Pocatello Well Chloride Concentrations

Two principal sources of chloride contamination are: 1) storm water and road runoff, and 2) septic systems. Either or both are believed to be responsible for the gradual increase in chloride with time. Septic leachate percolates from septic drain fields (along with nitrate, sulfate and other constituents), whereas storm water enters through parking lot drains, roadside ditches, and other road runoff control structures which promote infiltration. By itself, salt is not a health hazard (other than its sodium constituent, for people on sodium-restricted diets). Rather, the threat posed by such runoff control structures is from other constituents of road and parking lot runoff, such as oil and grease, lead, and chemical spills along transportation corridors. The fact that salt is reaching the aquifer is the real cause for concern, because it indicates that pathways exist by which other, more harmful compounds can enter.

Both Pocatello and Chubbuck have numerous drain structures called dry wells which are designed to allow street and parking lot runoff to drain into the ground. No one knows how many of these control structures exist although some estimates range into the hundreds (City of Pocatello Engineering Dept., pers. comm., 1999). Because these structures are located directly over the aquifer, because they promote large amounts of infiltration at point locations, and because the highly permeable Bonneville gravels underlie most of the valley floor, this source of potential contamination is considered a priority for future management action. The City of Pocatello is already taking steps in this direction through its new Storm Water Master Plan.