The effect of watershed, reservoir volume, and rainfall on nitrate levels in surface drinking water supplies

Three separate water sources provide drinking water to about one million people in Columbus, Ohio. About 80 percent of the city's drinking water is supplied by two surface water sources, and the remainder by groundwater. Three instream reservoirs, managed by the city, retain runoff from two separate watersheds. Agricultural production predominates in both watersheds, yet only one of the two watersheds, the Scioto River watershed, has a history of exceeding the Safe Drinking Water Act Nitrate-N MaximumContaminant Level (MCL) standard of 10 mg/L. We review the trends in nitrate-N concentration and loading in this watershed from 1982 to 1995.Study site Originating near Roundhead, the Scioto River flows south 231 miles through central Ohio to the confluence with the Ohio River at Portsmouth. As Ohio's second largest watershed, the Scioto River drains nearly 6510 mi2 (16,800 km2), 16 percent of Ohio's total land area. The Upper Scioto River (USR) and Big Walnut Creek (BWC), referred to as subbasins, are the two surface water sources that supply drinking water for greater Columbus.Quantitative differences between sub basins, reservoirs, and water quality characteristics are presented in Table 1.