Thu, Dec 23, 2021 9:57 AM

Council tackles Sound’s sludge

The growing and lasting impacts of sedimentation in Pelorus Sound has been investigated in a new report commissioned by Marlborough District Council. Photo: Supplied.

Erin Bradnock

A new study shows how goldmining, native forest clearing and dairy pasture continues to impact Pelorus Sound’s water quality.

The National Institute of Water and Atmospheric Research was commissioned by Marlborough District Council to investigate the sources of sedimentation on the inner Te Hoiere (Pelorus Sound).  

The sources of sediment in the inner Pelorus Sound Te Hoiere report showed sediment accumulation rates are 10 times greater than that before European settlement there.  

Sediment is one of the main threats to Aotearoa’s marine environment and impacts on freshwater ecosystems.  

The erosion of soil from the land and its deposition in estuaries and the sea is a natural process, but the rate sediment is being deposited is higher than before human activities disturbed the natural land cover.

Council’s coastal scientist Oliver Wade says the study has identified a complex dynamic of material deposited in the sound.  

“Around 70 per cent is termed ‘legacy sediment’ that, although originating from the land, has been in the sound for a long time and continues to move around.”

The main contributors to the sediment in Te Hoiere are subsoil, stream bank erosion, dairy pasture, harvested pine and native forest.  

“Thirty per cent of the sediment deposited comes from contemporary sources. Subsoils and streambank erosion make up the largest proportion, with smaller proportions attributed to erosion from land associated with primary industry and native forest,” says Oliver.  

Goldmining, native forest clearance, pastoral farming and more recent agricultural and forestry activities have all left their legacy in the Marlborough Sounds waters.

According to the study, the impact of increasing soil erosion, sedimentation and harvesting have all had a profound effect on the ecology of the subtidal environment in Mahau Sound, with shellfish diversity is now at its lowest point at any time in history.  

“This suggests that despite our best efforts to improve land management, the marine environment will take longer to recover as this sediment continues to have an impact well into the future.”

Oliver says integrated catchment and marine management will be needed in the future to halt any further degradation and help realise measurable improvements in the system’s environmental state.  

The Te Hoiere/Pelorus Restoration Project was launched late 2019 as a full landscape scale proposal focused on improving freshwater and land resources in the Te Hoiere and Kaituna River Catchments.  

Council, Ngāti Kuia, the Department of Conservation and the community have collaborated with the Te Hoiere Pelorus Project to restore the system’s environmental state.

“The project provides a platform for the community to come together and begin that process,” says Oliver.