Bacterial Movement Through Fractured Bedrock

Abstract

ABSTRACTThe movement of bacteria‐laden waters percolating through fractured crystalline bedrock in mountainous terrain was examined to determine whether effluent originating from domestic waste disposal systems could contaminate shallow ground‐water supplies. Inoculated waters were injected into holes and/or wells at two geologically different test sites (granitic, metamorphic) to evaluate the extent of microbial filtration in or along bedrock fractures. Microbiological examination of tracer waters, sampled both above and below the zone of saturation, was made.Field studies showed that the direction and rate of movement of contaminated ground waters were controlled largely by the anisotropic nature of the geologic stratum, particularly by the orientation of major bedrock fracture sets. Inoculated waters were found to be readily transported by the ground‐water gradient into a downslope well. At one test site, a tracer bacterium traversed a horizontal distance of 94 feet in 24‐30 hours. Continued bacteriological analysis of the contaminated well found the organism to be present for at least five days after inoculation of the upslope well.In the zone of aeration, bacteria‐laden effluent was found to percolate in or along fractures with inadequate filtration prior to entering the ground water. Studies conducted in metamorphic rock demonstrated that while fecal‐type bacteria decreased slightly during percolation through bedrock fractures, total bacterial densities were generally unchanged.From the hydrogeological and microbiological data obtained at both test sites, it can be concluded that moderate percolation rates and minimum distances between water wells and leachfield type waste disposal units are inadequate to protect potable ground‐water supplies from contamination in mountainous terrain.