Scientific Investigations Report 2006–5137
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006–5137
Ground-water quality is an ongoing concern in Idaho. In 1990, ground water accounted for nearly 85 percent of the State’s drinking water; in 1995, it accounted for almost 95 percent (Solley and others, 1993, 1998). Previous studies conducted in Idaho detected high concentrations of nitrate in many of the State’s aquifers (Rupert, 1994; Crockett, 1995; Rupert and others, 1996). Previous studies also detected increasing concentrations of nitrate in a growing number of public- and domestic-use wells (Parliman and Young, 1987; Young and others, 1987a; Young and others, 1987b; Rupert, 1994; Clark and Ott, 1996; Rupert and others, 1996). The predominant sources of nitrate throughout much of the State are inorganic fertilizer, cattle manure, and legume crops (Rupert, 1996).
These increasing nitrate concentrations are of particular concern in the mid-Snake region of south-central Idaho. The region is experiencing substantial growth in some agricultural industries associated with the discharge of nitrogen to the environment, including dairies and other confined animal feeding operations. Gooding, Jerome, and Twin Falls Counties are experiencing particular growth in dairy operations (Idaho Agricultural Statistics Service, 1999; U.S. Department of Agriculture, 1999). These potential new sources of nitrogen to ground water are additional to those sources from existing, long-term agricultural activities in the region. Because of declining ground-water quality in parts of the mid-Snake region, the Idaho Department of Environmental Quality (IDEQ) designated those areas as priority water-quality management areas and targeted them for further study and assistance to improve land-use planning, nitrogen monitoring, and nitrogen management. At present (2006), planners and resource managers in the region lack analysis tools that could assist them in land-use planning and in managing and protecting water resources. When they attempt to evaluate how new and proposed changes to land use might affect their water resources, they must do so on an operation-by-operation basis without the ability to consider the cumulative effects from all nitrogen sources.
In response to these concerns and needs, the IDEQ proposed a project to study nitrogen loading effects and to develop planning tools to help assess how various land-use decisions might affect nitrate concentrations in the region’s ground water. The IDEQ asked the U.S. Geological Survey (USGS) to conduct an investigation in the mid-Snake region of south-central Idaho that comprises Jerome and Minidoka Counties and parts of Blaine, Bingham, Butte, Camas, Cassia, Elmore, Gooding, Lincoln, Power, and Twin Falls Counties (fig. 1).
The purpose of the study was to improve understanding of past and future effects of land-use-related nitrogen loading on ground water in the study area. Specific objectives were:
Objective 1 was addressed by the investigation reported in Skinner and Donato, 2003. Objective 2 was addressed by the development of the simulator that accompanies this report. The simulator’s graphical user interface (GUI) is enabled for use with geographic information systems (GIS). Objective 3 is addressed by this report, which documents the construction and calibration of the subregional flow and transport model, provides instructions for using the GIS-enabled GUI, and offers guidance for modeling the differences in nitrogen loading resulting from changes in land use.
To meet Objective 2, we developed a subregional model of the study area, based on a regional, three-dimensional ground-water flow model of the eastern Snake River Plain (ESRP model) that was developed and calibrated during the USGS Regional Aquifer System Analysis (RASA) program (Garabedian, 1992). Flow boundary conditions and initial estimates of aquifer properties for the new model were derived from the calibrated ESRP model.
Estimates of nitrogen loading to the aquifer, as well as measured distributions of nitrate concentrations in ground water, were used as data sources in a three-dimensional ground-water flow and transport model. The transport of dissolved nitrate in the ground-water system was simulated using the USGS three-dimensional solute-transport model MOC3D (Konikow and others, 1996, Kipp and others, 1998). Nitrogen input, developed as a separate part of the project and described in Skinner and Donato (2003), was used to help define nitrogen flux at the surface. Ground-water nitrate concentration data were used to calibrate and evaluate the transport model.
The scope of computer model development and assessment in this component of the study was limited to evaluating conceptual models of nitrogen loading, nitrate transport, and nitrate distribution in the region’s ground-water system. General spatial and temporal trends observed in nitrate concentrations were compared with those estimated by the simulation model to evaluate the model’s ability to represent loading and transport characteristics of the ground-water system, including concentrations of nitrate in a series of about 300 wells throughout the study area, and temporal nitrate concentrations from 11 springs that discharge into the Snake River.
Success in reproducing observed ground-water nitrate concentration trends using flow and transport models and estimates of nitrogen loads indicates that there is utility for applying computer nitrogen loading and nitrate transport models to assess the effects of land use and changes in land use on nitrate concentrations in ground water. This finding meets Objective 3 of the study. Other ground-water flow models that use MODFLOW as a basis for simulations, such as the one developed by the State of Idaho for the Snake River Plain (Cosgrove and others, 1999), could be adapted for use with the GIS enabled graphical simulator that was developed for this study.
Jerome and Minidoka Counties and parts of Blaine, Bingham, Butte, Camas, Cassia, Elmore, Gooding, Lincoln, Power, and Twin Falls Counties fall within the study area, which comprises 5,341 mi2 in south-central Idaho (fig. 1). The study area is in the downstream part of the eastern Snake River Plain. About 66 percent of the area is rangeland, 11 percent is flood-irrigated land, 21 percent is sprinkler-irrigated land, and the remaining 2 percent is comprised of several land-use types. The area is predominantly semiarid; mean annual precipitation ranges from 8 to 16 in. Therefore, most of the agricultural land in the study area lies near the Snake River, a major source of water for irrigation.
The ground-water system in the study area is made up of two types of aquifers: a regional basalt aquifer and a local perched alluvial aquifer. The regional basalt aquifer underlying the eastern Snake River Plain provides most of the ground water that moves through the study area. The local perched alluvial aquifer (fig. 1) overlying the eastern Snake River Plain in Minidoka and Cassia Counties is a lesser source of ground water and is not included in the flow and transport modeling.
The eastern Snake River Plain aquifer, which encompasses the study area, is composed primarily of vesicular and fractured olivine basalt flows (Quaternary age) of the Snake River Group (Whitehead, 1992). These basalt flows average from 20- to 25-ft thick but can, in places, exceed 1,000-ft thick. The top of the basalt is generally less than 100 ft below land surface throughout this part of the plain.
Layered basalt flows in the eastern Snake River Plain aquifer yield exceptionally large volumes of water to wells and springs. Individual well yields are some of the highest in the Nation, typically ranging from 2,000 to 3,000 gal/min to as much as 7,000 gal/min with minimal drawdown (Whitehead, 1992; Lindholm, 1996). Transmissivity is commonly 100,000 ft2/d and can be as high as 1,000,000 ft2/d (Whitehead, 1992). Locally, aquifer properties can vary greatly; on a regional scale, however, the variability is minimal.
Regional ground water in the eastern Snake River Plain aquifer moves from the northeast to the southwest (Rupert, 1997). Ground water is discharged as springs and seeps to the Snake River along the reach bordering Twin Falls, Jerome, and Gooding Counties. Ground-water discharge to this reach of the Snake River increased considerably from about 1910 through the early 1950s (Kjelstrom, 1992). The increase is attributed to recharge from surface-water irrigation north of the Snake River. Since the early 1950s, ground-water discharge to the Snake River has decreased as a result of (1) increased ground-water withdrawals for irrigation (Moreland, 1976), (2) introduction of more efficient irrigation practices such as the conversion from flood to sprinkler irrigation, and (3) local droughts (Kjelstrom, 1992).
In the Burley area, an alluvial aquifer is perched above a blue-clay layer about 60 to 120 ft below land surface (Rupert, 1997). Water in this perched aquifer moves northward at the southern boundary of the perched aquifer and westward near the western boundary. The water level in the perched aquifer is about 100 ft above that of the Snake River Plain aquifer. The perched aquifer is recharged predominantly from the infiltration of irrigation water. Several wells completed in this aquifer go dry seasonally after irrigation ceases and become operational again after the start of the next irrigation season (Rupert 1997).
U.S.
Department of the Interior | U.S. Geological
Survey
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