Assessing microbial pollution of rural surface waters: A review of current watershed scale modeling approaches

Liquid and solid wastes generated from both animal and domestic sources can significantly impair drinking, irrigation and recreational water sources in rural areas. The assessment and management of non-point sources of microbial pollution, in particular, is an issue of great interest. A representative watershed scale water quality model would be an invaluable tool in addressing microbial pollution issues. The objective of this review is to present and evaluate current approaches to modeling the microbial quality of surface waters in rural watersheds. A complete watershed scale microbial water quality model includes subroutines which (i) characterize the production and distribution of waste and associated microorganisms, (ii) simulate the transport of microorganisms from the land surface to receiving streams, and (iii) route microorganisms through stream networks. Current watershed scale models only account for microbial transport to surface waters through overland flow and ignore subsurface transport. The movement of microorganisms on the soil surface is predicted using simple empirical equations or by assuming that microorganism transport is only associated with sediment erosion. However, several studies have indicated that the assumption that microorganism transport is directly linked with sediment transport may not be valid. The simulation of microorganism survival and transport in receiving streams is complicated by sediment/microorganism interactions. More research is needed to be able to quantitatively assess and model microbial processes in alluvial streams.