Economic Impact Analysis: 1st Progress Report, January 4, 2001
Section III: Baseline Water Quantity & Quality Conditions


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:

 

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.

 

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.

 

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.

 

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:

 

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.

 

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: