Transport of nutrients and eutrophication control by an urban runoff diversion system protecting a drinking water reservoir

Urban runoff from five storms during the 2003–2004 and 2004–2005 rainy seasons was sampled at the exit point of runoff diversion forebays leading to engineered retention ponds to protect a drinking water reservoir. Samples were collected from three different drainage areas both years and were analysed for water quality parameters including total dissolved solids, electrical conductivity (EC), dissolved organic carbon, bacterial count and nutrients in the water phase. In the second year of the study, samples were also taken at the entry point into the forebays and analysed to determine if the forebays contributed to removal of analytes prior to diversion in the retention ponds. EC, which had been used as the determining factor of whether runoff is used to recharge or diverted to holding ponds, did not relate to nutrient levels. This indicated that EC is insufficient to determine water quality because runoff with low EC may contain high levels of nutrients that can support eutrophication. Monitoring of nutrients themselves is essential for decisions.