VOCs are present in many natural and synthetic materials and commonly are used as fuel additives, solvents, and aerosols. Despite their high evaporation rates, VOCs have been detected in natural waters (Kotzias and Sparta, 1993). The likely sources of VOCs in surface water are industrial and wastewater discharges, accidental fuel and oil spills, and chlorinated municipal drinking water. However, all of the water sources that sustain streamflow are susceptible to contamination by VOCs. VOCs in rainfall originate from vehicle and industrial emissions; direct runoff from streets and paved surfaces is another source of VOCs (Terracciano and O'Brien, 1997). Contamination of water by VOCs is a concern because of the threat to human health. Possible effects to humans ingesting water containing VOCs include eye, nose, and throat irritation; central nervous system responses such as dizziness, headaches, and loss of short-term memory; and carcinogenesis (Wallace, 1993).
Samples collected for VOCs were analyzed for 86 compounds (table 4). The VOCs detected at the three intensive fixed sites and the range in concentrations are listed in table 9. Small concentrations of carbon disulfide, toluene, and xylene were detected in VOC field blanks. These same VOCs also were detected in source water for VOC field blanks, indicating that the source of the VOCs in the field blanks likely was not associated with sample collection and processing. The agriculture indicator site had the least number of VOCs detected with 15 in the 7 samples collected. The most commonly detected VOCs at the agriculture site were bromoform, carbon disulfide, chloroform, p-isopropyltoluene, and meta- and para-xylene, each detected in at least three samples. Concentrations of all VOCs detected at the agriculture site were less than 1.0 mg/L. More VOCs were detected at the site in samples collected during storm events than during base-flow conditions--six compounds (benzene, ethylbenzene, naphthalene, 1,2,4-trimethylbenzene, 1,3,5-trimethylbenzene, and xylene) were detected only in storm-event samples.
More VOCs were detected at the urban indicator site (21 in 22 samples, table 9) than at the agriculture site, probably because the sources of VOCs are more likely to be in urban areas. The most commonly detected VOCs at the urban site were cis-1,2-dichloroethene, methyl tert-butyl ether (MTBE), and toluene--all detected in at least 12 samples. Concentrations of all VOCs detected were less than 1.0 mg/L except those for acetone, which were less than 7.0 mg/L (table 9). Correlation between discharge and number of VOCs detected was not apparent at the urban site.
The integrator site had the most VOCs detected, 33 in the 13 samples collected (table 9). The most commonly detected VOCs at the integrator site were bromoform, chlorodibromomethane, chloroform, 1,4-dichlorobenzene, and dichlorobromomethane, each detected in at least 10 samples. Four of these VOCs (bromoform, chlorodibromomethane, chloroform, and dichlorobromomethane) commonly are referred to as trihalomethanes (THMs), which are generated when water is chlorinated at treatment plants. THMs are present in chlorinated water worldwide in concentrations typically ranging from 10 to 30 mg/L (Thurman, 1986). Concentrations of THMs in samples collected from the integrator site were less than the typical range. More VOCs were detected in samples collected at the integrator site during winter storm events than in other samples. Two of the largest VOC concentrations from the study were chloroform, 4.6 mg/L, and tetrahydrofuran, 3.9 mg/L, detected in samples collected at the integrator site. The concentrations of some VOCs detected at the integrator site were about equal to or larger than the combined concentrations of the same VOCs detected at the agriculture and urban indicator sites upstream of the integrator site.
Not all VOCs analyzed in the samples collected at the intensive fixed sites have an MCL listed in the EPA drinking water regulations. Concentrations of all VOCs that were detected at the intensive fixed sites and have an MCL were less than the MCL.
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