the last century has been artificially induced, caused by flood irrigation, and leakage from irrigation ditches and canals conveying water diverted from the Snake River. Initially, irrigation resulted in a net increase in groundwater recharge that can be seen in the historical graph of outflow from springs such as the Thousand Springs area. As noted above, most of the new irrigation supply since the 1960s has been from groundwater. Several municipalities also developed groundwater supplies in the 1900s and many are 100 percent reliant upon groundwater. Examples include the City of Idaho Falls which operates 18 wells and Twin Falls which operates 10 wells. The Idaho Falls 2005 Water Quality Report is a typical example of a report prepared for water consumers and provides further details about source water quality in that Snake River Plain community.
The aquifer is highly productive, with 66 percent of irrigation well yields exceeding 1,500 gpm (Goodell 1988). Net pumpage in 1980 (Garabedian 1992) was 1.14 million acre-feet.
The history of water use in the region and its effects on groundwater and surface water resources is summarized below (from IDWR 1999):
1. The first half of the 20th century was a period of expanding construction of surface water irrigation diversions and conveyance systems.
2. The large-scale irrigation projects increased the irrigated acreage resulting in rising groundwater levels due to recharge from flood-style irrigation practices, and a corresponding increase in spring discharge (see hydrograph of Thousand Springs discharge, below).
3. Beginning in the 1960s, continued expansion of irrigation began to rely more and more on groundwater. During this period, efforts to make surface irrigation more efficient (for example using sprinkler systems instead of flood irrigation, and using lined instead of unlined canals) resulted in less 'incidental' groundwater recharge from irrigation. These changes combined to cause a decline in groundwater levels (up to 12 feet) and a drop in spring discharge (see photo below of Thousand Springs to get an idea of the size of these springs).
4. Between 1975 and 1995, groundwater storage in the basin declined by an estimated 7 million acre-feet from previous highs (Johnson and Cosgrove, 1997); see figure below illustrating changes in groundwater levels 1980-1998.
5. Late 1900s drought conditions resulted in decreased diversions of surface water flows, which further reduced incidental recharge and also led to more groundwater development.
Thousand Springs, Idaho photo from Idaho Department of Water Resources
The median total dissolved solids content from more than 1,000 wells across the basin as reported by Low (1987) is 293 mg/L indicating the aquifer water quality is generally quite good. There are localized areas of elevated nitrate which is the result of agricultural land use practices (e.g. use of fertilizers). In general (as would be expected) water quality is better near the recharge area (eastern end of the basin). The groundwater is characterized as a calcium-sodium-bicarbonate chemical type (Wood and Low, 1986) that is relatively enriched in silica, which is common for aquifers hosted in basic volcanic rocks.
The US Department of Agriculture undertook a groundwater quality monitoring and agricultural best management practices demonstration project in south-central Idaho in an effort to understand and manage the effects of non-point source pollution (primarily from agricultural land use practices) on the aquifer system. This project was funded under Section 319 of the Clean Water Act. The State of Idaho has an online map server that can be used to lookup groundwater quality monitoring data for the entire state including the Eastern Snake River Plain Aquifer. IDWR Groundwater Quality Map Server
IDWR, along with other participants and stakeholders, has been studying the possible large-scale implementation of groundwater management in the ESRP. Building on previous studies, the feasibility of managed recharge was investigated and deemed feasible (IDWR 1999). This scheme would involve using existing irrigation infrastructure to convey surplus flows (for part of the year) to a series of surface infiltration basins to enhance groundwater recharge. This managed approach to recharge would be designed to fulfill multiple objectives, including stabilizing groundwater levels and spring discharge, providing a measure of predictability for water right holders, and helping to support efforts at protecting and enhancing fish habitat. The amount and timing of surplus flows that would be used continues to be a matter of discussion and negotiation amongst water right holders, notably, for Idaho Power (IDWR 1999). This is because upstream water diversions would affect power generation, and thus the program might require mitigation for lost hydropower revenues as well as consideration of the effects the diversions may have on downstream water users.
A groundwater flow modeling assessment was used to forecast the hydraulic response of the aquifer from four possible recharge scenarios, so that the hydrologic benefits could be evaluated (IDWR 1999). Each scenario was designed to take into consideration a number of factors including institutional controls on diversions, existing irrigation canal infrastructure, and seasonal water availability to conduct the recharge. The so-called 'Thousand Springs' scenario involved the maximum artificial recharge with a strong emphasis on diversion of surplus flows mainly in the winter months. After 20 years of recharge at a rate of 416,000 acre-feet per year, springflows at targeted locations were predicted to increase by up to 450 cfs, with a corresponding rise in groundwater levels across the central part of the plain of up to 15 feet. In April 2006, another chapter in the saga of the aquifer passed when a legislative proposal (House Bill 800) to implement managed recharge was defeated in the Idaho Senate, following an intensive campaign against the program, funded in part by Idaho Power. Most of the opposition to the project reportedly came from areas outside the ESRP. U.S. Water News Article
GROUNDWATER CONTAMINATION ISSUES
The ESRA has been ranked by the State of Idaho as the second most vulnerable aquifer to potential contamination (after the Boise Valley Aquifer in western Idaho). However, there are no known widespread problems with groundwater contamination in the ESRA. By far the most well - documented threat to the aquifer system is the Idaho National Engineering and Environmental Laboratory (INEEL, now called INL) located between Arco and Idaho Falls in the middle part of the basin. Here, the federal Department of Energy has stored, buried and disposed of waste products associated with weapons production, including radioactive and industrial chemicals. The contamination has affected the vadose zone and the aquifer and includes a variety of constituents including heavy metals and volatile organic compounds. USEPA INL Summary For more INEEL information click HERE.
CONCLUSIONS image from IDWR
The Eastern Snake River Plain Aquifer (ESRA) system is an extensive and regionally significant source of groundwater in southeast Idaho, as well as a major contributing source of flow to the Snake River. The ESRA is highly productive and regional groundwater flow is generally from east to west. For the past century, surplus irrigation flows have been the dominant form of aquifer recharge. Most of the natural discharge from the aquifer system is via series of very large springs adjacent to the Snake River, as shown below at American Falls and Thousand Springs.
Historic irrigation practices in the first half of the 20th century caused unmanaged (or incidental) aquifer recharge to increase, which resulted in higher groundwater levels and increased discharge at the extensive downgradient spring complexes. Beginning in the 1950s, shifts in water use practices resulted in an increase in groundwater withdrawals and a reduction in the unmanaged incidental recharge, leading to a declining groundwater level and spring discharge trend that continues to the present day. The water quality of the aquifer is generally good, with isolated occurences of nitrate associated with historical land use practices. The aquifer is vulnerable to contamination but there are no widespread pollution problems. A significant contamination threat is associated with the DOE nuclear waste facility at INEEL. Water management in the basin has grown increasingly concerned with fulfilling multiple objectives and is driven by a number of complex stakeholder issues, including those of water right holders as well as ecological concerns such as critial fish habitat. The potential for implementing large-scale managed recharge of the aquifer system has been under consideration for many years and has been the subject of a number of modeling studies. Large scale recharge has been determined to be feasible.
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Goodell, S.A. 1988, Water Use on the Snake River Plain, Idaho and Eastern Oregon, U.S. Geological Survey Professional Paper 1408-E.
Idaho Departmentof Water Resources, 1997, Upper Snake River Basin Study.
Idaho Department of Water Resources, 1999, Feasibility of Large-Scale Managed Recharge of the Eastern Snake River Plain Aquifer System.
Johnson, G.S. and D.M. Cosgrove, 1997, Recharge Potential on the Snake River Plain, Idaho: A Drop in the Bucket?, Biennial Symposium on Artificial Recharge of Ground Water, Tempe, Arizona.
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