Why are the USGS study findings on contributions from urban sources substantially lower than those estimated by the EPA Science Advisory Board?
|Back to Top| Point source contributions estimated by the EPA Science Advisory Board are 22 to 34 percent (versus USGS estimates of about 9 to 12 percent). The difference is probably due, in large part, to the ability of the enhanced model to account for transformations during transport, including, for example, the capture of phosphorus in large reservoirs, particularly in the Tennessee River and Missouri River Basins. In general, recommendations by the EPA Science Advisory Board (SAB) do not consider the effects of nutrient decay in streams and reservoirs. Thus, the SAB assumed that all nutrients that are input to streams from wastewater treatment plants in the Mississippi River Basin are delivered to the Gulf of Mexico without any loss. In addition, the methods for computation of urban sources differed; EPA estimates were based on discharges as reported in their database and USGS estimates were based on population.
