During November 2021, sediment flowing off this coastal farm property headed directly for a nearby estuary.Picture by Sarah Curtis
There’s a pollutant rapidly accumulating in our waterways and seabed — one that’s arguably more serious than even forestry slash but features far less in media reports. It’s sediment.
New Zealand is losing 192 million tonnes of soil each year due to erosion, the equivalent of more than seven million dump-truck loads, the Ministry for the Environment (MfE) says.
As a result, New Zealand contributes an astonishing 1.7 percent of all the sediment pollution in the world’s oceans, even though this country accounts for just 0.2 percent of global land area.
According to the MfE’s Environment Aotearoa 2019 report, almost half (44 percent) of this loss is from pastoral land.
“It’s accelerated by the loss of native vegetation, heavy rain causes landslips, slumps and streambank collapses, all of which send huge amounts of fertile soil and sediment into waterways,” the report says.
“The land-based erosion and transfer of sediment is a huge problem for New Zealand. It doesn’t just reduce land productivity — it also affects water quality. Fine sediments are the most common cause of contamination in New Zealand’s rivers and estuaries.
“Too much sediment in rivers, lakes, and coastal environments can smother freshwater and marine habitats.
“Sediments can carry other pollutants, such as heavy metals, nutrients and microbes. Sediment also hampers the growth of aquatic plants and animals, and sediment-choked waterways increase the risk of flooding in towns and cities,” the Environment Aotearoa report states.
An Auckland University professor Troy Baisden, who studies flow of carbon, nitrogen and water through ecosystems, says the North Island’s east coast is among the world’s leading hotspots for the uplift erosion and deposition of sediment.
In areas such as the Waipaoa catchment above Gisborne, sediments from the 20th century were more than 5m deep.
While the sediment is often generically called silt, technically silt is particles of sediment that range in size from 0.002mm (or 2 microns) to 0.063mm.
Larger particles up to 2mm are sand. Sediment particles smaller than silt – under 0.002mm or 2 microns – are referred to as clay or mud.
The public’s lack of awareness about sediment pollution in our waterways and coastal environment was raised by Gisborne District Council prosecutor Adam Hopkinson during a sentencing hearing in Gisborne District Court, last week for an errant forestry company.
Mr Hopkinson noted sediment is the most pervasive contaminant in New Zealand’s fresh and marine water systems. Sediment pollution often goes hand-in-hand with forestry offending, yet unless there's monumentally shocking build ups of it - as there was in Cyclone Gabrielle - news cameras usually only focus on the devastating accumulations of slash.
The forestry company sentenced last week had caused sediment pollution by using machines in a stream, creating tracks with no water controls alongside it, dragging logs through the stream, and destroying riparian planting in the process. Those activities directly breached conditions the company had agreed to abide by in a resource consent for harvesting.
It had also constructed forestry roads in contravention even of the industry’s own code for best practice. The roadway, landing edges, and spoil dump sites hadn’t been compacted, armoured or benched as was necessary to guard against erosion and side cast material had been allowed to slump down slopes towards waterways.
A large amount of fill had been deposited in a stream to replace a culvert crossing — one that was already in the wrong place — a red zone area (some of the region’s most erosion-prone).
These poor practices had a “significant effect on the mobilisation of sediment”, Mr Hopkinson said.
“Sediment as a stressor has particularly negative consequences for aquatic ecosystems. Sediment can smother aquatic species and habitats, reduce oxygen (and light) levels, create stress on the ecosystem and generally make it more difficult for aquatic species to live.”
The stream involved in this company’s offending is known to be a home for long-finned tuna (eel) and a tributary of the Waipaoa River.
Dr Andrew Swales, an estuarine physical processes scientist who leads the National Institute of Water and Atmospheric Research’s (Niwa) catchments to estuaries programme, points to the steepness of much of New Zealand’s terrain as the reason for our sediment problem.
“Our hills are erosion-prone due to their slope, their geology, high-intensity rainfall and earthquakes,” he says.
“It’s a potent mix. Many places have highly erodible sedimentary rocks, earthquakes destabilise hillslopes, and rain triggers slope failure and landslides,” Dr Swales says.
Tairāwhiti and the East Coast are perfect examples of that type of terrain. More than a quarter of Gisborne district’s land is susceptible to severe erosion (26 percent to be exact), compared with only 8 percent of the rest of New Zealand.
A persistent easterly weather system (La Nina) in recent years has resulted in heavy rainfall events, slope instability, and flooding. With climate change, these weather events are predicted to become more extreme and potentially more frequent.
Severe erosion includes large-scale gully erosion, earth-flow erosion and deep-seated slumps, and causes long-term damage to the productivity of rural land by threatening communities and rural businesses, including farms and orchards, roads and bridges; lowering water quality, and harming the natural and cultural values of the land and the coastal environment.
All this has a negative economic impact on the district’s hill country farms, infrastructure, and high-quality land on floodplains.
Using estuaries as an example of the specific adverse effects of sedimentation, Dr Swales said: “Prior to human arrival, sedimentation rates in our estuaries were typically much less than 1mm per year on average. As catchments were deforested, the average sedimentation rates increased 10-fold.
“Typical rates in upper North Island estuaries over the last century are in the order of 2–5mm per annum.”
The result of this accelerated sedimentation has been the formation of intertidal mudflats in place of the sandflats that characterised many estuaries before catchment deforestation.
In upper North Island estuaries, these rapidly accreting mudflats have been colonised by mangroves, where shellfish previously lived.
“It’s a double whammy,” he says. “We’re losing productive soils from the land, and these eroded soils are having adverse effects in estuaries.
“Increased turbidity (murkiness) of water affected the survival of many species of predatory fish and birds, which used their eyes to find their prey.
“Fine sediments carry other contaminants with them, including heavy metals, organic contamination from stormwater runoff, and microbes which are harmful to human health.
“In rural settings, the fine sediments carry phosphorus with them as well,” Dr Swales says.
So what can be done?
In the nation’s courthouses, prosecutors are calling on judges to impose fines for environmental offending that are “more than just a cost of doing business”.
Two bills that had their third readings in Parliament during August — the Natural and Built Environments Bill and the Spatial Planning Bill were designed to include significant hikes in the level of penalties, the courts could impose. However, the bills were hotly contested with National vowing to repeal them both if it could form a government.
Escalating concern over major storm-induced damage, caused by inundations of floodwaters full of forestry and other wood, culminated this year in a Ministerial Inquiry into Land Use.
It confirmed a growing realisation that exotic plantation forestry isn’t the answer for the country’s most highly erodible land, that there needs to be much more oversight and careful design to commercial harvesting regimes, and that some forested lands should be retired to revert to native cover.
Niwa researchers have been gathering data since Cyclone Gabrielle, but can’t yet define the amount of sediment levels produced in that event.
They also have research under way for Hawke’s Bay and Tairāwhiti to help create models of where the sediment has ended up in the marine environment and its impacts on the health of fisheries’ habitats and seabed ecosystems. Those impacts are much harder to gauge than the damage caused by sediment on land, project leader Dr Daniel Leduc says.
“One of the most shocking sights from Cyclone Gabrielle was those huge sediment deposits that buried houses to their rooftops. The home of our marine life is also impacted, only it is harder to map and is constantly moved around by water currents, waves, and tides.
“Our job is to assess where the impacts have been felt and to what extent, particularly in vulnerable habitats and places where species like flounder and gurnard live close to the seafloor.”
Back on land, there is funding available through the Ministry for Primary Industries and Gisborne District Council for erosion control initiatives, including the Erosion Control Funding Programme (ECFP). Established in 1992 as the East Coast Forestry Project to address this district’s severe erosion problems, it was later renamed.
While other ECFP funds are closed, there’s still funding available for ECFP community projects. Projects may involve a community, or be on a regional scale. They may also have other environmental, social and economic benefits for the region.
In addition, Gisborne District Council’s Sustainable Hill Country (SHC) programme also aims to increase protection of highly erodible land.
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