Three rivers in the Shenandoah Valley of Virginia are contaminated with mercury and have been designated as "impaired" on Virginia's 303d list of contaminated waters due to fish consumption advisories issued by the Virginia Department of Health. These rivers, the South River, South Fork Shenandoah River, and the Shenandoah River between Front Royal and the confluence with Craig Run (fig. 1), are regulated by the Virginia Department of Environmental Quality (VDEQ) under the Total Maximum Daily Load (TMDL) Program, which develops plans to restore and maintain water quality for impaired waters.