Atmosphere-Water Interaction of Chloroform, Toluene, and MTBE in Small Perennial Urban Streams

An initial goal of the U.S. Geological Survey’s National Water-Quality Assessment (NAWQA) Program is to determine the frequency of occurrence and concentrations of a wide range of anthropogenic and natural compounds in the Nation’s surface and ground water. A large number of contaminants are being monitored including volatile organic compounds (VOCs), pesticides, nutrients, and trace elements. As the NAWQA Program enters its second decade of studies, increased emphasis is being placed on identifying the source(s) and pathway(s) of frequently detected compounds. Chloroform, toluene, and methyl tert-butyl ether (MTBE) are frequently detected VOCs in the atmosphere, surface water, and ground water in urban watersheds and are the focus of this preliminary evaluation of the interaction of these compounds between the atmosphere and perennial urban streams. Air VOC samples were collected from four sites, and stream-water VOC samples were collected from two sites. The air concentrations were used to calculate water-equilibrium concentrations. These waterequilibrium concentrations then were compared to the measured stream-water concentrations to infer the predominant source of the VOCs. Results suggest that the atmosphere may not be the predominant source of chloroform and toluene in the two urban streams. In contrast, MTBE may be coming from the atmosphere, but further study would help to confirm that contributions from other sources are not important.