Daily Detroit river total phosphorus loads to Lake Erie from water treatment plant turbidity

The Great Lakes Water Quality Agreement established the western and central basin of Lake Erie total phosphorus (TP) target of 6000 metric tons per year. Models that develop load-response curves showed that daily loads and annual loads of the Detroit River are important. Direct measurements near the river mouth are difficult due to Lake Erie seiches and surface oscillations. Therefore, an alternative approach for estimating daily loads is needed. We show that existing turbidity-TP relationships can be applied to water treatment plant (WTP) turbidity to develop daily load estimates by those responsible for routine monitoring. We show how turbidity measured at WTP are comparable to those measured in the river. WTP uses an existing infrastructure that is of high temporal resolution, so agencies charged with determining loads may use this network. By using daily water flux and the WTP-based observations, daily TP concentration estimates and loads are applied upstream near Belle Isle and downstream near Fighting Island. By adding the respective loads to the river, we obtain daily TP flux estimates to Lake Erie. Due to well known turbidity gradients, the Windsor WTP needs an adjustment to better reflect turbidity in the entire river. After adjusting, the summed daily rates to Lake Erie from both stations are comparable each other and to other rates. We also show the 2019–2024 annual averages are approximately 15 % greater than that estimated by the Environmental Protection Agency (EPA) and Environment and Climate Change Canada (ECCC). These daily and annual load estimates can be useful augmentations of more traditional monitoring efforts that provide only annual loads.