Economic Impact Analysis: 1st Progress Report, January
BASELINE WATER QUANTITY AND QUALITY CONDITIONS - OVERVIEW
This section characterizes the LPRV aquifer as a basis for later projection tasks in this study. The geography, geology and hydrology of the aquifer are summarized. Water use trends point to an aquifer in rough balance or in a small deficit between withdrawals and recharge. Baseline water quality conditions acknowledge certain well-publicized water quality issues for which certain progress is being made. Other issues such as levels of nitrate, chloride and sulfate are not presently being addressed.
DESCRIPTION OF THE LPRV AQUIFER
The LPRV aquifer is a highly prolific, alluvial valley-fill aquifer, situated in the Portneuf Valley beginning at the Portneuf Gap and grading into the Eastern Snake Plain aquifer northwest to Tyhee. The contributing watershed includes portions of the Bannock Range and the Pocatello Range, as well as the Upper Portneuf River Valley. The LPRV aquifer is the source of supply for all human needs in the study area.
The general flow path of groundwater in the LPRV system is from the Portneuf Gap toward Pocatello (from southeast to the northwest) (see Exhibit III-1). Northwest of Pocatello, toward American Falls Reservoir, groundwater in the LPRV aquifer converges with groundwater flowing from the north out of the Fort Hall Reservation and groundwater from the Eastern Snake Plain Aquifer.
Much of the current understanding of LPRV aquifer hydrogeology is based on the work of John Welhan and Chris Meehan, who divide the LPRV aquifer into four hydrologic subdivisions for purposes of analysis. These hydrologic subdivisions are shown on Exhibit III-1 and discussed below:
The Southern Aquifer is the principal source of water supply for Pocatello and Chubbuck.
The Eastern Aquifer, smaller than the other hydrologic subdivisions, roughly parallels the southern aquifer, but is distinguished based on a different water quality.
The Central Aquifer acts as a transition from the southern aquifer to the northern aquifer.
The Northern Aquifer extends from the bedrock high located about mid way down the lower Portneuf Valley to the northwest. This portion includes the Pocatello Creek tributary.
GEOLOGY OF THE LPRV AQUIFER
The high productivity and the vulnerability to contamination from surface and near surface sources are rooted in the LPRV aquifer's geology. The following is a greatly simplified overview of that geology.
The geology of the LPRV aquifer is comprised of sands and gravels ranging in thickness from approximately 100 to more than 500 feet and blanketed in areas by 5 to 20 feet of silt and loess. Interbedded units of clay are common toward the northern end of the system.
Changes in sand and gravel composition, depth to bedrock, and bedrock type are evident along the aquifer system. The subdivision of the aquifer into four smaller areas, described on the previous page, helps account for the geologic variability along the river valley.
The Southern Aquifer has a history of excellent water yields, which are derived from coarse, gravels at depths less than 100 to 150 feet below surface. Very permeable, unconfined gravels overlying a section of low-permeability, basin fill sediments dominate this portion of the LPRV aquifer.
The Eastern Aquifer is unconfined and is composed of silty gravels of low permeability.
The Central Aquifer is also unconfined and is comprised of a thin sedimentary layer overlying shallow bedrock.
The Northern Aquifer is comprised of multiple confined silty gravel and sand aquifers hosted in stratified, but poorly sorted, sedimentary basin fill more than 2,000 feet thick.
LPRV AQUIFER EXCHANGE
The recharge characteristics of the LPRV aquifer provide both information about its sustainability as the sole water source for the study area and about the areas that may be particularly susceptible to contamination from surface or near surface sources.
Annual recharge of the LPRV aquifer is estimated at approximately 7.4 billion gallons (23,000 acre-feet) per year. The following discussion focuses primarily on the southern division of the aquifer, which initially receives the bulk of the recharge. The other aquifer hydrologic subdivisions are recharged principally through intra-basin flows from south to north.
An estimated 70 percent of primary recharge to the southern portion of the LPRV aquifer is lateral groundwater flow from the Bannock Range. This recharge area includes the Mink Creek and the Gibson-Jack Creek sub-basins (see Exhibit III-1). The recharge originates from the snowpack and precipitation in the southern Bannock Range.
Approximately 15 percent of southern LPRV aquifer recharge is derived from the upper Portneuf River basin through the Portneuf Gap, and over ten percent is derived from the other drainages, principally the Eastern Aquifer and Pocatello Creek. The evidence suggests that the Portneuf River does not significantly recharge the aquifer.
Other recharge sources include the Pocatello Creek drainage, Pocatello Range, direct precipitation and intra-basin flow. Groundwater flowing into the LPRV watershed from areas outside of the boundaries (intra-basin flow) is unknown, but is potentially a source of recharge.
LPRV AQUIFER WATER USE
The LPRV aquifer provides the sole source of potable water supply for the cities of Pocatello and Chubbuck, as well as supplies for self-supplied industrial operations, rural households with their own wells and agricultural operations in the valley. Current annual water use is rapidly approaching the annual recharge estimate described previously. If water demands continue to rise at the rates experienced over the past decade, groundwater mining resulting in declining aquifer levels may begin to occur.
Published statistics on LPRV aquifer water use are available only for the Pocatello and Chubbuck municipal water utilities. The Study Team has estimated household water use in rural areas based on a local per capita water use factor and an estimate of the self-supplied population served. Total withdrawals for municipal system and household self-supply in the year 2000 are estimated to be about 6.9 billion gallons (21,400 acre-feet).
The LPRV aquifer also supports self-supplied industrial and agricultural operations. The study team estimates that 0.6 to 1.3 billion gallons (2,000 to 4,000 acre-feet) per year are withdrawn from the aquifer for these non-potable purposes.
WATER USE TRENDS
In the past decade, there has been modest growth in the amount of LPRV aquifer water withdrawn by municipal water utilities and by self-supplied households in aquifer-served rural areas.
The accompanying chart illustrates the following water use trends:
Pocatello used 5.6 billion gallons (17,580 acre-feet) per year in 2000, up from 5.43 billion gallons (16,920 acre-feet) in 1990, an increase of about 4 percent. Current Pocatello water use is about 300 gallons per capita per day.
Chubbuck used 0.64 billion gallons (2,000 acre-feet) per year in 2000, up from 0.55 billion gallons (1,720 acre-feet) in 1990, an increase of about 16 percent. Chubbuck water use is about 190 gallons per capita per day.
Self-supplied households in rural areas used 0.57 billion gallons (1,780 acre-feet) per year in 2000, up from 0.50 billion gallons (1,560 acre-feet) in 1990, an increase of about 15 percent.
LPRV AQUIFER WATER QUALITY - OVERVIEW
Historically, the study area has had numerous commercial or industrial sites that have at one time caused water quality degradation. There have been 13 U.S. EPA Superfund sites identified in the study area but, through remediation and other efforts, all except one of these sites have been removed from the Superfund list.
LPRV water quality concerns became highly publicized with the discovery of Trichloroethylene (TCE) in a number of Pocatello municipal wells in 1991 and Perchloroethylene (PCE) in Chubbuck municipal wells. The source of the TCE is generally believed to be from older areas in the Fort Hall Mine Landfill and work is currently underway to pinpoint the source and develop a plan to limit further contamination. The source of PCE is still unknown and being investigated by EPA and its consultants.
The cities of Pocatello and Chubbuck water quality records show that the overall quality of the LPRV aquifer is good, although the water is very hard and has a relatively high total dissolved solids (TDS) content. However, more recent water quality monitoring by the IDEQ, Idaho Department of Water Resources, and the City of Pocatello indicate several areas of the LPRV aquifer system have degraded water quality. TCE, PCE, nitrate and sulfate are the primary constituents of concern (COCs) that have been identified to date, while rising chloride levels indicate potential pathways for contamination by surface spills or runoff. The COCs vary with each portion of the aquifer, as discussed on the following page.
LPRV AQUIFER WATER QUALITY - BY LOCATION
Principal locations of water quality concern are shown on Exhibit III-2. The observations regarding LPRV water quality are based upon the Idaho Statewide Groundwater Monitoring Program, IDEQ studies and various reports by John Welhan and Chris Meehan.
Southern Aquifer The greatest concern in the Southern Aquifer has been the TCE plume, discovered in 1991, discussed earlier. Nitrate levels in five of the six monitoring wells used in the Idaho Statewide Groundwater Quality Monitoring Program appear fairly constant. The sixth well indicates a substantial increase in concentration between 1993 and 1997, although only two data points are available. An area near the northern end of the hydrologic subdivision is experiencing increasing salt concentrations, likely due to drawing water from the Eastern Aquifer as a result of cones of depression from municipal drinking wells in this area. Several localized, high-chloride anomalies have been observed in this aquifer and may stem from surface, or near surface, sources such as salt runoff from road de-icing, septic field leaching and/or agricultural/animal waste.
Eastern Aquifer This area is known to have degraded water quality and a portion of the hydrologic subdivision has been identified by IDEQ as a nitrate priority area. One municipal well was drilled in this area, but was never used regularly and was eventually abandoned because of chronically high nitrate levels. Flow of contaminated groundwater into the Southern Aquifer is a potential concern.
Central Aquifer This area is not well researched, but, as in parts of the Southern Aquifer, there is concern over migration of high salt concentrations from the Eastern Aquifer.
Northern Aquifer IDEQ has identified a high nitrate concentration area near the Pocatello Creek mouth as a nitrate priority area. Corresponding increases in chloride, sulfate, sodium, calcium and magnesium in the same area suggest that septic leachate may be the cause. The City of Chubbuck and the EPA have contracted for a groundwater monitoring program in this area, largely due to concerns about the nearby PCE plume affecting the Fort Hall Reservation and municipal wells in Chubbuck.
CURRENT EFFORTS MADE TO ADDRESS WATER QUALITY CONCERNS
Efforts are being made by Bannock County and the cities of Pocatello and Chubbuck to address several of the water quality issues mentioned above. For example, the TCE contamination in the Southern Aquifer is currently being studied. The PCE contamination in the Chubbuck area has also been monitored for several years and work is in progress to address the contamination.
In contrast, proactive programs have not been implemented to address the concentration, distribution, and sources of other COCs -- such as sulfate and nitrate -- and indicators of potential problems such as increased chloride levels. Rising chloride levels are an indicator of potential pathways for contamination from surface spills or contaminated runoff. These contaminants are likely the result of non-point sources such as septic leachate, road salting, stormwater runoff and runoff from agriculture and ranching operations. Without proactive measures, these contaminants are likely to continue to degrade LPRV groundwater quality.
AVAILABLE DATA SOURCES
Much of the core knowledge of the LPRV hydrogeology is based on extensive research conducted through Idaho State University by John Welhan and Chris Meehan. Additional data is available from the following sources:
Idaho DEQ Regional and Local Monitoring Data IDEQ maintains several databases that house data about various potential contaminant sources. These include: 1) Primary Contaminant Inventory; 2) Drinking Water Management System; and 3) Wastewater Application Permit Database.
Idaho Department of Water Resources Ambient Monitoring Data Idaho Department of Water Resources (DWR) in cooperation with the Idaho Department of Environmental Quality (DEQ) and the Idaho Department of Agriculture (IDA) designed and maintains a statewide groundwater quality monitoring network. The objectives of the statewide program are to: 1) characterize the groundwater quality, 2) analyze for trends, and 3) identify areas where concentrations of constituents are anomalous.
EPA Regional Geographic Initiatives Database - A geographic information systems (GIS) database of wells, water quality data, soils, geology, land use, etc.
Other Research in the Area Several research projects have been conducted in the vicinity of the LPRV, but are focused on specific areas and issues.