Ten production wells drilled in a highly productive sand and gravel aquifer and recharged by an adjacent river were evaluated for potential Giardia and Cryptosporidium contamination. The goal of this study was to determine whether riverbank filtration could achieve significant reduction to a level at which no additional engineered filtration would be required for pathogenic protozoa. Pathogen monitoring was conducted sporadically over 10 years. Intensive monitoring was conducted for a 20‐month period at 10 “flowpath wells” and two production wells. Algae, diatoms, and other surface water indicators were found in 57% of 128 groundwater samples. Of 285 groundwater samples collected and analyzed for Giardiao or Cryptosporidium, no pathogens were detected. No correlation existed between Giardia, Cryptosporidium, and surface water indicators. All surrogates demonstrated a minimum 4‐log reduction. Even though there is hydrologic influence, riverbank filtration is highly effective in removing pathogenic protozoa.

Riverbank filtration relies on the streambed and aquifer matrixes to reduce pathogens under varying conditions of induced infiltration resulting from changes in river stage and flow velocity. The Greater Cincinnati (Ohio) Water Works monitored hydrologic parameters and water quality at its Charles M. Bolton well field during a comprehensive flowpath study. The study determined the frequency of occurrence of high river-stage events from historical data and monitored hydrologic parameters to estimate the potential unit infiltration rate. Giardia, Cryptosporidium, algae, spores, particle counts, and turbidity were also monitored. The project investigated potential pathogen/surrogate breakthrough during several high river-stage and infiltration events. High-stage events occurred less than 4% of the time. Giardia and Cryptosporidium were not detected in any groundwater samples. Increases in surrogate concentrations were minimal and maintained >3.5-log reduction. The streambed and aquifer have the ability to buffer water quality effects from major increases in the infiltration rate.

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