Spatial and temporal evolution of trihalomethanes in three water distribution systems

Spatial and seasonal changes in trihalomethane (THM) concentrations were in vestigated in three distribution systems of Quebec (Canada) which are suppl ied by different surface waters and which use a variety of physicochemical treatment strategies. The investigation was based on an intensive 25-week s ampling programme, undertaken at a time of year when the temperature of sou thern Quebec surface waters exceeds 4 degreesC (April-November). THMs and o ther water duality and operational parameters were monitored at points alon g the distribution system-between the treatment plant and the system extrem ities-which represented variable residence times of water. Results showed t hat THM concentrations vary significantly (from 1.5 to 2 times, depending o n the utility) between finished waters as they leave the plant and water at the system extremities. When water temperature exceeds 15 degreesC, spatia l THM variations are particularly high (from 2 to 4 times, depending on the utility). The development of multivariate regression models showed that wa ter temperature was a better predictor of THM seasonal variability than chl orine dose, surrogates of natural organic matter and pH. Also; initial THM formation (in finished waters leaving the plant) was a good predictor of TH M levels at distribution system extremities. (C) 2001 Elsevier Science Ltd. All rights reserved.