Scientific Investigations Report 2007–5041
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2007–5041
The Spokane Valley-Rathdrum Prairie (SVRP) aquifer is the sole source of drinking water for more than 500,000 residents in Spokane County, Washington, and Bonner and Kootenai Counties, Idaho (fig. 1). Those counties include the rapidly growing cities of Spokane, Spokane Valley, and Liberty Lake, Washington, and Coeur d’Alene and Post Falls, Idaho. Concerns have been expressed about the potential effects of the recent growth and of projected urban, suburban, and commercial growth on water availability and water quality in the aquifer and in the Spokane and Little Spokane Rivers.
The SVRP aquifer consists primarily of thick layers of coarse-grained sediments — gravels, cobbles, and boulders — deposited during a series of outburst floods that resulted from repeated collapse of the ice dam that impounded ancient Glacial Lake Missoula (Bretz, 1930). Sources of recharge to the aquifer include infiltration from precipitation, return flow from water applied at land surface, leakage from the Spokane and Little Spokane Rivers and adjacent lakes, and surface-water inflow from tributary basins. The aquifer discharges primarily into the Spokane and Little Spokane Rivers and through withdrawals from wells. The aquifer was designated a “Sole Source Aquifer” by the U.S. Environmental Protection Agency (USEPA) in 1978 (under the provisions of the Federal Safe Drinking Water Act of 1974) in response to local concerns about aquifer vulnerability to water quality degradation. The U.S. Environmental Protection Agency (2000) defines such an aquifer as one that supplies at least 50 percent of the drinking water consumed in the area overlying the aquifer. Communities that depend on a sole-source aquifer generally do not have a viable alternate drinking-water source.
Water-management issues in this rapidly growing bi-State area have become increasingly regional in nature. Several groups have initiated a comprehensive, regional study of the SVRP aquifer to serve as a scientific basis for addressing regional water concerns. In 2004, the Washington State Department of Ecology (WDOE), the Idaho Department of Water Resources (IDWR), and the U.S. Geological Survey (USGS) in consultation with local stakeholders developed a comprehensive work plan for a study to gain a better understanding of ground-water and surface-water resources in the SVRP area. The first study objective is the development of a comprehensive knowledge base and accompanying data to provide an improved scientific basis for water management of the SVRP aquifer. This report, which describes the hydrogeologic framework and updates to the ground-water budget of the aquifer, along with six recently published reports (Campbell, 2005; Hortness and Covert, 2005; Kahle and others, 2005; Gregory and Covert, 2006; Oldow and Sprenke, 2006; Bartolino, 2007), provides comprehensive information based on historical and current investigations. The final and concurrent phase of the study uses this information to develop a numerical ground-water flow model to support the conjunctive management of ground water and surface water in the SVRP aquifer. The model is described in a separate report (Hsieh and others, 2007). The results of the overall investigation are intended to provide tools for the evaluation of alternate water-resource management strategies throughout the SVRP aquifer.
The purpose of this report is to describe the current knowledge base for the hydrogeologic framework and ground-water budget of the SVRP aquifer. The description of the hydrogeologic framework is based on a review and interpretation of well logs, geologic maps, and geophysical studies available as of September 2006. The description of the hydrogeologic framework includes the approximate depth to the base of the aquifer, the approximate thickness of the aquifer, and the occurrence and thickness of fine-grained layers within the aquifer. Brief descriptions of the hydrogeologic units that bound the aquifer also are provided. The description of the ground-water budget is based on new information compiled during this investigation and updates to water-budget components developed by previous investigators.
The scope of this report includes the regional and local geologic history, the surficial and subsurface geology, selected physical characteristics of the SVRP aquifer and adjacent units, and an updated ground-water budget for the aquifer. Additional data needs that could improve understanding of the hydrogeologic framework and ground-water budget of the aquifer also are discussed.
The SVRP aquifer underlies about 370 mi2 of a relatively flat, alluvial valley surrounded by bedrock highlands (Kahle and others, 2005). The aquifer extends south from Lake Pend Oreille to Coeur d’Alene Lake and west across the Washington-Idaho State line to near Nine Mile Falls northwest of Spokane. Land-surface altitudes in the area range from about 1,500 to nearly 2,600 ft. Several lakes, the largest of which are Coeur d’Alene Lake and Lake Pend Oreille in Idaho (pl. 1), are located along the margins of the aquifer. The area generally is devoid of surface drainage other than the Spokane and Little Spokane Rivers (pl. 1).
Ground water is the primary source for public-supply, domestic, irrigation, and industrial water use in the study area (Hutson and others, 2004). Estimated ground-water use in 2000 for Spokane, Bonner, and Kootenai Counties was more than 188 Mgal/d (Hutson and others, 2004). In Spokane County alone, estimated ground-water use in 2000 was about 110 Mgal/d for public supply, 12 Mgal/d for domestic use, 9 Mgal/d for irrigation, and 8 Mgal/d for industrial use (accessed September 13, 2004 at http://water.usgs.gov/watuse/data/2000/). Peak summer daily ground-water withdrawals from the aquifer are estimated to be about 450 Mgal/d (MacInnis and others, 2000).
Primary land uses in the study area include urban and agriculture. Agricultural land is used predominantly for pasture or the production of hay, wheat, grass seed, barley, and oats (fig. 2). Urban areas supplied by the aquifer include the Spokane metropolitan area in Washington and Coeur d’Alene and Post Falls in Idaho. Residential and commercial development is increasing rapidly in the area as evidenced by a 16 percent population increase in Spokane County, Washington and nearly 56 percent increase in Kootenai County, Idaho, during the 1990s (U.S. Census Bureau, 2002).
The climate within the study area varies from subhumid to semiarid and is characterized by warm, dry summers and cool, moist winters (Molenaar, 1988). Mean annual (1971–2000) precipitation values for weather stations in the area were 16.7 in. at the Spokane International Airport, 25.9 in. near Bayview, Idaho, and 28.1 in. at Coeur d’Alene, Idaho (Western Regional Climate Center, 2005). Most of the precipitation falls as snow during the 5-month period from November through March (Molenaar, 1988). The distribution of average annual precipitation for 1961–90 in the study area is shown in figure 3.
Preparing the basic data required to characterize the hydrogeologic framework of the SVRP aquifer involved the review of existing hydrogeologic data as summarized in Kahle and others (2005) and the collection of more recently available drillers’ logs. Methods used to compile the hydrogeologic data are presented in this section. Methods used to describe the ground-water budget of the aquifer are included in the section, “Ground-Water Budget.”
The first step in identifying available well data involved obtaining drillers’ logs for previously inventoried (field verified) wells from records housed in USGS offices in Boise, Idaho, and Tacoma, Washington. These data were augmented by obtaining additional well records from the WDOE and IDWR on-line well logs databases and from other sources. In the well log selection process, preference was given to logs of deeper wells in order to characterize as much of the SVRP aquifer thickness as possible. Approximate locations (latitude and longitude coordinates) were assigned for the non-inventoried wells using public land survey locations (township, range, section, and quarter-quarter), well addresses, and (or) parcel number for each well included on drillers’ logs. To the extent possible, paper maps (USGS 7 ½-minute quadrangles and City or County road maps) and on-line maps (Spokane County Assessor, Mapquest®, and Google™Earth) were used to verify drillers’ locations and to estimate latitude, longitude, and land-surface altitude for the non-inventoried wells.
Data for 587 wells (table 3, at back of report) were used to characterize the hydrogeology of the study area. Information including site location, land-surface altitude, well-construction details, and available water levels for each well was entered into the USGS National Water Information System (NWIS) database (U.S. Geological Survey, 2007).
Lithologic data from drillers’ logs for the 587 project wells were entered into the Rockworks 2002® software, a stratigraphic analysis package. A total of 26 hydrogeologic sections were constructed and plotted using Rockworks® to identify and correlate hydrogeologic units based primarily on lithology and stratigraphic position. Where data were sparse or unavailable, stratigraphic contacts between hydrogeologic units were inferred. The base of the aquifer was, in places, inferred from geophysical data and corresponding transects described in Kahle and others (2005). Of the original 26 sections, 15, which are representative of the 26, are published in this report (pl. 2). Three distinct hydrogeologic units were differentiated on the sections; the SVRP aquifer, the Basalt and fine-grained interbeds unit, and the Bedrock unit. Information from the hydrogeologic sections and data from additional wells were used to identify significant fine-grained layers within the aquifer and to describe the approximate altitude of the base of the aquifer and the thickness of the aquifer.
The well-numbering system (fig. 4) used by the USGS differs slightly for the States of Washington and Idaho, but both systems are based on official rectangular subdivisions of the public land survey system. In both States, wells are assigned numbers that identify their location within a township, range, section, and 40-acre tract. Washington well number 25N/44E-14G01 (fig. 4) indicates the well is in Township 25 North and Range 44 East, north and east of the Willamette Base Line and Meridian, respectively. The numbers immediately following the hyphen indicate the section (14) within the township; the letter following the section indicates the 40-acre tract of the section. The two-digit sequence number (01) following the letter indicates the well was the first one inventoried in that 40-acre tract.
Idaho well number 54N 04W 31DDD1 (fig. 4) indicates the well is in Township 54 North and Range 4 West, north and west of the Boise Base Line and Meridian, respectively. The numbers immediately following the hyphen indicate the section (31) within the township; the letters following the section indicate the quarter section (160-acre tract), quarter-quarter section (40-acre tract), and quarter-quarter-quarter section (10-acre tract). In Idaho, quarter sections are designated by the letters A, B, C, and D in counterclockwise order from the northeast quarter of each section. Within the quarter sections, 40-acre and 10-acre tracts are lettered in the same manner. For example, well 54N 04W 31DDD1 is in the SE ¼ of the SE ¼ of the SE ¼ of section 31. The number following the letters (1) represents the serial number of the well within the tract.
In the illustrations in this report, wells are identified individually by only the section and 40-acre tract, such as 14G01 or 31DDD1. Township and range are shown on the map borders.
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