The Atmosphere as a Source of Water-Soluble Volatile Organic Compounds Found in Urban Streams

Volatile organic compounds (VOCs) are frequently detected in surface waters of the United States. To reduce their occurrence in urban streams, it is imperative to understand the sources of these pollutants. The atmosphere is a potential source of selected water-soluble VOCs in urban streams and was investigated in this study. Air and surface-water VOC data collected during 1996 to 2002 from 10 monitoring sites from the U.S. Geological Survey's National Water-Quality Assessment (NAWQA) Program were examined. These sites were selected to evaluate the potential for direct transport of VOCs from the atmosphere to urban streams. Concurrent samples (air and surface water) were collected from eight sites, but two sites had available data that were compared based on the closest sampling dates. Analytical results of 87 VOCs were screened by evaluating the occurrence and concentrations in both air and surface water using defined categories, and equilibrium concentrations in surface water (C_w^s) were calculated based on the measured air concentrations. Four VOCs (acetone, methyl tert-butyl ether (MTBE), m- and p-xylene, and toluene) were in the category of detected in more than 20 percent of surface-water samples and in more than 10 percent of air samples, with at least 10 percent of C_w^s concentrations greater than the aqueous long-term method detection level (LTMDL). One VOC, benzene, was in the category of detected in more than 20 percent of surface-water samples and in more than 10 percent of air samples, with at least 2 percent of the C_w^s concentrations greater than one-half the aqueous LTMDL. Six VOCs (chloroform, methylene chloride, perchloroethene, p-isopropyltoluene, 1,1,1-trichloroethane, and trichloroethene) were in the category of detected in more than 20 percent of surface-water samples and in more than 10 percent of air samples, with less than 1 percent of C_w^s concentrations greater than one-half the aqueous LTMDL. Five VOCs were identified as having the atmosphere as a possible source of contamination to small urban streams: acetone, benzene, MTBE, m- and p-xylene, and toluene. Of these five VOCs, acetone, benzene, and MTBE were consistently in equilibrium with the atmosphere. Depending on the site and season, the atmosphere can serve as a probable dominant source for VOC contamination in urban streams.