Net nutrient uptake in the White River, downstream of a municipal wastewater treatment plant

Wastewater treatment plays a crucial role in preserving water quality in receiving streams; however, continuous nutrient enrichment diminishes a river's ability to take up and transform nutrients. The longitudinal gradient in nutrient concentrations downstream from a wastewater treatment plant (WWTP) effluent discharge can be used to evaluate nutrient retention and more specifically the net nutrient uptake length. Nutrient retention can be analyzed by comparing the input to output nutrient load of a study reach. The net nutrient uptake length is an average distance that nutrients travel in the dissolved inorganic form before sediment or biotic assimilation occurs. The objectives of this study were to: (i) determine if the Paul R. Noland WWTP in Fayetteville, AR affects water quality among differing river discharges of the White River, AR, (ii) determine the fate and transport (i.e. retention efficiency) of nutrients added in the WWTP effluent, and (iii) determine if the retention efficiency of nutrient uptake is correlated with water quality parameters. A two-year study (2006--2007) was conducted along a 6.1-km reach of the White River. The fate and transport of added nutrients were examined by comparing nutrient load inputs to outputs and by using the nutrient spiraling framework for calculating net nutrient uptake lengths. The WWTP effluent discharge increased stream nitrite, soluble reactive phosphorus, and total organic carbon concentrations as well as conductivity. Increasing river discharge raised dissolved oxygen concentrations and turbidity, but lowered temperature, conductivity, and total organic carbon concentrations. Significant interactions between site location and river discharge were observed with nitrate, total nitrogen, total phosphorus, and chloride concentrations. The longitudinal pattern in nutrient concentrations fits the net nutrient uptake model only 33% of the time. Net nutrient uptake lengths were inconsistent for nitrate, ammonium, or soluble reactive phosphorus. Interpretations for nitrate and soluble reactive phosphorus suggested that the fluvial channel acted as both a sink and a source of nutrients, but the channel always acted as a sink for ammonium. When retention of nutrients was analyzed by the whole-study-reach approach, only the mean ammonium retention coefficient was numerically different from zero suggesting that only 12% of added ammonium was retained in the study reach. The effluent discharge increased the concentrations of several water quality parameters, and it appears the long-term enrichment has rendered the immediate-downstream reach ineffective as a nutrient sink. Nutrients added in the WWTP discharge were transported downstream with little to no uptake or transformation before reaching Beaver Lake.