Microbial contamination of subsurface tile drainage water from field applications of liquid manure

Application of liquid manure to fields is a common practise to efficiently utilize and dispose of manure in many North American farm operations. Guide lines given to farm operators frequently focus on the amounts of manure req uired to provide certain nutrient requirements given the source of the manu re and plant requirements. Under proper conditions, this is an efficient us e of manure with few adverse environmental impacts. However, under adverse conditions, excessive application rates can result in significant ground an d surface water contamination by bacterial and other contaminants in the ma nure. A key problem is that the pathways by which these contaminants reach surface waters are not well understood. Because of this, not enough informa tion is available on the amounts which can be safely spread and those condi tions to avoid in order to prevent inadvertent contamination by the spreadi ng of liquid manure. The experiments described in this paper report on bact erial contamination at a single field site due to liquid manure spreading u nder accepted practises in Ontario over a two year period. A nalidixic acid resistant strain of Escherichia coil was used as a biotracer to quantify t he degree of transport from the field level to nearby receiving waters. The results show that significant amounts of bacteria can reach surface water by infiltrating through the soil and travelling through sub-surface tile dr ains to the receiving water. Rain shortly after manure application is sugge sted to be the most important indicator of bacterial contamination rather t han spreading rate (volume applied per unit area) or condition of the field prior to spreading.