Microbial and chemical tracer movement through two Southland soils, New Zealand

Abstract

There has been a recent, rapid increase in both land application of dairy shed effluent in Southland, New Zealand, and the microbial load in ground and surface waters. We investigated the fate of faecal coliforms, a host-specific Salmonella bacteriophage, and a non-reactive chemical tracer (Br–), when applied to large intact lysimeter soil cores (500 mm diam. by 500 mm high), to determine the pattern of microbial transport through typical Southland soils. The soils were a poorly drained Fragic Perch-gley Pallic Soil, and a well-drained Typic Firm Brown Soil. A depth of 25 mm of dairy shed effluent containing faecal coliforms and spiked with bacteriophage and Br– was applied to the soil at a rate of 5 mm/h followed by ~1 pore volume of simulated rainfall applied at 5 mm/h. Resulting leachates, collected continuously over ~1 pore volume, were analysed for the microbial and bromide tracers. The microbial tracers moved rapidly through both soils, peaking early in the leachate at ~0.15 pore volume and then tailing off in a pattern indicative of bypass flow. Bromide moved more uniformly through the soils but peaked at ~0.5–0.8 pore volume. The microbial flow pattern observed indicates that the structure in these soils makes them vulnerable to leaching of microbes into local surface and ground water. The large difference between the rate of microbial and chemical tracer transport indicates chemical tracers should only be used with caution to model microbial transport parameters.