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The first round of low-dose chlorine treatment was added to the Dunsandel water supply from January 7-10 and saw a significant reduction in the number of coliforms in the water supply.
However, the Selwyn District Council has now added a second low-dose of chlorine after testing showed some coliforms remain.
This low-dose chlorine treatment is precautionary only, and no E. coli has been detected within the water supply.
Residents do not need to boil their water.
The Dunsandel supply provides water to about 200 households.
