Resident Geoff Grocott who lives nearby surveying the silt build-up after flooding at Motueka River. Photo: Samantha Gee / RNZ
The Tasman District Council is asking people to hold off on disposing of silt over concerns about a fast-growing noxious weed spreading across the district.
People are being told to leave silt where it is, if safe to do so, until it has a management plan in place and a drop-off site organised.
It was also asking residents that use bore water to check for potential contamination from floodwater and agricultural chemicals and get their water tested if they had concerns.
The council said the highly destructive Asiatic Knotweed had been caught up in silt and debris in recent weeks.
The prolific plant could grow 3 metres deep and any vegetation fragments can grow into new plants.
It was known to push through concrete, asphalt and building foundations.
Seeds were not the issue with knotweed.
It was the prolific underground rhizome material (that can grow 3m deep and up to 7m out from the parent material) which may have been exposed by floodwaters at sites in the river catchments and washed downstream, the council said.
The plant was very persistent and difficult to destroy due to the large mass of underground plant material.
It had been known to push through concrete, asphalt and building foundations and liked damp conditions, but could grow in dry areas too, the council said.
It was deciduous so wouldn't be obvious at this time of year but would show itself in the coming summers.
The council said the weed could remain unnoticed underground for 20 years.
The council requested no soil, sediment or gravel was taken off-site from Sherry River, Motueka River downstream of the Wangapeka/Motueka River confluence, Pigeon Valley Stream downstream from Pigeon Valley Road and Riwaka River downstream of the North and South Branch confluence.
It said bacterial contamination remained a major concern in areas that had been flooded and boiling water minimised the risk.
Numerous agri-chemical containers were carried away by floodwaters, where the contents spilt into waterways and onto land.
Boiling water would not make chemically contaminated water safe and anyone who discovered these containers near their bore water supply, should not use it for drinking or supplying animals.
It was recommended residents get their water tested if they had any concerns.
The Water Services Authority - Taumata Arowai had advice about drinking water safety on its website. https://rebrand.ly/synwrt6.
