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Abstract
Spatial and seasonal changes in trihalomethane (THM) concentrations were investigated
in three distribution systems of Quebec (Canada) which are supplied by different surface
waters and which use a variety of physicochemical treatment strategies. The investigation
was based on an intensive 25-week sampling programme, undertaken at a time of year
when the temperature of southern Quebec surface waters exceeds 4 degrees C (April-November).
THMs and other water quality and operational parameters were monitored at points along
the distribution system--between the treatment plant and the system extremities--which
represented variable residence times of water. Results showed that THM concentrations
vary significantly (from 1.5 to 2 times, depending on the utility) between finished
waters as they leave the plant and water at the system extremities. When water temperature
exceeds 15 degrees C, spatial THM variations are particularly high (from 2 to 4 times,
depending on the utility). The development of multivariate regression models showed
that water temperature was a better predictor of THM seasonal variability than chlorine
dose, surrogates of natural organic matter and pH. Also, initial THM formation (in
finished waters leaving the plant) was a good predictor of THM levels at distribution
system extremities.