Sediment phosphorus release at a small impoundment on the Illinois River, Arkansas and Oklahoma, USA

The Illinois River in northeast Oklahoma and northwest Arkansas has been the focal point of environmental, political and legal debate over elevated P concentrations and loads transported across the Arkansas and Oklahoma border. The Oklahoma Water Resources Board has adopted numeric total P criteria (0.037 mg P L−1) in Oklahoma's Scenic Rivers, including the Illinois River. The US Geological Survey has reported flow-weighted total P concentrations approximately an order of magnitude greater than the newly adopted criterion. Furthermore, elevated dissolved P concentrations have been traced over 45 river kilometers upstream to municipal wastewater discharges in the headwaters of the Illinois River. The point of regulation regarding the total P criteria will be where the Illinois River flows from Arkansas into Oklahoma; however, a small impoundment exists at the Arkansas and Oklahoma border and the states respectively monitor water quality upstream and downstream from this impoundment. The purpose of this study was to evaluate P release from sediments accumulated in this small impoundment. Sediment P release measured in laboratory incubations was as much as 4 mg P m−2 day−1 under aerobic conditions and approximately 15 mg P m−2 day−1 under anaerobic conditions. Sediment equilibrium P concentrations (EPC0) in laboratory equilibration studies were 0.05–0.20 mg P L−1, which is greater than the total P criteria for this river in Oklahoma. Thus, it is conceivable that P released from bottom sediments in this small impoundment may, in fact, increase dissolved P concentrations in the Illinois River. The effect of internal P cycling at Lake Frances on P concentration in the Illinois River downstream might be greatest in the near future, because municipal discharges have recently reduced effluent P concentrations.