Operating rules for an off-stream blending reservoir to control nitrate in a municipal water system

Municipal water supply systems dependent on surface water sources that are susceptible to both low flows and contamination can use off-stream blending reservoirs (OSBR) both to increase their yield and avoid investments in expensive treatment technology. Three water quality operating rules (WQORs) are simulated from a retrospective planning perspective using the OSBR system that controls nitrate contamination for Pontiac, Illinois as an example case. These rules regulate pumped reservoir inflow based on (1) a maximum river concentration (Riv-CR), (2) a maximum reservoir concentration (Res-CR), and (3) a hybrid rule incorporating both maximum reservoir and river concentrations (Hyb-CR). The Hyb-CR rule typically registers the highest yield when nitrate is modeled as a conservative contaminant, while Res-CR tends to produce the highest yields when nitrate losses are considered. In general, reservoir concentration-based WQORs are effective for increasing the yield of OSBR systems that control the concentration of nonconservative contaminants, and often produce higher yields than a traditional Riv-CR, even with conservative contaminants. However, numerous circumstances are identified under which Riv-CR may outperform both Res-CR and Hyb-CR.