Microbial Contamination of Groundwater under Agricultural Fields, Sources and Pathways
University of Guelph, Kemptville Campus
The application of animal manure and human waste to provide nutrients to field crops has been widely practiced for centuries. Increased interest in recycling other organic materials, such as paper mill, distillery and vegetable waste from supermarkets, has broadened the range of possible sources of pathogenic bacteria applied to soil. In addition, on-site waste treatment systems, such as septic units, are more prevalent in rural than other areas. Grazing domestic and wildlife may also pose a threat. Whether or not pathogenic microorganisms, such as bacteria, viruses and protozoa, are able to travel through the soil and superficial rock layers to groundwater depends on the form and the way these organic materials are applied, the partitioning of precipitation into surface runoff and infiltration, survival and persistence of pathogens outside a suitable host, soil type, soil structure, and the type of flow that dominates water movement down through the soil. The same soil-focused considerations apply to the outflow from the weeping beds of septic waste systems. Solid organic materials act somewhat like mulch and encourage infiltration of water, whereas liquid and slurry forms can clog finer pores at the soil surface, thereby encouraging runoff, but the water entering the soil is likely to do so through large diameter pores that can facilitate rapid flow to depth. Once in the soil, bacteria from solid manure appear to be less able to compete with indigenous soil organisms that those from liquid manure. Bacteria and viruses are strongly adsorbed to metal oxides, clay and organic matter, and so are less likely to be removed from sandy soils. However, even though sandy soils are more permeable, unsaturated flow is more likely than movement under saturated conditions. Well surveys in Ontario, Canada showed much less contamination of groundwater under coarse textured soils than under soils with more clay and silt. Saturated flow conditions can occur in these finer textured soils, especially as they are often well structured. The presence of macropores, whether formed by soil organisms such as earthworms or larger burrowing animals, or produced by wetting-drying or freeze-thaw cycles, are very important for the rapid transport of viruses, bacteria and protozoa to depth (Fig. 1.) There is evidence that the walls of these pores are lined with organic matter and also more mineral nutrients are available than in the bulk soil, which will tend to enhance pathogen persistence. This paper reviews the processes important for pathogen transport and persistence in soil and considers the consequences for quantitative risk assessment and agricultural management practices that can enhance the protection of groundwater resources.