We would like your input to develop and action a plan to remediate the Tokomaru Bay legacy landfill.

The closed landfill is vulnerable to erosion when the Mangahauini River is in flood and washes over the site. Erosion has exposed waste to the environment that's ended up on the beach. This has happened 7 times in the last 5 years. With climate change, floods are likely to increase in frequency.

Removal of the landfill material is recommended in a controlled way to minimise further release of waste into the environment.

Last year the government granted us funding of nearly $5m from the Contaminated Sites Remediation Fund to remediate the landfill site to prevent further erosion and waste ending up in the environment.

The project

This is a 2-year project following engagement to work through details with hapū and residents of Tokomaru Bay. The remediation of the legacy landfill site means to restore it by reversing or stopping environmental damage.

The 4 initial remediation options

Each option has been initially assessed for risks and information gaps which may impact the ability to complete a robust assessment. If an option is determined not feasible or to have a “fatal flaw” (meaning it's not practicable or doesn't meet Council's remedial objectives), the option is discounted at this review stage and will not be further assessed. A high-level planning assessment is required as the next stage.

Option 1 - no remedial works, active monitoring
Option 2 - rock revetment
Option 3 - partial waste removal
Option 4 - full waste removal

No remedial works, active monitoring

This option involves leaving the landfill as is with ongoing monitoring and collection of waste that's eroded from the landfill following future flood events.

It's considered a baseline option as waste will continue to erode into the awa and migrate to Tokomaru Bay.

While there's no initial active remedial works, the regulatory, maintenance and cleanup requirements are likely to be substantial including:

• apply for a long-term discharge consent for the site.
• Develop a Site Management Plan (SMP).
• Regular monitoring of the site in accordance with the SMP and discharge consent requirements.
  This is likely to require the advancement of monitoring wells at the site, site walkovers and the collection of groundwater/leachate, surface water and sediment samples on a 6-monthly basis.
• Collection and disposal of eroded waste material following flood events.

Planning assessment consents are likely to be required for discharges from the landfill to land or water, and for maintenance activities within the riparian management area.

Key risks and information gaps

• This option will not address the complete-source-pathway-receptor linkages identified.
• The risks from landfill gas have not been assessed with respect to ongoing use of the site as a Council transfer station.
• The contaminant concentrations of the silt stock piles to the north of the landfill have not been assessed. The risk to surface water and sediment quality from the stock piles is unknown.

From a Te Ao Māori perspective, the further discharge of waste to the awa is not in keeping with the principles of Te Mana o te Wai. There's potential for further regulator infringements if waste continues to erode.

Rock revetment

This option includes the reinstatement and bolstering of the rock revetment installed in 2020 and encapsulation of the landfill.

This includes:

• applying for short-term resource consents authorising rock revetment works and a long-term discharge consent for the site.
• The construction of a rock revetment structure along the landfill edge slope to withstand the erosion forces of the awa.
• Covering the landfill surface with a low permeability soil cover and vegetated with native species. Cover materials would be tested prior to import to ensure they meet engineering permeability and environmental quality requirements. The final surface would be graded to allow for surface water to shed.
• Develop a site management plan.
• Regular site monitoring in terms of SMP and resource consent, with ongoing repairs to revetment when required.

Planning assessment consents are likely for discharges from the landfill to land or water, to erect a structure within the awa, vegetation clearance and for maintenance activities within the riparian management area.

Key risks and information gaps

• Modeling completed, indicates further erosion of the waste to the awa despite the predicted life span of 50 - 100 years. Therefore, the likely long-term success of this option is low. Future climate change and associated change in flood magnitude and frequency was also identified as a risk.
• An ecology assessment would be required to assess the risk to ecological receptors from the establishment of a revetment structure and to support resource consent applications.

Partial waste removal

Includes excavation of waste to reshape the landfill and remove exposed waste along the bank and transport to a licenced disposal facility.

A low permeability landfill cover would be installed to minimise stormwater infiltration and a rock revetment structure would be installed to protect from erosion during flood events. Ongoing monitoring and maintenance would be required including:

• developing a Site Management Plan (SMP).
• Regular monitoring of the site in accordance with the SMP and resource consent and repairs to revetment when/if required.

Planning assessment consents are likely to be required for discharges from the landfill to land or water, to erect a structure within the awa, vegetation clearance and maintenance activities within the riparian management area.

Consent may also be required for disturbing contaminated soil under the National Environmental Standard for Assessing and Managing Contaminants in Soil to Protect Human Health (NES CS).

Key risks and information gaps

• Modeling completed, indicates further erosion of the waste to the awa despite the predicted life span of 50 - 100 years. Therefore, the likely long-term success of this option is low. Future climate change and associated change in flood magnitude and frequency was also identified as a risk.
• A suitable disposal facility needs to be confirmed.
• An ecological assessment would be required to assess the risk to ecological receptors from the excavation and establishment of any revetment structure(s) and to support the resource consent application.

Full waste removal

Includes the removal of all waste from the landfill, including the silt stockpiles.

The waste would be transported to a licenced landfill facility (location to be confirmed). No ongoing site management or monitoring would be required for this option.

Resource consent is required to approve the works.

Key risks and information gaps

• A suitable disposal facility needs to be confirmed.
• Following the removal of the landfill, the flow path of the awa may change. An assessment has been completed which indicates the removal of the landfill is unlikely to have any impact on the awa mouth or beach.