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Step 5: Reconcile the hydrologic and phosphorus budgets to observed in-lake conditions using a lake water quality model Expected Cost: $18,110 Lakes receive phosphorus loads from both external sources (stormwater runoff and atmospheric deposition, primarily) and internal sources (recycle of plant- or sediment-bound phosphorus). While stormwater runoff volumes and phosphorus loads will be estimated with P8, a separate model is required for simulating the in-lake nutrient processes and resulting phosphorus budget of the lake. An in-lake mass balance model developed by Barr will be used to estimate phosphorus concentrations in Lake Owasso. The in-lake model takes into account the impacts of external (watershed runoff, atmospheric deposition, septic systems, etc.) and internal phosphorus sources (oxic and anoxic sediment-release, aquatic plant decay, rough fish, etc.) on the predicted in-lake phosphorus concentration. The in-lake model can be calibrated by comparing the predicted phosphorus concentrations in Lake Owasso with monitoring data and adjusting model parameters to closely match monitored lake conditions. This partitioning of phosphorus sources allows for better assessment of BMP benefits and more precise estimates of in-lake concentrations at various times during the summer recreational period. In order to accurately model the potential internal load of phosphorus that the lake receives from its seasonally anoxic sediments, Barr will collect five sediment core samples from Lake Owasso. These sediment cores will be analyzed for mobile-phosphorus, the fraction of sediment phosphorus that may be recycled back into the water column, from anoxic lake sediments. This recycled phosphorus generally becomes available for alga] uptake in the late-summer when thermal stratification breaks down and the lake mixes. Step 6: Using the lake model, assess likelihood of beneficial use attainment under ultimate watershed land use conditions Expected Cost: $3,050 The calibrated P8 and in-lake models will be used to estimate phosphorus loads and lake phosphorus concentrations assuming ultimate watershed land use conditions. Runoff and lake water quality during wet, dry, and average precipitation conditions will be evaluated to better represent the full range of possible climatic conditions. The estimated in-lake conditions will be used to determine the future attainability of the established beneficial use goals. Step 7: Recommend feasible alternative remedial measures Expected Cost: $5,040 If in-lake water quality modeling results suggest that water quality goals will not be attained under ultimate watershed land use conditions, the water quality model will be used to predict the benefits of implementing watershed or in-lake BMPs on Lake Owasso water quality. BMP alternatives will be selected based on several factors, including characterization of the in-lake phosphorus budget (the contribution of watershed versus internal phosphorus loads), hydrologic considerations (stormwater Barr Engineering Company :ODMA\PCDOCS\DOCS\247725\ I