Marlborough District Council says a contamination plume in groundwater is largely to the south of Blenheim's town supply bores.
A Defence Force investigation has found low levels of firefighting foam pollution in the aquifer which supplies the bores.
The council says one of its nine supply bores tested positive for very low levels of P-F-A-S compounds in 2018.
It shut that bore, until later testing came back clear.
The council says it will keep monitoring for the foreseeable future.
California this month moved to slash the state's allowable limits for PFAS in drinking water to well below New Zealand's guideline levels.