Ji-Hee Son

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 in warm water and 0.11 mg L 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.

Will stringent total nitrogen wastewater treatment plant discharge regulations achieve stream water quality goals

The Colorado Department of Public Health and Environment (CDPHE) proposed the in-stream numeric nutrient criteria as 2 mg TN per L and 0.16 mg TP per L for warm surface waters and 0.40 mg TN per L and 0.11 mg TP per L for cold surface waters. Consequently the department presented the nutrient limits for the municipal wastewater treatment plants (WWTPs) as annual averages of 0.7 mg TP per L and 5.7 mg TIN per L and quarterly averages of 1.0 mg TP per L and 9.0 mg TIN per L. Implementing stringent nutrient reduction at point sources is unlikely to result in improvements to the environment without non-point source controls. In this study, total nitrogen (TN) load inputs from known point source, WWTPs, and other non-point sources at six sub-basins of the Cache La Poudre (CLP) River Basin were estimated and compared under various hydrologic conditions. Significant loading exceedance from the proposed limits was observed during lower flow conditions and other sources dominated during events when the exceedance was observed except for one point. The point receives direct TN inputs from a WWTP which has the highest TN concentration in its effluent among all WWTPs in the study area; however, TN loads entered the point from other sources were significant during higher flow conditions. TN loads in the CLP River were simulated to determine whether the loads meet the proposed in-stream limits in a case in which all WWTPs comply with the proposed regulations for WWTPs. From this study, it was observed that reducing TN concentrations only at WWTPs merely impacts total TN loads in the river.