Scientific Investigations Report 2007–5041
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2007–5041
A series of geologic events has defined the surface and subsurface geologic framework in the study area. A basic description of these events was provided in Kahle and others (2005). That description has been modified slightly and repeated here to provide the reader with a comprehensive understanding of the geologic framework that affects the occurrence and movement of ground water in the study area. Although descriptions of the region’s geologic history are available at various levels of detail in numerous documents, the summary that follows is based in part on descriptions contained in Conners (1976), McKiness (1988), Molenaar (1988), Adema (1999), Breckenridge and Othberg (2001), Kiver and Stradling (2001), and Lewis and others (2002).
The simplified geologic history presented in this report describes three major time periods. The pre-Tertiary geology includes mostly Precambrian sedimentary rocks that have been metamorphosed and disrupted in places by igneous intrusions. The Tertiary geology includes the Columbia River basalts and interbedded lacustrine deposits of the Latah Formation. The Quaternary geology includes mostly glacial and catastrophic flood deposits of varying grain size that overlie the older rocks. A simplified geologic time scale (table 1) is provided to aid the reader in understanding the sequence of geologic events and the magnitude of geologic time during which they occurred. A map of the extent of late-glacial ice and glacial lakes in northern Washington, Idaho, and western Montana is shown in figure 5.
The oldest rocks in the region surrounding and underlying the study area are metamorphosed, fine-grained sediments that originally were deposited in a large, shallow north-south-trending marine basin during the Precambrian. These rocks are present in outcrop today as low-grade metasedimentary rocks, including argillite, siltite, and quartzite, which grade locally into more highly metamorphosed schists and gneisses (pЄm, pl. 2).
Following deposition and metamorphism, as much as 20,000 ft of the Precambrian rocks were eroded before the Paleozoic Era began (Conners, 1976). During the Cambrian, additional sedimentation occurred in shallow seas that resulted in shale, limestone, and sandstone being deposited over the Precambrian rocks. However, from the end of Cambrian time to the present, the region mostly has been emergent and much of the post-Cambrian sediments have been eroded from the area leaving few surface exposures (Єs, near southern end of Lake Pend Oreille, pl. 2).
Emplacement of various igneous intrusive bodies, along with associated metamorphism and deformation, occurred during a long period of time between the Jurassic and Tertiary. During the Cretaceous, faulting and emplacement of large granitic bodies (TKg, pl. 2) resulted in the formation of the north-south-trending Purcell Trench, a geomorphically low feature that extends from north of the Canadian border south through the Cocolalla Valley and into the Rathdrum Prairie (pl. 1, fig. 5).
In pre-Tertiary time, the region’s surface-water drainage was from a vast area to the north and east of the study area. Streams flowed south from the Purcell Trench and Clark Fork Valley into presumably a large river that flowed through the Rathdrum Prairie and then west through the Spokane Valley to the ancient Columbia River. The pre-Tertiary landscape was characterized by ridge crests and valley bottoms that had considerable relief, probably 4,000 ft or more in places (Molenaar, 1988).
During the Miocene, basalt flows of the Columbia River Group spread northeast from the Columbia Plateau and filled the deep canyons of the pre-Tertiary landscape. Drainage systems that previously had transported sediment out of the area now deposited sediment at the margins of the basalt flows. The Early Miocene basalt flows dammed drainages, including the ancient Rathdrum-Spokane River, and created lakes in which sand, silt, and clay of the Latah Formation were deposited. The earliest basalt flows apparently did not extend to the eastern and northern areas of the Rathdrum Prairie, and a relatively thick section of sediment accumulated in those areas. With the northeastward flow of basalt, Late Miocene basalt eventually overrode the entire Rathdrum Prairie region and created alternating layers of basalt and Latah Formation interbeds as recorded in drillers’ logs for wells located in the northeastern Rathdrum Prairie (Hammond, 1974).
During a period of slow downcutting from the Late Miocene to the Early Pleistocene, as much as 590 ft of Latah Formation sediments were removed from the region (Anderson, 1927). Streams in the developing drainages eroded much of the exposed Latah Formation beds and some of the younger basalt near the margins of the basin. Accurate estimates of the thickness and extent of the remaining Latah Formation sediments are difficult to determine because of the cover of Pleistocene drift and a scarcity of boreholes that penetrate below the water table. Anderson (1940) discovered a 980-ft-thick bed of Latah Formation beneath an exposed basalt flow when drilling a well west of Hayden Lake.
The late Tertiary landscape likely was characterized by the ancestral Spokane River, which followed a course similar to that of today’s Spokane River except in north Spokane where the ancestral river’s course probably was through the Hillyard Trough on the east side of the basalt plateau of Five Mile Prairie (Newcomb and others, 1953). The river then flowed west along the present reach of the Little Spokane River Valley toward the present main valley near Long Lake. Tertiary sediments associated with the ancestral river may occur at depth along its historic course, now buried by Pleistocene drift.
Today, the Latah Formation has limited surface exposures near Hayden and Coeur d’Alene Lakes and near Spokane and occurs mostly as deeply weathered, yellow to orange silt and clay (older sediments, Ts, pl. 2). Surface exposures of the Columbia River basalt are common in the upland areas surrounding the SVRP aquifer in Washington. In Idaho, the largest exposures of basalt occur near Hayden and Coeur d’Alene Lakes (Tb, pl. 2).
During the Pleistocene, the study area was subjected repeatedly to the erosional and depositional processes associated with glacial and interglacial periods. Although as many as six major glaciations affected the area, only the most recent can be described with any level of certainty. Sediments from earlier periods probably are encountered locally in some wells, but little surface evidence remains to reconstruct the depositional history of those sediments.
During the climax of the most recent Pleistocene glaciation (about 15,000 years before present), much of northern Washington, Idaho, and westernmost Montana was covered by lobes of the Cordilleran ice sheet (fig. 5). The large ice sheet formed in the mountains of British Columbia and flowed south, filling valleys and overriding low mountain ranges in the northern parts of Washington, Idaho, and Montana. The Pend Oreille River and Purcell Trench lobes contributed vast quantities of sediment to the study area via meltwater streams from the glacial lobes. The Okanogan and Columbia River lobes affected the study area by occasionally blocking westward drainage of the ancestral Columbia and Spokane Rivers and creating large ice-age lakes. The Columbia River lobe created Glacial Lake Spokane; later, the Okanogan lobe created Glacial Lake Columbia (fig. 5), which inundated the smaller area covered by Glacial Lake Spokane. When the Purcell Trench lobe in northern Idaho blocked the drainage of the ancestral Clark Fork in northwestern Montana, Glacial Lake Missoula was created (fig. 5).
Glacial Lake Missoula had a maximum surface altitude of about 4,200 ft, a maximum depth of 2,000 ft, and a maximum surface area of 3,000 mi2. Catastrophic failure of the Clark Fork ice dam released as much as 500 mi3 of water at a rate 10 times the combined flow of all present-day rivers on Earth. The torrent of floodwater crossed parts of Montana, Idaho, Washington, and Oregon before reaching the Pacific Ocean. The continuous southward flow of ice repeatedly blocked the Clark Fork allowing Lake Missoula to refill multiple times. This cycle may have been repeated as many as 100 times (Atwater, 1986) before the end of the last glaciation. The largest of the Missoula floods, many of which probably occurred relatively early in the lake-filling and flooding cycle, overwhelmed local drainages and topped the 2,400-ft divide west of Spokane, spilling south towards Cheney and beyond and creating the Channeled Scablands (fig. 5). Smaller floods that spread through the Rathdrum Prairie and Spokane River Valley likely discharged through lower altitude drainages, including the present-day Little Spokane River, Long Lake, and Hangman (Latah) Creek (pl. 1).
The south end of Lake Pend Oreille at Farragut State Park marks the location of the outbreak of the Missoula floods (pl. 1). Most of the floodwaters flowed south through the Rathdrum Prairie and then west toward the Spokane area. The flood deposits consisted mostly of gravels of glaciofluvial origin derived from glacial outwash of the Purcell Trench lobe and reworked by the flood events. Near-surface deposits include coarser gravels located in the center of the valley and finer sands and gravels located along the margins. Flood bars of these deposits occur along the margins of the Rathdrum Prairie and Spokane Valley and dam the outlets of Spirit, Twin, Hayden, Coeur d’Alene, Hauser, Liberty, and Newman Lakes.
Glacial Lake Columbia, impounded by the Okanogan lobe, was the largest glacial lake in the path of the Missoula floods (fig. 5). This lake was long-lived (2,000–3,000 years) and had a typical surface altitude of 1,640 ft; however, the altitude reached 2,350 ft during maximum blockage by the Okanogan lobe and rose as high as 2,460 ft during the Missoula floods (Atwater, 1986). The higher level of Glacial Lake Columbia probably occurred early, whereas the lower and more typical level of the lake occurred in later glacial time (Richmond and others, 1965; Waitt and Thorson, 1983; Atwater, 1986). At the lower level (1,640 ft), Glacial Lake Columbia extended east to the Spokane area, where clayey lake sediment is intercalated with Missoula flood sediment (Waitt and Thorson, 1983). At the higher level of Glacial Lake Columbia (2,350 ft), the glacial lake would have flooded the Rathdrum Prairie to within a few miles of the Purcell Trench lobe that dammed Glacial Lake Missoula.
Sedimentation associated with Glacial Lake Columbia resulted in thick, fine-grained sediments throughout much of the region. Within the study area, clay and silt deposits, presumed to be Glacial Lake Columbia sediments, have been identified in deep boreholes in the Hillyard Trough and north Spokane areas and in the Hangman (Latah) Creek Valley. At least 16 beds of Glacial Lake Missoula flood deposits have been identified within Glacial Lake Columbia deposits in the Hangman (Latah) Creek Valley just south of Spokane (Waitt and Thorson, 1983). These fine-grained deposits generally occur at depth beneath late glacial deposits of the Missoula floods and likely occur elsewhere in the study area. Alternating beds of lake and flood deposits may occur at considerable depth (400–600 ft) throughout parts of the study area.
Although Glacial Lake Columbia apparently inundated most of the study area at least periodically, the last of the Missoula floods may have spilled through an area devoid of a glacial lake. A complex of flood bars that develop only when standing water is very shallow or absent is present from the Spokane River Valley to its confluence with the Columbia River 65 mi downstream (Kiver and Stradling, 2001). The present surface morphology of the Rathdrum Prairie and Spokane River Valley developed during the last outburst floods between 13,000 and 11, 000 years ago (Waitt, 1985). These late glacial-outburst-flood deposits constitute much of the upper part of the SVRP aquifer.
Surface exposures of Quaternary deposits within the study area include:
Hydrogeologic data compiled from 587 wells (table 3) was used to describe the hydrogeologic units of the study area. Although characterization of the SVRP aquifer is the focus of this investigation, the Basalt and fine-grained interbeds unit, which includes Columbia River basalt and interbedded lacustrine deposits of the Latah Formation and the Bedrock unit, which includes metasedimentary and igneous intrusive rocks, also are described. A simplified conceptual model of the hydrologic conditions in the study area is shown in figure 6. Using the data described in the section “Methods”, hydrogeologic sections were constructed and used to describe the SVRP aquifer and the surrounding hydrogeologic units. Using these sections and additional wells, the approximate base and thickness of the aquifer were estimated and the significant fine-grained layers within the aquifer were described.
Although the SVRP aquifer is known for its extremely coarse-grained texture and high transmissivity, several recent investigators have described fine-grained or confining layers within the aquifer, specifically in the Hillyard Trough and Little Spokane River valley (CH2M HILL, 1998; Golder Associates, Inc., 2004). Identification of the location and thickness of significant fine-grained layers within the aquifer is important for appropriate representation in computer models being developed to simulate the movement and storage of ground water in the aquifer. As part of this investigation, an attempt was made to identify and map significant (more than 5 ft thick) fine-grained layers within the aquifer on the basis of available well data.
During the process of identifying fine-grained layers within the SVRP aquifer, it became apparent that some of the fine-grained material recorded in drillers’ logs probably was not glaciolacustrine but rather older lacustrine deposits of the Latah Formation at the base of the aquifer. In some cases, fine-grained layers were fully penetrated in boreholes, and the aquifer was easily identifiable and described as continuous below the fine-grained layer. For example, see wells 26N/43E-08E04 [hydrogeologic section A-A’]; 26N/45E-24C02 [hydrogeologic section F-F’]; and 51N 04W 35DDA1 [hydrogeologic section I-I’] on plate 2. In other cases, however, fine-grained layers were not fully penetrated and were encountered at or near the base of the well, making it difficult to determine whether the fine-grained layer was within the aquifer or was older lacustrine material below the aquifer. For example, see wells 51N 04W 35BBA1 [hydrogeologic section I-I’], 51N 05W 11ADB1 (hydrogeologic section J-J’), and 53N 04W 25DDC2 [hydrogeologic section L-L’] on plate 2.
Although these two types of fine-grained layers are nearly impossible to differentiate on the basis of lithology alone, a method was devised using stratigraphic position, color, and the occasional presence of organic matter to help differentiate glaciolacustrine fine-grained layers within the aquifer from older lacustrine fine-grained material below the aquifer. A description of the fine-grained deposits—clay and (or) silt with some sand and occasional gravel—that occur in the study area follows.
Deeply weathered or organic-rich clays commonly described in drillers’ logs as red, orange, yellow, brown, or green (with or without associated wood) were interpreted to be Miocene non-glacial lacustrine deposits of the Latah Formation. Within boreholes in the study area, the fine-grained deposits of the Latah Formation generally occur along the perimeter of the aquifer, especially near the three-channel area in Idaho, and commonly are associated with basalt. The top surface of the Latah Formation in this area represents the bottom extent of the SVRP aquifer. Latah Formation sediments undoubtedly exist throughout much of the aquifer area at greater depth than indicated by the well data. This unit is difficult to distinguish from glaciolacustrine deposits in drillers’ logs unless the driller recorded an “organic-rich” color, organic matter (such as wood), or basalt layers.
Most of the deposits described in drillers’ logs as blue or gray clay likely are glaciolacustrine deposits in the SVRP aquifer. These fine-grained layers were deposited in the previously described large proglacial lakes caused by damming of the ancient Rathdrum-Spokane and Columbia Rivers by ice lobes downstream of present-day Spokane. Long-lasting stands of glacial lakes would have resulted in great thicknesses of fine-grained material being deposited over large areas. In places, the immense floods from Glacial Lake Missoula could have deeply eroded these fine-grained layers. Outside the main path of the floodwaters, remnants of the fine-grained layers may occur. The repetition of the depositional and erosional processes for thousands of years would have caused multiple episodes of fine-grained sedimentation and then subsequent removal and scouring in places during the Missoula floods as well as deposition of coarse-grained material. This history has resulted in the variable nature of the fine-grained layers within the aquifer that have been recorded in drillers’ logs. Additional uncertainty associated with well locations, and variability in the level of detail recorded by different drillers, also contributes to the difficulty of mapping the fine-grained layers. The cross sections on plate 2 illustrate the great variability of the altitude and thickness of the glaciolacustrine deposits within the aquifer and of the Latah Formation adjacent to or beneath the aquifer.
The SVRP aquifer consists of unconsolidated, coarse-grained gravel, cobbles, boulders, and some sand primarily deposited by a series of catastrophic glacial outburst floods. The material deposited in this high-energy depositional environment is coarser grained than is typical for most basin-fill deposits and forms one of the most productive aquifers in the United States (Molenaar, 1988). Fine-grained layers of clay and silt are scattered throughout the aquifer and likely were deposited in large proglacial lakes in the path of the Missoula floods. The aquifer extends from Lake Pend Oreille through the Rathdrum Prairie and Spokane Valley to near Spokane where it is divided by Five Mile Prairie (pl. 1). On the west side of Five Mile Prairie, the Western Arm of the aquifer follows the course of the present-day Spokane River from near downtown Spokane to the community of Seven Mile. On the east side of Five Mile Prairie, the main body of the aquifer extends through the Hillyard Trough and then west through the Little Spokane River Valley to Long Lake, an area referred to as the Little Spokane River Arm of the aquifer (pl. 1). To the south of Five Mile Prairie, the aquifer is separated by a buried basalt ridge that extends about 2 mi south to an area referred to as the Trinity Trough, a breach in the basalt ridge that connects the east and west parts of the aquifer in that vicinity.
Owing to the depositional history described previously, the aquifer generally has a greater percentage of finer material near the margins of the valley and becomes more coarse and bouldery near the center throughout the Rathdrum Prairie and Spokane Valley. In the Hillyard Trough, the deposits generally are finer grained and the aquifer consists of sand with some gravel, silt, and boulders.
Approximate water levels within the aquifer, based on the September 2004 water-level map (Campbell, 2005), are shown in the hydrogeologic sections on plate 2. The greatest depths to water, about 500 ft, occur in the northwest part of the Rathdrum Prairie (hydrogeologic section M-M’, pl. 2). Depth to water in the downgradient part of the aquifer in Washington is about 200 ft or less (hydrogeologic section D-D’, pl. 2). The shallowest depths to water occur along the Spokane and Little Spokane Rivers; near the outlets of Lake Pend Oreille (hydrogeologic sections L-L’ and M-M’,pl. 2) and Hayden and Coeur d’Alene Lakes (hydrogeologic sections J-J’ and I-I’, respectively, pl. 2); and near Long Lake (hydrogeologic section A-A’, pl. 2). In the northern Rathdrum Prairie where bedrock “highs” protrude up into the aquifer, the aquifer may have very thin or seasonally saturated zones (hydrogeologic sections K-K’, L-L’, and O-O’, pl. 2). Although most of the SVRP aquifer is unconfined, the lower unit of the aquifer is confined in the Hillyard Trough and along the Little Spokane River Arm below the extensive fine-grained layer (hydrogeologic sections A-A’ and B-B’, pl. 2). In the Little Spokane River Arm, the altitude of the fine-grained layer is sufficiently high that most of the upper unit of the aquifer is unsaturated.
Numerous fine-grained, low-permeability, interbedded deposits occur within the SVRP aquifer at considerably different altitudes (sections A-A’ through J-J’ and M-M’ through O-O’, pl. 2). General observations about these fine-grained layers, each at least 5 ft thick, are presented by geographic area in the following sections.
As reported in CH2M HILL (2000), an extensive fine-grained layer separates the SVRP aquifer into upper and lower units in the Hillyard Trough (pl. 2, hydrogeologic sections A-A’ through D-D’). Based on observations made during this investigation, altitudes of this layer range from about 1,660 to 1,720 ft. Most of this layer, however, occurs at an altitude of about 1,670 ft (fig. 7). The thickness of the layer ranges from 162 to 265 ft and averages 215 ft as indicated by logs for five wells that fully penetrated the layer. To the south, this layer is estimated to extend within about 2 mi of downtown Spokane (fig. 7). Based on drillers’ logs used during this investigation, a local fine-grained layer occurs at higher altitudes within the Hillyard Trough; however, this layer is thinner and less continuous than the layer at the 1,670-ft altitude. Although the top of the upper fine-grained layer ranges from about 1,790 to 1,840 ft, it generally occurs at an altitude of about 1,820 ft and has a thickness of about 30 ft. Site-scale hydrogeologic characterization of the Kaiser Mead Plant in the north end of the Hillyard Trough (T 26 N, R 43 E, section 16) suggests the presence of numerous thin clay layers, some less than 5 ft thick, in the upper part of the aquifer (HartCrowser, 1988).
The extensive fine-grained layer in the Hillyard Trough also is present in the Little Spokane River Arm of the SVRP aquifer that extends into the Long Lake area. However, the top of the layer in this part of the aquifer is more variable than in the Hillyard Trough and occurs at an altitude of about 1,500–1,700 ft. The thickness of the layer ranges from 20 to 280 ft and averages 130 ft as indicated by logs for 30 wells that fully penetrated the layer.
Too few data are available for the Western Arm of the SVRP aquifer to identify the presence or absence of continuous fine-grained layers. The driller’s log for well 26N/42E-35M01 indicates two clay layers within the aquifer were encountered; one 5-ft thick layer with an altitude of 1,744 ft and a lower 10-ft thick layer with an altitude of 1,644 ft. CH2M HILL (1988) reports that near the former City of Spokane North Landfill site (pl. 1), flood deposits of the aquifer are underlain by glacial lake deposits (silt and clay) except in lower altitude areas near the Spokane River where the aquifer is underlain by basalt (hydrogeologic section C-C’, pl. 2).
Based on available well-log data, the central axis of the Spokane Valley appears to be devoid of extensive fine-grained deposits (hydrogeologic section E-E’, pl. 2). Isolated fine-grained deposits occur locally but mostly are limited to the valley’s margins and at the outlet of Liberty, Newman (hydrogeologic section F-F’, pl. 2), and Hauser Lakes.
Multiple fine-grained deposits are scattered throughout the SVRP aquifer in the Rathdrum Prairie (hydrogeologic sections G-G’ through J-J’ and M-M’ through O-O’, pl. 2). However, the extent of these deposits are difficult to map because of their discontinuity and variable altitudes and thickness. Of the 321 project wells located in Idaho, 52 fully penetrated a clay layer within the aquifer. The altitude of these clay layers range from 1,653 to 2,392 ft with thicknesses ranging from 5 to 98 ft (fig. 8). Another 22 project wells in Idaho partially penetrated 1–135 ft of fine-grained material at their completion depths (fig. 8). Nearly 77 percent of the project wells in Idaho did not penetrate fine-grained material, even though several were in close proximity and depth to wells that did penetrate the fine-grained material. The discontinuity of the fine-grained deposits probably is attributable to this area being along the principal path of repeated Missoula floods in contrast to the Hillyard Trough where more of a slack water environment would have allowed the preservation of more preexisting fine-grained deposits. The locations of wells that fully penetrate fine-grained deposits in the Rathdrum Prairie part of the aquifer are shown in figure 8.
More extensive fine-grained deposits at various altitudes appear to be somewhat common on the east margin of the Rathdrum Prairie between Coeur d’Alene Lake and the south end of the Middle (Ramsey) Channel than elsewhere in the area (fig. 8). A fine-grained deposit with an altitude ranging from about 1,860 to 1,980 ft occurs in the area just west of Hayden Lake (hydrogeologic section I-I’, pl. 2). Well 51N 04W 35DDA1 fully penetrated 90 ft of clay above the lower part of the aquifer; wells 51N 4W 14AAC1, 14DBB2, 23DCB1, 26ACC1, and 35BBA1 partially penetrated clay at the base of each well (hydrogeologic section I-I’, pl. 2). A higher and apparently thinner fine-grained deposit was penetrated in wells 50N 04W 12BCD1 and 51N 04W 11DDA1 and 23ABAC1 (hydrogeologic section I-I’, pl. 2).
On the west margin of the SVRP aquifer near Rathdrum, fine-grained deposits were noted in several wells. Altitudes of the fine-grained deposits in those wells range from about 1,963 to 2,234 ft (fig. 8). Well 51N 04W 06CCDD1 (pl. 1) fully penetrated 80 ft of clay above the lower part of the aquifer.
Southwest of Rathdrum, two deep grounding wells, 51N 05W 11ADA1 (620 ft) and 51N 05W 11ADB1 (650 ft), encountered clay at an altitude of 1,671 and 1,673 ft, respectively. The deeper of the two wells, 51N 05W 11ADB1, only partially penetrated 135 ft of the clay at the base of the borehole (hydrogeologic section J-J’, pl. 2). The altitude and thickness of this clay are similar to those of the extensive fine-grained layer in the Hillyard Trough. These two boreholes are the only ones in the central part of the Rathdrum Prairie that provide information on the lower part of the aquifer. Only coarse-grained material was recorded by the driller for the upper 515 ft of each of the boreholes, indicating the upper and thinner fine-grained deposits that occur elsewhere in the Rathdrum Prairie are absent at this location.
Immediately north of Round Mountain, wells 53N 04W 27DBD1 (hydrogeologic section L-L’, pl. 2), 53N 04W 22CBD1 (pl. 1), and 53N 04W 28CAB1 (hydrogeologic section N-N’, pl. 2) fully penetrated clay at altitudes of 2,271, 2,262, and 2,224 ft, respectively. Thicknesses of the clay layers are 16, 10, and 71 ft, respectively.
Near Athol, fine-grained deposits were penetrated in several wells between altitudes of 2,007 and 2,125 ft. The thickness of the deposits in well 53N 03W 08AC1 is 94 ft (hydrogeologic section O-O’, pl. 2). A 39-ft-thick clay layer at an altitude of 2,392 ft also was penetrated in this area in well 53N 03W 05DCC1 (section O-O’, pl. 2). Near the southern ends of Hoodoo and Spirit Valleys, numerous wells penetrated fine-grained deposits at altitudes that range from 1,983 to 2,258 ft. Thicknesses of the layers range from 12 to 80 ft (fig. 8).
A description of the hydraulic properties of the SVRP aquifer is included here in a generalized fashion in order to provide the reader with an understanding of the highly transmissive nature of the aquifer. Several previous studies including Drost and Seitz (1978), Bolke and Vaccaro (1981), and CH2M HILL (1998) estimated aquifer characteristics based on aquifer tests and ground-water model simulations. Although hydraulic properties of the aquifer were variable, most results indicated that hydraulic conductivity (a measure of the ability of the aquifer material to transmit water) and transmissivity (the rate at which water is transmitted through a unit width of the aquifer under a unit hydraulic gradient, equal to the hydraulic conductivity multiplied by the aquifer thickness) values were on the upper end of values measured in the natural environment (Kahle and others, 2005). Drost and Seitz (1978) reported transmissivity values that ranged from less than 130,000 ft2/d in the western part of the aquifer to more than 13 million ft2/d near the Washington−Idaho State line. Bolke and Vaccaro (1981) estimated hydraulic conductivity values of 2,600–6,000 ft/d for most of the aquifer on the Washington side and about 860 ft/d in the Hillyard Trough. CH2M HILL (1998) reported hydraulic conductivity values ranging from about 100 to 6,200 ft/d, with most values greater than 1,000 ft/d.
In most places, the SVRP aquifer is bounded laterally by metamorphic or igneous intrusive rocks. In places, such as near Spokane and Coeur d’Alene, the aquifer is laterally bounded by basalt and fine-grained interbeds. The bottom boundary of the aquifer generally is unknown except along the margins or in shallower parts of the aquifer where wells have penetrated the entire aquifer thickness and reached bedrock (metamorphic or igneous intrusive rocks) or basalt and fine-grained interbeds. The upper boundary of the saturated portion of the aquifer is represented by the regional water table as described by Berenbrock and others (1995) and Campbell (2005). In the northern Rathdrum Prairie, the water table can be as deep as 500 ft below land surface; near the Washington-Idaho State line the water table is about 150 ft below land surface. Reported ground-water divides approximately represent the aquifer boundary in the Hoodoo and Spirit Valleys and near Careywood, Idaho (Kahle and others, 2005). Upgradient areas of the aquifer also are bounded by tributary lakes, including Pend Oreille, Spirit, Twin, Hayden, Coeur d’Alene, Hauser, Newman, and Liberty. Streams tributary to the aquifer include Lewellen, Sage, and Rathdrum Creeks in Idaho, and Chester and Saltese Creeks in Washington. Streams tributary to the Spokane River in the aquifer extent include Hangman (Latah) Creek near Spokane, Washington, and the Little Spokane River north of Spokane. The aquifer’s lower discharge area is poorly defined, but is believed to be near Long Lake at the confluence of the Spokane and Little Spokane Rivers.
A contour map of the approximate altitude of the base of the SVRP aquifer is shown in figure 9. The base of the aquifer is defined as where the aquifer lies on top of bedrock (granite or metamorphic rock), basalt, or fine-grained deposits that probably are the Latah Formation. Contours were manually drawn and lie within the 2005 revised extent of the aquifer (Kahle and others, 2005). Data used to construct the map include data for the 587 project wells. Of these wells, slightly more than 100 fully penetrate the base of the aquifer and generally are located along the aquifer’s margin. For these wells, the altitude of the base of the aquifer was obtained by subtracting the depth to the base of the aquifer from the digital-elevation-model (DEM) derived land-surface altitude for the well. The inferred base of the aquifer drawn on the hydrogeologic sections also was used for contouring. Data from existing geophysical transects described in Kahle and others (2005) were used as a guide to estimate the base of the aquifer on the sections but not directly on the base-of-aquifer map. If well data appeared to contradict the geophysical transects, preference was given to the well data. Along the aquifer margin, subsurface contours were tied to DEM-derived land-surface contours, also in 200-ft intervals. The altitude of the base of the aquifer ranges from less than 1,800 ft near Lake Pend Oreille to less than 1,200 ft near the aquifer’s outlet near Long Lake (fig. 9).
There is general agreement between the base of aquifer depicted on the map in this report and other more site specific maps (Graham and Buchanan, 1994; Boese and Buchanan, 1996; Golder Associates Inc., 2004; Baldwin and Owsley, 2005; Stevens, 2005). Similarly, there is general agreement between the map in this report and Buchanan’s (2000) map, the first aquifer-wide map produced for the study area. Notable differences are related largely to the different aquifer boundaries used for the two maps. Buchanan used an aquifer boundary similar to the Sole Source boundary (shown in Kahle and others, 2005, fig. 5) that excludes the Middle and Chilco Channels, excludes the southern end of Hoodoo Valley, and connects the aquifer near Nine Mile Falls.
There also is general agreement between the map in this report and a basement-altitude map produced using gravity data to constrain depth to basement modeling (Oldow and Sprenke, 2006). The difference between the two maps is related largely to the different assumptions used in the construction of each map and the uncertainties inherent to each method. An important difference in these maps is that the map in this report is a base-of-aquifer map, whereas the map constructed by Oldow and Sprenke (2006) is a basement map based on a model of a pre-Tertiary basin filled with gravel. The pre-Tertiary basement surface would be expected to differ from the base-of-aquifer surface where the aquifer is underlain by Tertiary basalt and (or) associated fine-grained interbeds of the Latah Formation. The two maps may have better agreement where the basalt and Latah Formation interbeds have been eroded fully from the pre-Tertiary surface. This may be the case where the two maps appear very similar at the outlet of Lake Pend Oreille and throughout most of the Spokane Valley. In areas where the gravity-derived map (Oldow and Sprenke, 2006) indicates a lower surface, such as in the Hillyard Trough and central Rathdrum Prairie, the base-of-aquifer estimates shown in figure 9 infer a shallower base. Both maps and (or) methods would benefit from deep borehole information in these areas.
The extent of the SVRP aquifer used in this report remains the same as that used in Kahle and others (2005). Although much of the aquifer extent is fairly easily defined and well accepted, an exception to this is in southern Bonner County near the south end of Hoodoo and Spirit Valleys (pl. 1). Recent evaluation of water levels in wells in Township 54 N and Range 4 West indicates the ground-water divide near the southern end of Hoodoo Valley may be farther south than shown on plates 1 and 2 (Hsieh and others, 2007). Further analysis of water levels in existing wells and the possible addition of monitoring wells in this area are needed to better characterize the aquifer extent.
An aquifer thickness map was constructed using the same data set described in the previous section to illustrate the approximate thickness of the SVRP aquifer (fig. 10). In Idaho, areas of greatest thickness, more than 800 ft, occur in the northwest part of the northern Rathdrum Prairie, through the West Channel area, and through the west-central part of the Rathdrum Prairie. In Washington, the areas of greatest thickness, more than 600 ft, occur in the central part of the Spokane Valley, in Spokane, and in the Hillyard Trough. Near the Washington-Idaho State line, the thickness of the aquifer is about 400–600 ft. Aquifer thickness estimates are more reliable in areas where wells fully penetrate the aquifer than in areas where the thickness was inferred.
Of the 587 project wells, 24 are completed in the Basalt and fine-grained interbeds unit. These wells are located near Spokane, northwest of Hayden Lake, and near Middle (Ramsey) and Chilco Channels (pl. 1). Although this unit can yield sufficient quantities of ground water for domestic use, it is discontinuous and not considered an important aquifer within the study area. As illustrated on plate 2, some wells completed in this unit are open to the Latah Formation interbeds (25N/43E-27B02 on hydrogeologic section D-D’, 26N/43E-15H02 on hydrogeologic section A-A’, and 52N 04W 24DDB1 on hydrogeologic section K-K’); others are open to the Columbia River basalt (26N/42E-23E01 on hydrogeologic section C-C’, 51N 04W 12ABA1 on hydrogeologic section I-I’, and 52N 04W 24DDB1 on hydrogeologic section K-K’). The total thickness of the Basalt and fine-grained interbeds unit is quite variable as are the individual layers of basalt and fine-grained material within the unit. The total thickness of the unit is shown on only two hydrogeologic cross sections (E-E’ and K-K’, pl. 2) where wells 26N/44E-34B01 and 52N 03W- 19DD1 penetrated 450 and 495 ft of this unit, respectively.
Of the 587 project wells, 67 are completed in the Bedrock unit. Many of these wells are located along the perimeter of the SVRP aquifer near Nine Mile Dam, Hillyard Trough, and Spokane Valley (pl. 1). Others are located in the three-channel area of the Rathdrum Prairie and near Sage and Lewellen Creeks where the overlying aquifer is thin or has a thin saturated zone (pl. 1; hydrogeologic sections K-K’, L-L’, N-N’, and O-O’, pl. 2). The Bedrock unit includes the Precambrian to Tertiary metamorphic and intrusive igneous rocks that laterally bound and underlie the aquifer. The crystalline structure of these rocks generally inhibits their ability to store and transmit water. However, weathered or fractured zones within the rocks can transmit useable amounts of ground water to wells completed in the unit.
A summary of well yields, as reported on drillers’ logs used during this investigation, is shown on the left side of the following table by hydrogeologic unit. Well-yield testing is done to determine if an adequate and sustainable yield is available from a well. Driller-reported well yields are not only dependent on the productivity of the unit to which the well is open, but also are a function of the design and purpose of the well. During well-yield testing, a well intended for municipal water supply likely would be pumped at a higher rate and have both a larger diameter casing and a longer open interval than one intended for single-family use, thereby having an apparent higher yield than that for the single-family well. Despite the fact that yields often are estimates, they are useful in comparing the general productivity of hydrogeologic units; they also illustrate the large amount of variability within a single unit. Based on the data set used for this study, the median yield for the SVRP aquifer, the Basalt and fine-grained interbeds unit, and the Bedrock unit are 100, 10, and 8 gal/min, respectively.
A summary of specific capacity information, derived from driller-reported yield divided by the drawdown measured in the well during pumping, is shown on the right side of the table below, by hydrogeologic unit. Specific capacity is often used to describe the productivity of a hydrogeologic unit. Based on the data set used for this study, the median specific capacity for the SVRP aquifer, the Basalt and fine-grained interbeds unit, and the Bedrock unit are 200, 8.3, and 0.66 gallons per minute per foot, respectively.
| Hydrogeologic unit | Yield (gallons per minute) | Specific capacity (gallons per minute per foot) | |||||||
|---|---|---|---|---|---|---|---|---|---|
| Minimum | Median | Maximum | Number of values |
Minimum | Median | Maximum | Number of values |
||
| SVRP aquifer | 0.08 | 100 | 8,000 | 322 | 0.22 | 200 | 5,500 | 142 | |
| Basalt and fine-grained interbeds | 0 | 10 | 2,280 | 19 | 0.43 | 8.3 | 510 | 4 | |
| Bedrock | 0 | 8 | 1,905 | 60 | 0.01 | 0.66 | 3,810 | 16 | |
U.S.
Department of the Interior | U.S. Geological
Survey
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