OVERCOMING MODEL UNCERTAINTY IN COPPER TMDL DEVELOPMENT: HAIWEE RESERVOIR

Haiwee Reservoir is located approximately 10 miles south of Owens Lake and 1.5 miles east of Highway 395 in Inyo County, California. The reservoir is a drinking water supply for the city of Los Angeles and is operated by the City of Los Angeles Department of Water and Power (LADWP). The reservoir is listed pursuant to the federal Clean Water Act, Section 303(d), for impairment caused by copper. One of the primary sources of copper to the reservoir is the application of copper sulfate to control algae within the reservoir system as well as the upstream portions of the Los Angeles aqueduct, which provides inflow to the reservoir. Fish kills have been associated with copper sulfate applications, and routine monitoring shows copper concentrations to be much higher than water quality standards.Lack of data, pressure from interested parties to utilize an off-the-shelf model, and a lack of understanding of modeling technical underpinnings often leads to the development of watershed and water quality management plans (TMDLs) that are difficult to explain to stakeholders and the public. The availability of data to represent the hydrologic and water quality characteristics of the Haiwee Reservoir system provided an opportunity to develop a site-specific model that could be used as an assessment tool for the TMDL process as well as a management tool for the primary stakeholder, LADWP. The Haiwee Reservoir Copper Model was developed using Microsoft Excel as the platform and user interface. The use of Excel ensured that the model was easily transferred to water quality managers and stakeholders and also provided for easy access to all equations and methods employed in the model. The model's computational scheme was comprised of three layered components. The first layer was the water balance for which sufficient inflow, outflow, and meteorological data provided an accurate accounting of water. The second layer is a sediment budget. The third layer was the copper mass balance, which was calibrated using in-lake copper data and accurate information on the timing, amount, and location of copper sulfate applications to the reservoir systems. The resulting calibrated model was useful for testing various operational scenarios and performing loading-source and water quality analyses.