Recommendations for TCE Plume Action


This report was prepared for the City of Pocatello by the Portneuf Ground Water Forum, July 7, 1999. Portneuf Groundwater Forum members involved were:



Recommendations for the TCE problem in the south valley area were requested by the Mayor and City Council to provide guidance for designing a performance-based request for proposals (RFP) for remediation alternatives. Additionally, this recommendation is designed to help the city council make critical decisions based on the proposals received from the RFP. It is important to note that the recommendations made in this memo are based on existing factual information only.



Trichloroethylene (TCE), a liquid solvent, currently occurs in eight city drinking water wells. In 1993 when extensive monitoring commenced, only three wells had solvent concentrations above 2 parts per billion. By 1998, all of the eight wells had concentrations above 2 parts per billion. All evidence studied to date indicates the number of wells and the affected area of the aquifer, are growing.



  1. Reduce solvent content in ground water to protect downstream wells from contamination (plume control).

  2. Identify the extent of the "data gap" in the south valley to identify and characterize the sources of contamination.



Detailed mapping of solvent distribution conducted in 1994-1995 as well as the geographic pattern of other wells affected since then, indicates that a steadily lengthening TCE plume occupies a large portion of the aquifer in the southern valley. Although solvent concentrations in wells most affected by the plume have varied since 1993, there is no evidence that concentrations overall are decreasing. Additionally, the geographic pattern of wells affected by increasing concentrations indicates the plume is not dissipating. Using the past six years of analytical records and having a good picture of the hydrology and geology of the area, it is estimated that more than 100 gallons of liquid solvent have already moved past the LDS ballfield. This suggests that the source contains a large quantity of solvent with the potential for continued high concentrations downgradient.

Recommendation 1 above will address the urgent water supply and water quality situation in the south valley. The Forum believes that without plume control the city will have fewer options and less flexibility in where to drill new wells to supply clean drinking water for future demand. Recommendation 2 above reflects the need to develop a long-term remediation strategy based on "source control" which must be predicated on better information which will enable everyone involved to remediate the source. There is evidence to suggest a source other than the landfill, therefore it would be prudent to design a remediation that would take into consideration all sources which, consequently, requires determining if there are other sources present.