Relative Phosphorus Load Inputs from Wastewater Treatment Plants in a Northern Colorado Watershed

Excess nutrients are among the leading sources of water quality impairment in the Unites States, and the USEPA has been working with state regulatory agencies to develop nutrient criteria for wastewater treatment plants (WWTPs). The Colorado Department of Public Health and Environment is scheduled to establish nutrient regulations in 2013, and stream total P (TP) concentration standards of 0.16 mg L−1 in warm water and 0.11 mg L−1 in cold water have been proposed for the rivers in the state. The objectives of this study were to monitor TP concentrations and loads along the Cache La Poudre River as it flows from the pristine upstream area through urban regions and finally through a mixture of agricultural and urban land uses. The study attempts to evaluate the sources and influences of TP under different hydrologic conditions. Twelve sampling events were completed from April 2010 to August 2011 to assess the influence of various flow and precipitation conditions on aqueous TP concentrations. During midrange flows and dry conditions, WWTPs were the major sources of TP, but other sources were more significant under high‐flow and wet conditions according to a load analysis. The analysis indicates that reducing the TP load from WWTPs will only marginally affect the TP load in the river, and therefore it appears that other sources (e.g., stormwater and agricultural runoff) need to be addressed before the aquatic life–based stream standard can be achieved.