Runoff from cattle feedlots is collected and held within large holding ponds prior to land application, but in rare circumstances these ponds can leak and affect the subsurface environment. A site identified by unusually high chloride and ammonium concentrations was identified and studied over several years. Additional monitoring wells were installed, and cores were collected along the presumed groundwater flow path. Groundwater chemistry demonstrated significant vertical and longitudinal changes in chloride, ammonium, and nitrate. Up-gradient of the holding pond chloride and ammonium concentrations were low (13 and 1 ppm, respectively) and nitrate concentrations were high (30 to 65 ppm). At a groundwater depth equal to the bottom of the holding pond, ammonium and chloride increased to 12 and 270 ppm, respectively, and nitrate decreased to below detection (less than 0.5 ppm). Further down-gradient, nitrate has increased back to 30 ppm, and ammonia has decreased to less than 1 ppm. Chloride levels remain elevated (greater than 100 ppm). Potential nitrification and denitrification activity was observed in the cores collected down-gradient. A culture-based most probable number assay confirmed the presence of nitrogen-transforming microorganisms with denitrifiers most abundant (up to 92,000 per gram), followed by nitrite-oxidizing bacteria (up to 27,000 per gram), and finally ammonia-oxidizing bacteria (2600 per gram). Molecular studies are ongoing in an effort to resolve whether the community is dominated by bacteria or archaea. Geochemical and microbiological evidence, so far, indicates that intrinsic bioremediation is possible at this site.