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The Opequon Creek watershed is located in northern VA and the eastern panhandle of WV. Currently, the main creeks in the watershed do not meet VA or WV state water quality standards for recreational uses and aquatic life. In both states, the creeks are listed as impaired due to high levels of nutrients, bacteria, benthic and biologic impairment. The Opequon Creek is part of the upper Potomac River watershed, and ultimately impacts water quality in the Chesapeake Bay watershed. The main aim of this study was to develop a methodology that can be used to reduce nutrient loadings entering the bay area and improve water quality in Opequon watershed by implementing four innovative agricultural BMPs. The study develops an integrated approach to nutrient reduction incorporating three models involving water quality modeling, nutrient fate and transportation and an optimization model to recommend a least cost strategy for nutrient reduction. Four optimization scenarios were evaluated, involving a uniform, holistic, prioritization, and targeted reduction approaches. A uniform reduction approach evaluated each subwatershed to meet a reduction goal. Using specific land use contributions, an annual cost of $5.9 million would be required to meet N and P reduction goals on 14 of the 17 subwatersheds. The holistic approach is a scenario whereby the entire watershed’s nutrient reduction strategy is evaluated to meet the nutrient reduction goal at the Opequon watershed mouth. However, no optimal solution was found for this approach using agricultural BMPs. When BMPs were implemented on all acres of crop and pasture land, a total cost of $19.3 million was computed with only 43% of the reduction goal is achieved for P and 42% for N. In the third scenario, a prioritization approach targets priority subwatersheds. High priority subwatersheds were identified using the WCMS nutrient levels and public participation prioritization exercise in watershed management. The same three subwatersheds were identified as high priority by both methods: Mill, Tuscarora and Middle Creeks. Using P as the only constraint, the total cost of BMP implementation for these three subwatersheds under the Chesapeake Bay values was approximately $1.1 million compared to $282,000 using specific land use specific values. This result showed that nutrient reduction costs are much lower under specific land use contributions than using the Chesapeake Bay wide averages. The final scenario involved a targeted approach where reduction goals are to be met for both the Virginia and West Virginia parts of the Opequon watershed. No optimal solution exists for these two points of evaluation. As with the second scenario, when BMPs were implements on all agricultural land, VA had 69% and 63% of reduction goals achieved for N and P while WV had 36% and 49% of reduction goals achieved for N and P, respectively. From a perspective of water resource policy, this study showed that: (1) P goals are more attainable at reasonable cost than N goals so that trading on the Opequon watershed is more likely to be feasible for P than N; (2) compliance with WV and VA reduction goals across all subwatersheds is more achievable than meeting a holistic reduction goal for the entire watershed; and (3) local knowledge gives comparable information on priority subwatersheds as does watershed modeling.

A spatial optimization approach to watershed water quality management: A case of the Opequon Watershed