[en] During the summer of 1993, groundwaters and surface waters were found to have anomalous sulfate concentrations in the Southern Pocatello municipal aquifer in an area known as the Highway Ponds. Leach tests performed on a large pile of road aggregate stockpiled near the Highway Ponds have been identified as the most likely source for the sulfate. Correlating trends of sulfate and chloride concentrations can be found both in the main Pocatello aquifer and in Pocatello Creek groundwaters. The chloride contamination at Pocatello Creek has previously been suggested to be derived from road salt. It is hypothesized that aggregate used in roadbed construction may be responsible for elevated sulfate in the areas groundwater. Chemical modeling has eliminated carbonate precipitation/dissolution reactions in buffering the chemistry of sulfate-impacted groundwater. Ion-exchange with clays is hypothesized to be a more significant process and is being investigated further. 12 refs., 3 figs