Bannock County Groundwater

Technical Advisory Committee & Findings

A Commissioner-appointed Groundwater Technical Advisory Committee met for approximately 10 months during 2009-2010 to review existing groundwater protection regulations and the data collected over many years of scientific studies of the Lower Portneuf  Valley Aquifer. After evaluating this large body of information, the Committee recommended a series of, what they believed would be, effective and meaningful groundwater protection measures for the aquifer. These recommendations were passed on to the Citizen’s Advisory Committee in fall 2010 for evaluation.

Committee Members

  • Paul Andrus, Planner, City of Chubbuck
  • Shannon Ansley, Environmental Hydrogeologist, Idaho Department of Environmental Quality
  • Justin Armstrong, Superintendent Water Department, City of Pocatello
  • Dennis Dunn, Idaho Department of Water Resources
  • Steve Ernst, Planner, Bannock County
  • Jeff Hammes, District Ranger, U.S. Forest Service
  • Tom Hepworth, Idaho Department of Environmental Quality
  • Jon Herrick, Superintendent Water Pollution Control, City of Pocatello
  • Emma George, Planning Department, Shoshone-Bannock Tribes
  • Matthew Lewis, Planning Division Manager, City of Pocatello
  • Therese Marchetti, Superintendent Landfill, Bannock County
  • Jerry Mason, Attorney at Law, Coeur d’Alene
  • Sean Mottishaw, Sportsmen for Fish and Wildlife representative
  • Tom Mullican, Idaho Department of Environmental Quality
  • John Norstog, Planning Director, Shoshone Bannock Tribes
  • BJ O’Doherty, District Conservationist, U.S. Department of Agriculture
  • Dave Pacioretty, Bureau of Land Management
  • Steve Pew, Environmental Division Manager, Southeast District Health Department
  • Hannah Sanger, Environmental Educator, City of Pocatello
  • Tim Shurtliff, Engineer, Bannock County
  • John Sigler, Senior Environmental Coordinator, City of Pocatello
  • Kevin Spencer, Private Water System Representative
  • George Spinner, Managing Scientist, Cascade Earth Sciences
  • Glenn Thackray, Hydrogeologist, Idaho State University
  • Linda Tigert, Office Coordinator (now County Planner), Planning and Development Services, Bannock County
  • John Welhan, Affiliate Faculty & Research Hydrologist, Idaho Geological Survey

Findings Submitted to the Citizen’s Advisory Committee

  • Septic systems are a significant groundwater recharge source for the Lower Portneuf Valley Aquifer, particularly in the Black Cliffs, Mink Creek and Johnny Creek areas.
    • Each septic system discharges about 250-350 gallons/day.
    • In areas having many closely spaced septic systems, the cumulative amount of contaminated water into the subsurface is very large.
  • Each septic system discharges wastewater with levels in excess of 40 mg/L of nitrate into groundwater.
  • Nitrate levels in groundwater of 3 mg/L (ppm) or above indicate man-made sources.
  • Pocatello has 17 active public water supply wells.
    • 11 of them have nitrate levels regularly above 3 mg/L (ppm).
    • 2 additional wells have had at least one sample near or above 3 mg/L in recent history
    • Thirteen of seventeen wells (76%) in the Pocatello municipal water system are impacted by man-made sources of nitrate contamination
  • Chubbuck has three active municipal wells.
    • All three (100%) of them have nitrate levels above 3 mg/L.
  • Principal recharge areas are the mountains and benches surrounding Pocatello and Chubbuck. As this recharge flows through the subsurface, it carries contaminants to the main body of the aquifer.
  • Pocatello and Chubbuck public water supply wells are located in the main body of the aquifer.
  • Ground water movement in the central part of the aquifer body (the valley floor) is more rapid than it is through the mountains and benches.
  • The dilution capacity of the rapidly moving part of the main aquifer body cannot keep up with the high volume contaminant input from unsewered and densely developed areas along the sides of the valley (Mink Creek, Johnny Creek, Gibson Jack). The Black Cliffs area (South 5th Avenue) contributes lower water volumes but higher nitrate concentrations to the main valley aquifer.
  • Nitrate levels increase toward the northern end of the aquifer indicating a cumulative effect on nitrate concentrations from the Portneuf Gap to Chubbuck.
  • Other known contaminants and their plumes provide an additive or cumulative adverse effect. (TCE, PCE, diesel/fuel, EDB, other chemicals).
  • The southern part of the aquifer, from Red Hill southeast toward Portneuf Gap, is especially vulnerable to contamination due to a thin protective soil cover and shallow depths to groundwater.


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