Tāhunanui Beach. Photo: Nelson Regional Development Agency / Supplied
Part of Nelson's Tāhunanui Beach will close this week after sawdust contaminated with arsenic and other chemicals was exposed by erosion.
The sawdust, from a local timber mill, was used as fill for the raised carpark in the 1960s at the back beach, a popular off-leash area for exercising dogs.
Nelson City Council community services group manager Andrew White said when it was first exposed last year, the sawdust was not considered a pollutant and a decision was made to leave it in place, with regular monitoring.
"We understood it was not considered a pollutant to the beach and inlet and would break down over time. We were also concerned that removing the sawdust would hasten the erosion occurring in the area.
"However, we took the prudent approach to continue to monitor the erosion of the material and regularly test for contaminants."
Recent monitoring found offcuts of sawn timber that had been chemically treated among the exposed sawdust.
The contaminants include arsenic, chromium and copper, which the council said are unlikely to pose a risk to human health, as they are "on average below the applicable human health protection standards for recreation land use".
But they could pose a risk to the marine environment, as they exceed the Australian and New Zealand Environment and Conservation Council (ANZECC) sediment quality guidelines.
The carpark at the back beach will close from Wednesday to Friday while the area is excavated.
"We are now going to excavate pits across the area to determine how widespread the chemically-treated offcuts are throughout the filled area, and this will determine our next steps," White said.
"Depending on the results of this investigation, it is quite possible that we will need to remove some or all of the filled area."
People and dogs are advised to keep away from the area. The front face of the carpark is already taped off because of the risk of collapse due to erosion.