Why don’t you leave the sediment in place? Won’t most of it have washed into the sea or broken down?
Dioxins bind strongly to the sediment and, once bound, do not release to the water. The flat grade of the Kopeopeo Canal means the sediment doesn’t readily move downstream. This, coupled with the slow breakdown of dioxins in this environment, mean the problem will remain unless we do something about it. Dioxins can also accumulate in the food chain which is one of the key drivers for carrying out the remediation project. The sediment has now built up so it is affecting the canal’s ability to convey floodwater. Removal of the sediment is also necessary to ensure an extreme flood event does not result in the canal water overtopping the flood banks and affecting surrounding land.
Why not take the sediment to landfill instead of remediating it?
Landfill burial was evaluated along with many other remediation options in 2010, prior to the current consents being applied for and granted. This option was eliminated from further consideration due to costs and potential traffic effects. Due to the type of contaminant, the dewatered sediment would need to go to a landfill at Hampton Downs (North Waikato) or Tirohia (near Te Aroha). The long cartage distances involved increase the risk of spilling as the result of an accident. Cultural beliefs have also been taken into consideration during final selection of the remediation process. Ngāti Awa are strongly opposed to removing material from their rohe and prefer that the process of ‘healing the land’ occurs locally.
Is there any urgency to get sediment removed?
As well as addressing the contaminant problem, the build-up of sediment in the canal reduces its drainage capacity and is contributing to drainage problems on the plains that flow into the Kopeopeo Canal. Typical drain maintenance - removing sediment to restore drainage capacity - cannot take place as long as contaminated sediments are in place.
The Consent Holder (Bay of Plenty Regional Council’s Rivers and Drainage Group) has the necessary funding and consents to undertake the remediation. Significant delays could jeopardise the funding agreements or result in the works not being completed within the consented time period. Removal of the sediment is also necessary to ensure an extreme flood event does not result in the canal water overtopping the flood banks and impacting surrounding land.
Why not realign the canal and fill it in? Wouldn’t that be cheaper?
This option was also considered. However physical constraints on moving the canal and underground infrastructure (wastewater, fibre optic, power cables) along the canal corridor mean this is not viable and the expense would be prohibitive.
Why are you putting the sediment back onto rural land? We have read in the Whakatāne Beacon that this breaches United Nations Environment Programme practices. Is this true and could it impact on our ‘clean/green’ image which is important to international trade?
A large number of sites were considered during the early stages of the project. The original method needed to have three sites close to the canal because sediment was being excavated and transported to the containment sites by road transport. These sites were then negotiated and either purchased or leased prior to getting resource consents for containment of the sediment. While the new method is not reliant on road transport, a decision was made to retain and use two of the three containment sites (CS1 and CS3) for the safe storage of dredged sediment. While one containment site (CS1) is located in a rural setting, the containment measures at the sites have been enhanced to ensure no contaminants will discharge onto adjoining rural land. A stringent monitoring regime is also in place to ensure that the containment systems are working adequately. While the UNEP has developed requirements for the management, treatment and disposal of persistent organic pollutants (POPs) such as dioxins, these requirements only apply to the bulk storage of concentrated wastes that contain concentrations of dioxins above 50 milligrams per kilogram (mg/kg), which is around 10,000 times higher than the concentrations of dioxins identified in the sediments within the canal. For more information, see page 41 of the Environmental Management Tool Kit for Obsolete Particles.
Will the treatment sites or the groundwater below them become contaminated?
The sediment will be placed into geotubes (geotextile bags) which will be placed within fully enclosed ‘containment cells’. These cells will be formed using high density polyethylene (HDPE) liner which will eliminate sediment-bound contaminants’ movement into the surrounding soil or groundwater. There are extensive monitoring conditions the consent holder needs to comply with, including the monitoring of the containment sites and the groundwater beneath them.
The contaminant is hydrophobic and does not result in impacts to groundwater. Results recently obtained from historical dredge spoil placed along a stopbank indicate dioxins at the surface have not migrated through the soil profile, even when exposed to the environment for at least 30 years.
Did you trial a new methodology?
Many submissions on the consent indicated the community was concerned about various elements of the project. The key concerns were associated with traffic, dust and spillage during removal and transportation of partially dewatered sediment to the containment sites. Additional concerns were raised about potential releases to the environment during the filling and addition of bioremediation components before the containment sites were sealed.
Cutter suction dredging and geotube dewatering has been available in many parts of the world for decades but has only become available in New Zealand during recent years. The technique involves removing the sediment using a mechanical cutter head directly adjacent to a suction intake. As the sediment is disturbed, the suction pump draws it up into a pipeline and transfers it to the containment site. At the containment site the water/sediment slurry is dosed with commonly used chemicals, called flocculants, which separate the solids from the water. The separation occurs in the pipeline prior to it being pumped into geotubes. These tubes are designed to allow the water out but keep the sediment in. The discharged water (filtrate) collects in the base of the HDPE-lined containment site before it is tested. It then discharges back into the canal. Further details of this process can be viewed here.
On the basis of the trial results and positive community feedback, a variation to the existing consents was applied for. The variation was granted on a non-notified basis by an independent commissioner.
What will happen to the sediment after treatment?
Sediment will be left in place at the containment sites following treatment. The final land use of each site will be determined by the treatment level achieved and other site specific controls put in place.
What will this work mean for the drainage scheme?
When the sediment has been removed the water flow in the canal will be increased. This means better land drainage and greater flood protection.