| Description |
Waste load allocations in the United States have traditionally been based on historical data. Total Maximum Daily Load (TMDL) policies and stormwater permits are often based on steady-state low flow analysis without considering unsteady flow and pollutant conditions prevalent during stormwater events. Questions surrounding remedial investments for conditions that do not exist during low flow events have been raised. Furthermore, the frequency of violations under various pollutant removal scenarios has been hard to determine. The Spokane River TMDL used a 2001 low flow event as the basis for planning multimillion dollar investments in wastewater treatment systems and stormwater plan. This study modified the original CE-QUAL-W2 model to simulate hydrology and water quality over a 1999-2009 period with attention to phosphorus, nitrogen, and dissolved oxygen. Calibrating and applying the model for an extended period enabled better prediction of nutrient dynamics under varying flow conditions, which simplified the investigation of permissible nutrient levels. Model results showed that hydrologic conditions outside the low flow period may be cause for concern. Violation of water quality standards occurred outside the 2001 duration with both phosphorus and nitrate concentrations being much higher in years with higher flows. Results demonstrated the need for better understanding of the influence of nonpoint sources on Spokane River-Long Lake water quality, and that additional information concerning stormwater inputs and nutrient cycling would permit better decisions in the future. Finally, the calibrated CE-QUAL-W2 model was applied to the Spokane River-Long Lake as a case study to simulate water quality changes in response to various climate change and population growth scenarios. Model results indicated the disproportionate increase of streamflows during winter, which can overwhelm the existing stormwater flow and pollution control infrastructure through exceedingly high nutrient loadings. Moreover, model simulations revealed extreme nature of climate change impacts, where streamflow increase seemed to have some positive effect on water quality at surface layers, but nutrient and dissolved oxygen concentrations in the deeper layers did not experience any improvement. While the current TMDL proposes a 50% reduction of nonpoint loading, results indicated that this will not be adequate. Such multifaceted nature of climate change effect on water quality makes management decisions more complex for water managers, and indicates the need for revisiting the citations of point and nonpoint source loading reduction in the existing TMDL based on low flow analysis, targeted towards meeting dissolved oxygen standards in the Spokane River-Long Lake system. |
