Pollutant Loads Returned to the Lower Murray River from Flood-Irrigated Agriculture

Pollutant concentrations and loads returned to the lower Murray River (South Australia) from flood-irrigated agriculture were monitored over a period of 2 years at six locations. This monitoring programme was designed to provide a baseline prior to environmental improvements being undertaken to reduce pollutant loads returned to the river. Pollutant (Escherichia coli, total nitrogen, oxidised nitrogen, total phosphorus, filtered reactive phosphorus and total organic carbon) concentrations were significantly (p < 0.01) higher in the drainage water than the river water and increased during the irrigation season. Salt concentrations were also significantly (p < 0.01) higher in the drainage water than the river inflow water but decreased during the irrigation season because of dilution of the saline groundwater inputs. Pollutant loads exported to the river were significantly higher (p < 0.01-0.05) during the irrigation season for all water quality parameters except oxidised nitrogen. Levels of oxidised nitrogen, filtered reactive phosphorus and E. coli increased in the river downstream of the where the agricultural pollutant inputs begin. Load calculations indicated that this increased water pollution is likely due to the pollutant contributions from the flood-irrigated areas and biogeochemical processing of dissolved nutrients in the river.