AWWA SRC57784

The City of Aurora, Colorado is concerned that residential development of the Aurora Reservoir watershed would likely result in a significant deterioration of water quality in the reservoir. The decline in water quality would interfere with the water supply and recreational uses of the reservoir. Residential development in the reservoir watershed is expected to increase nutrient loads unless best management practices (BMPs) are implemented. The City wishes to allocate to the watershed a portion of the total acceptable nutrient load to the reservoir. This load allocation would be used by developers as an allowable upper limit for designing appropriate BMPs. The City and Black & Veatch conducted a study to estimate the maximum nutrient loads the reservoir could assimilate and yet comply with numerical water quality objectives established for the reservoir. Significant sources of nutrients include imported water from Rampart Reservoir, discharge from three wells owned by the city, discharge from two wells owned by East Cherry Creek Valley Water and Sanitation District, storm water runoff from the Aurora watershed, geese feces, and atmospheric deposition (direct precipitation and dry-fall). The study approach involved use of empirical eutrophication model to validate estimates of acceptable nutrient loads to the reservoir from the watershed. The initial focus was on phosphorus because this nutrient rather than nitrogen appears to limit the growth of algae in the reservoir. Annual phosphorus loads from Rampart Reservoir and the wells were calculated on the basis of existing flow and phosphorus-concentration data. Estimates of phosphorus loads from geese and atmospheric deposition were based on data from other studies. Initial estimates of watershed loads were based on previous modeling studies. Annual total phosphorus (TP) loads were estimated for 1997, which appears to have resulted in the poorest water quality in the reservoir in recent years. TP loads were adjusted so that the eutrophication parameters predicted by the models were comparable to the observed reservoir water-quality characteristics. Includes 22 references, tables.