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Currently there is a wide array of water quality models to assist in establishing TMDLs for impaired waterways. However, once established, TMDL implementation must take into account economic and political factors, uncertainty, future population growth and land use changes, as well as issues of equitability and feasibility. Tools that can help incorporate these considerations into TMDL implementation are lacking. Virginia is currently implementing its Potomac Tributary Strategy to reduce nutrient loads to the river by 40%. This effort has many similarities to the implementation of a TMDL. In 1998, a study was completed, which assessed the optimal combination of load control efforts that achieve the nutrient reduction goal for the Potomac. Study objectives were to: project annual nitrogen inputs to Potomac over 15 years; estimate the minimum annual costs for attaining the goal; and assess potential cost savings from using alternative development patterns within rapidly urbanizing areas. The model developed for the study uses water quality modeling results in conjunction with land use and economic data to assess temporal and spatial distributions of costs for controlling loads from various point sources and land uses.

Model results were used to predict future costs and to assess tradeoffs between differing policy options. Results suggested meeting the goal in 1998, required at least $27 million, in additional annual expenditures. Point source retrofits accounted for two-thirds of the additional expenditures, additional agricultural controls accounted for the remainder. Under current demographic trends, annual costs for maintaining the goal would increase 40% over 15 years. Rising costs result primarily from wastewater disposal needs of the watershed's growing population. Planning future development within the watershed to promote the use of centralized water treatment would slow the pace of future cost increases.

The model developed for the study has the potential to support the TMDL allocation and implementation process. Potential uses are: assessing control costs for constituents, such as nutrients, sediment, or fecal coliform; examining tradeoffs between different reduction scenarios; targeting allocation of scarce funds; and assessing the feasibility of pollutant reduction trading, across geographic areas or economic sectors.