Overland flow transport of pathogens from agricultural land receiving faecal wastes

Considerable investment has been made in recent years in improvements to the microbiological quality of urban wastewater discharges to surface waters, particularly in coastal towns, with the aim of reducing the exposure of bathers and surfers to gastrointestinal pathogens. As this source of pollution has come under greater control, attention has started to focus on diffuse catchment sources of faecal contamination which have been shown to be dominant during high river flows associated with storm events. This association with storm events suggests that rapidly responding hydrological pathways such as overland flow are likely to be important. The aim of this paper is to establish the current state of knowledge of pathogen transport processes in overland flow. In addition, the paper will attempt to convey the way that soil erosion science may aid our understanding of this environmental problem. The scale and nature of faecal waste applications to land in the UK is briefly reviewed, with data presented on both livestock slurry and manure, and human sewage sludge. Particular emphasis is placed on factors influencing the likelihood of pathogens making their way from infected livestock and humans to the soil surface, and therefore the chances of them being available for transport by overland flow. The literature relating to pathogen transport in overland flow is reviewed. Existing pathogen transport models treat pathogens as particles and link pathogen transport models to pathogen die-off kinetics. Such models do not attempt to describe the interactions that may occur between pathogens and soil and waste particles. Although conceptual models describing the possible states in which pathogen transport may occur have been proposed, an understanding of the factors controlling the partitioning of the microorganisms between the different states is only just beginning to emerge. The apparent poor performance of overland flow mitigation measures such as grass buffer strips in controlling the movement of faecal indicators highlights the need for a better understanding the dynamics of microbial transport so that better management approaches may be developed. Examples of on-going research into overland flow transport processes are briefly described and gaps in knowledge identified.