GROUNDWATER BASINS IN BOUNDARY COUNTY
Groundwater is widespread in Boundary County, but has received very little study, and information about its quality or distribution is very limited. Neither the Idaho Department of Water Resources nor the Idaho State Geological Survey have published any reports on groundwater in the county. Neither these agencies nor the Panhandle Health District have organized the information in their files regarding groundwater quality throughout the county in a way that is accessible to their staff or the general public. However, the DEQ maintains a website that includes some of the water quality and other data on all public water supply systems in the county. The data are contained in “Source Water Assessment Reports” that were completed between 2000 and 2003 and can be viewed online. The Idaho Department of Water Resources maintains a website that includes a database of well driller’s logs that can be accessed for information on the depth to groundwater in individual wells. Due to the limited availability of official groundwater reports for Boundary County, these two databases were used to perform a cursory analysis of the occurrence of groundwater in the county and develop the following discussion of groundwater basins and aquifers. The analysis included the construction of well-log cross sections in various parts of the county. These sections can be viewed at the Boundary County Planning Department.
Although the DEQ map identifies watershed sub-basins (hydrologic units) and separate aquifers, the DEQ has not officially identified any distinct groundwater basins within Boundary County. However, examination of surface geological and topographic maps and subsurface well logs indicates that separate or partially separate groundwater basins probably exist.
Purcell Trench Groundwater Basin:
One of the main geographic features of Boundary County is the large valley that trends north-south through the center of the county from the Canadian border into Bonner County. This valley, commonly referred to as the Purcell Trench, is a deep, fault-bounded basin that lies between the Selkirk Range on the west and the Purcell Range on the east. Several hundred feet of glacial and an unknown thickness of pre-glacial sediment were deposited in this basin, and this sediment comprises the main aquifers in Boundary County. These aquifers receive their water from precipitation that falls in the mountains and on the valley floor and then percolates downward and flows slowly toward the basin. Regionally, groundwater flows eastward from the Selkirk Range and mostly westward from the Cabinet and Purcell Ranges. In the east-central portion of the county where the Kootenai River has eroded its valley through the Cabinet-Purcell Range, groundwater probably flows southward from the Purcell Range and northward from the Cabinet Range. The DEQ identifies the Purcell Trench as IDAQUIFER #3 and shows it as part of watershed sub-basin 17010104.
Various geologic features of the Purcell Trench interrupt this regional groundwater flow and separate the basin into smaller sub-basins that have locally different groundwater flow patterns. These sub-basins may have slightly different aquifers, water tables and water chemistry. A few of these are discussed below; others may exist that were not identified during this preliminary examination of groundwater resources in Boundary County.
Highland Flats Sub-Basin:
The southern portion of Boundary County is more mountainous than the rest of the county. The Selkirk Range occupies the west half of the county and the Cabinet Range encompasses the east half. The Selkirk Range consists primarily of very hard, crystalline granitic rock that is non-water bearing except very locally in fracture zones (the IDAQUIFER #2 portion of watershed sub-basin 17010104). The Cabinet Mountains are underlain by hard, well-cemented, fine-grained sedimentary rocks that have been slightly metamorphosed and recrystallized. These rocks also have little or no pore space, but like the granitic rocks of the Selkirk Range are locally water-bearing where fractured (IDAQUIFER #10 and #11). Because both ranges are elevated and contain few, poor-quality aquifers, they are not considered to be groundwater basins.
In the southern part of the county, the Purcell Trench groundwater basin is less than a mile wide. The western half of the basin in this area is known as the Highland Flats, which is a relatively flat plateau at a surface elevation of between 2,200 and 2,300 feet. Well-log data suggest that Highland Flats forms a small groundwater sub-basin that is partially separated from the main portion of the groundwater basin by a topographic ridge of granitic bedrock that is partly buried and partly exposed. This ridge is bounded on the west, and probably on the east, by faults (Miller and Burmester, 2003). The highest point on this ridge is Round Mountain at an elevation of 2,851 feet. The western margin of the sub-basin is the Selkirk Range.
Highland Flats is underlain by between 20 and 30 feet of fine-grained brown sediment that is generally non-water bearing. Beneath this is a zone of at least 200 feet of blue clay that was deposited in an ancient lake during glacial times more than 10,000 years ago. This clay is also non-water bearing. Hence, there are no aquifers in Highland Flats that are shallower than about 200 feet.
The lake deposit forms a confining layer for an aquifer that underlies it. This means that groundwater beneath the confining layer is under upward pressure, and when a well is drilled through the confining layer into the underlying aquifer, groundwater will rise in the well above the level of the aquifer. Therefore, the depth to water in the well is less than the depth of the water-producing zone. In Highland Flats, the aquifer that underlies the lake clay is a zone of sand and gravel. In the wells that have been studied, this zone occurs at a depth ranging from 250 to 400 feet and is 20 feet thick or less. Due to variations in pressure and aquifer characteristics, the depth to groundwater in these wells is highly variable. The aquifer produces groundwater at rates ranging from approximately 10 to 60 gallons per minute (gpm). Underlying it is a thin zone of weathered granitic bedrock, which also produces limited quantities of groundwater in some wells.
This zone is as much as 50 feet thick, but produces at a rate of only 1-2 gpm. None of the public water systems listed in the Public Services, Facilities and Utilities section are located within Highland Flats, and therefore the DEQ database provides no information about the chemistry of this groundwater.
Paradise Valley Sub-Basin:
North of Highland Flats, the Purcell Trench widens eastward and the western margin of the Cabinet Range has been eroded and partially buried by lake and other deposits that occur in Highland Flats. This broad plateau, sometimes referred to as a “bench,” is slightly lower in elevation than Highland Flats and is known as Paradise Valley. Deep Creek is the main surface stream in this portion of the trench. It flows northward near the western margin of the basin to its confluence with the Kootenai River west of Bonners Ferry.
In Paradise Valley, both the surficial brown sediment and the underlying blue lake clay are thicker than in Highland Flats. The brown sediment is 30-50 feet thick and the blue clay is more than 500 feet thick in the central portion of the Purcell Trench. Because the Purcell Trench is deep in this area, it is possible that multiple aquifers are present at depths that could exceed 1,000 feet in the center of Paradise Valley. However, none of the wells that have been examined are deep enough to have completely penetrated the lake clay, and therefore neither its total thickness nor the depth to an underlying aquifer or granitic bedrock are known at this time. Because these wells were not drilled through the clay, they were dry holes and they produce no water. As a result, many residents in this area utilize surface water provided by the Paradise Valley Water Association.
The DEQ map is generalized and does not identify any distinct or separate aquifers within the Paradise Valley Sub-basin. Paradise Valley is shown as part of the “Kootenai Valley Flow System.” However, wells along the eastern margin of Paradise Valley indicate that several small, partially separate aquifers are present. These wells penetrate little or no lake clay. Instead, this area is underlain by multiple zones of sand and gravel that have limited lateral extent and are probably lake margin stream deposits and/or beach deposits. Some wells produce water at low rates (2-5 gpm) from zones as shallow as 25 feet, while others produce water at rates of more than 10 gpm from zones deeper than 100 feet. The Cabinet Mountains Water Association produces groundwater from two very productive shallow wells in the Crossport area just south of the Kootenai River. These wells appear to be capable of producing more than 500 gpm from a gravel aquifer that was deposited by the river. Due to these variations, the depth to groundwater is highly variable in the eastern Paradise Valley.
Kootenai Valley Sub-Basin:
West of the City of Bonners Ferry, the Kootenai River turns northward to parallel the Purcell Trench and forms the broad Kootenai River Valley. The western margin of this valley is the steep eastern flank of the Selkirk Range. The eastern margin is a topographically high terrace that is locally referred to as the North Bench. It is the equivalent to the flat plateau that forms Paradise Valley, although it is crossed by more numerous creeks and streams that have eroded channels into its surface, making it more undulating and not as flat-lying as the surface of Paradise Valley. The Purcell Range lies east of the North Bench, and forms the eastern limit of the Kootenai Valley groundwater sub-basin. The DEQ map identifies this sub-basin as the “Kootenai Valley Flow System” and maps it as IDAQUIFER #3.
The Kootenai Valley occupies the central part of the Purcell Trench, which is probably more than 1,000 feet deep. The Purcell Trench Fault is a major fault that separates the Selkirk Range from the trench, and movement on this fault elevated the Selkirk Range and depressed the Purcell Trench (Doughty and Price, 2000). There are relatively few wells within the Kootenai Valley, especially on the floodplain of the Kootenai River, and most of these are shallow wells that were drilled only deep enough to reach the first groundwater zone. In 1971, the United States Geological Survey drilled 84 observation wells in the floodplain and monitored the depth to groundwater in these wells over a period of several months (Dian and Whitehead, 1973). Most of these wells were drilled to depths of 30 feet or less, and encountered groundwater at depths ranging from five feet to about 20 feet. Logs of these wells indicate that the water-bearing zones ranged mostly from clay to medium-grained sand, although two wells produced from coarse-grained sand and one well produced from gravel. These water-bearing zones could not be traced from well to well over even short distances, indicating that aquifers are probably local in extent near the valley’s surface.
Water quality in these near-surface aquifers is likely influenced by intense agricultural activity, but little information is available to quantify this effect. The DEQ Source Water Assessment Reports for some water systems in the Kootenai Valley do indicate that some of these wells are at moderate risk of organic contamination from drainage ditches and agricultural operations. During the spring season, when water levels in the Kootenai River are highest, surface water percolates through the river levees into the surrounding floodplain, which raises the water table. Farmers must pump this excess water into drainage ditches to lower the water table, and this drainage may carry agricultural wastes and various organic chemicals, such as fertilizers and pesticides, downward to groundwater. These chemicals may then migrate in the subsurface toward the Kootenai River (Dion and Whitehead, 1973). A subsequent study performed by Extension professor David Wattenbarger in 1990 and 1991 indicate that while some infiltration occurred, pollutant levels did not exceed clean water standards. In his study, Wattenbarger took water samples from the outlets of ditches in five drainages; District 12, Ball Creek, District 1, Deep Creek, District 6, Houcks, District 4, county road, and District 10, mouth of Long Canyon Creek. The ditches were sampled every two weeks from mid-April through early October, and the samples analyzed by the University of Idaho Analytical Laboratory. In 1990, samples were evaluated for nitrates. In 1991, samples were analyzed for both nitrates and phosphates. Nitrate levels were highest in April and decreased with each sample thereafter. The highest sample in April met clean water standards and by September nitrates fell below detectable levels. The phosphate level was also highest in April, yet also fell within clean water standards. Phosphorous fell below detectable level by mid-August.A number of wells have been drilled along the western margin of the floodplain, where homes have been built along the West Side Road. In this area, the granitic bedrock of the Selkirk Range is present at shallow depths (50 feet or less), and most wells produce either from the granite or from coarse gravel or sand that overlies the bedrock. Rainfall and snowmelt that falls in the Selkirk Range probably percolates downward and flows into fractures in the bedrock toward the Kootenai Valley and either emerges at the surface as springs or accumulates in the bedrock or in the gravel that overlies it. Some residents use these springs as their principal water source. Wells in this area may exhibit large variations in the depth to groundwater from year to year, depending on the amount of precipitation that falls in the mountains.
According to data obtained by the DEQ, water samples obtained from springs and wells near the Selkirk Range front have occasionally tested positive for elevated concentrations of uranium or arsenic. These metals are naturally occurring in the bedrock, and it is likely that they have been leached out of the rock and concentrated in surface and/or groundwater. They are the only known contaminants affecting the groundwater quality in the Kootenai River sub-basin, but in most cases concentrations have not exceeded the maximum contaminant limit allowed by law.
Round Prairie Groundwater Basin:
In the northern part of the county is the Round Prairie groundwater basin. The DEQ map shows Round Prairie as part of the Kootenai Valley aquifer system (IDAQUIFER #3) and watershed sub-basin 17010105, but examination of water well logs in Round Prairie indicates that it is probably separate from the Kootenai Valley sub-basin. Round Prairie Creek flows eastward through a narrow gap in the Purcell Range and joins the Moyie River in the northeastern part of the county, and it appears that groundwater in the aquifer also flows eastward rather than westward toward the Kootenai Valley Flow System.
Within the gap in the Purcell Range, Round Prairie Creek or a predecessor stream has built a floodplain and deposited sand and gravel that form the principal aquifer supplying residents in this part of the county. This gravel aquifer is generally about 50-feet thick or less, but is up to about 100 feet thick in some wells. Due to high runoff from the range, most of the aquifer is saturated and the water table in the Round Prairie Valley is very shallow (10 feet or less), or is even above the land surface in some areas, creating marshy wetlands that remain saturated much of the year. Private wells that tap the gravel aquifer yield good quality water at rates of five to 50 gpm. There are no public water systems that draw groundwater from this aquifer in the Round Prairie Basin.
In some parts of Round Prairie, especially near its north and south borders, the gravel aquifer is thin or absent. Wells in these areas tap very old metamorphosed sandstone, shale and igneous rock called the Pritchard Formation of the Belt Supergroup that underlies the gravel deposit and forms the bedrock in the Purcell Range. Obtaining usable quantities of groundwater from the Pritchard Formation requires intersecting a fracture zone with sufficient permeability, and finding a suitable drilling location is difficult. Fortunately, there are numerous faults in this portion of the Purcell Range, and it appears that these fault zones create sub-surface aquifers in local areas. The DEQ map identifies these aquifers as #4, #5, #7 and #8. These fractured-rock aquifers occur at different depths, so some wells produce groundwater at relatively shallow depths (200 feet or less), while others encounter water-bearing zones at depths that exceed 300 feet. Flow rates are also highly variable, with some wells tested at nearly 100 gpm.
Because there are no public water supplies in the Round Prairie basin, the DEQ has no information about differences in water quality between the shallow gravel aquifer and the deeper Pritchard Formation aquifers in this groundwater basin.
Moyie River Groundwater Basin:
Residents living north of the City of Moyie Springs rely primarily on groundwater along the course of the Moyie River, which flows southward out of Canada to its confluence with the Kootenai River near Moyie Springs. The Moyie River occupies a narrow canyon formed by the Moyie Fault, which has fractured the Pritchard Formation and made the rock more susceptible to erosion. This river follows this belt of fractured rock and has built a narrow floodplain that consists primarily of coarse gravel and boulders. The gravel yields groundwater at flow rates of 35 or more gpm at very shallow depths (50 feet or less). The aquifer is recharged by runoff from the mountains that border the valley, as well as by infiltration from the river itself. A few wells obtain their water from the Pritchard Formation at depths of more than 100 feet, such as the public water systems of Eastport, Good Grief, and Feist Creek Resort. The DEQ does not differentiate the Moyie River underground water basin from the Kootenai Valley Flow System.
Appendix V š