Water Quality Trends and Shifts Vary across Constituents in the Upper Poteau River Watershed

Highlights The relatively undisturbed Black Fork showed either decreases or no change in flow-adjusted constituents. Water quality was changing at the Poteau River and James Fork, both impacted by point and nonpoint sources. At the James Fork, total P did not change and OP increased, while P decreased at the Poteau River; N increased at both. After shift changes in flow-adjusted concentrations, often no monotonic trends occurred over time. Abstract . Water quality trend analyses seek to determine whether constituent concentrations or loads have increased or decreased over time, which can suggest the effectiveness of management practices or the need for pollutant reduction. The Poteau River watershed (PRW) is a transboundary watershed across Arkansas and Oklahoma, and the Arkansas portion is listed as a priority watershed. The Poteau River flows into Lake Wister, which is an important reservoir for recreation, fishing, and waterfowl hunting in eastern Oklahoma. The purpose of this study was to analyze long-term water quality trends across the upper PRW using the three-step trends method and to determine how water quality has evolved with increasing agriculture and changes in wastewater treatment discharges in Arkansas. The relatively undisturbed river in the PRW, the Black Fork, showed either significant decreases or no change in nutrient and sediment concentrations over time (i.e., ~1.0% year-1 decrease or less). On the other hand, water quality has changed over time in the James Fork and Poteau River watersheds, which are impacted by both point and nonpoint sources. At the James Fork, orthophosphate (OP) significantly increased over time, while total phosphorus (TP) and OP significantly decreased at the Poteau River; total nitrogen (TN) and nitrate plus nitrite (NN) significantly increased at both. Finally, sediment concentrations significantly decreased over time at both the Poteau River and James Fork, with decreasing shifts also occurring in the early 2000s. The changes over time were not necessarily monotonic, as shifts in flow-adjusted concentrations were observed. Effluent concentrations from point sources were found to be decreasing in the 2000s, and regulations were implemented in Arkansas to manage poultry little applications in nutrient surplus areas (i.e., the upper PRW) based on the P index. After those shift changes, water quality was not monotonically changing for most sites and parameters over time. Overall, continued monitoring should be prioritized to ensure that increasing or unchanging trends do not jeopardize water quality in the watershed. Keywords: Point and nonpoint sources, Water quality trends, Watershed management.