after multiple regression analysis of data from one-time sampling studies of many water systems. A study (7) of 10 treatment plants over a 10-month period concluded that no single factor always dominated THM formation but that total chlorine dosage was a principal factor. In the present study, when data for all three plants were combined, multiple regression analysis showed that chlorine dosages and demands were dominant in determining mean chloroform levels. Seasonal trends, with minimum T H M levels observed during the cold winter months ( 3 , 7 , 8 , 1 8 - 2 0 ) and with maximum THM levels during spring (18),summer ( 8 , 1 9 , 2 0 ) ,and fall ( 7 ) , have been reported. Water temperature ( 5 , 2 1 ) and variations in the organic content of the raw water supply ( 5 , 7 , 1 8 ) have been suggested as factors influencing seasonal T H M level fluctuations. In the present study, seasonal variation, with low values in the winter and high values in the summer, was observed for temperature, prechlorine and total chlorine dosages, chlorine demand 1 and total demand, and CHC13 and CHBrClz levels. High values in the winter and low values in the summer were evident for alkalinity measurements and alum dosages. The need to consider temporal and spatial variation in T H M levels ( 2 2 ) during planning of sampling schedules and in the interpretation of monitoring results was emphasized by the finding of a significant ( p < 0.05) variation in THM levels during a 24-h period and from day to day in water from a tap in the distribution system. Although changes in THM levels after water leaves a treatment plant have been reported ( 4 , 7 , I O ) , chloroform levels did not change significantly ( p > 0.05) between stations 2 and 5 in the present study. Over 13 months, trihalomethane levels in station 1 water for the three treatment systems ranged from 7.0 to 187 pg/L for CHCl3 and
