Trihalomethanes and Viruses in a Water Supply

In June 1975, a program was established for monitoring viruses and volatile organics in the Occoquan Reservoir and water service area supplying approximately 600,000 residents of northern Virginia. Of interest were: (1)The magnitude of and variations in total trihalomethanes (TTHM) during the year; and (2)whether viruses could be recovered. Discharges of chlorinated sewage did not appreciably alter TTHM concentrations in one of the reservoir tributaries, but, as others have shown, chlorination of water from the reservoir during routine treatment produced high TTHM concentrations, principally chloroform, in finished water. The highest concentrations, up to 459 ?g/l chloroform, were observed in summer when the raw water chlorine demand was high. The yearly mean concentrations of chloroform in finished water was approximately 275 ?g/l. Polio 1 was recovered on seven occasions, four times from finished water. This report has stimulated much interest and has resulted in further monitoring to try to verify the reported findings and in a joint experimental project to test methods of virus sampling and analysis.