PC-QUASAR

From the web site; "PC-QUASAR is a water quality and flow model for river networks. It is designed to be used by river regulatory authorities and water/sewerage utility companies to help manage river water quality. The software is now available to be downloaded at no charge, as it is no longer supported by CEH (see below). PC-QUASAR can provide distributions of flow and quality at key sites, allowing river regulators to set effluent consent levels designed to meet river quality objectives. PC-QUASAR can also provide river flow and water quality estimates at each reach boundary over a period of time, allowing proposed changes in the river's use, flow or quality to be assessed. The following determinands can be modelled: river flow, pH, nitrate, temperature, E coli, biochemical oxygen demand, dissolved oxygen, conservative pollutant or tracer."

http://www.ceh.ac.uk/products/software/CEHSoftware-PC-QUASAR_000.htm