Houghton Bay leachate

Houghton Bay has a long history of discharges of leachate, which originates from the former Houghton Valley landfill and is carried to the bay via the stormwater system.

Historical causes

The closed landfill extends up the original valley floor from Cave Rd to Houghton Valley School and on up to Sinclair Park. It was opened in 1951 and was filled in two stages. Stage 1 was completed in 1963 and stage 2 in 1971. The land is currently used as playing fields.

Since its closure, landfill management methodologies have improved considerably. Current best practice includes catchment lining, layering, diverting water flows, and capping.

These practices were not used in the Houghton Valley landfill. 

The result is that water percolating from rain and subsurface water picks up dissolved and suspended components from the biodegrading activity of anaerobic bacteria in the landfill. These components form leachate, which enters the stormwater network that lies beneath the landfill.

These conditions are exacerbated by field drains in place that capture groundwater from the landfill, and are connected to the stormwater system. This water then discharges onto Houghton Bay beach above the high water mark.

Investigations

This situation has been investigated a number of times since 1990, with various actions initiated.  Our most recent investigation was focussed on determining the state of the environment in the receiving waters adjacent to the stormwater outfall.

The investigation found the effects of the discharge in the receiving environment are minor. Mean concentrations of heavy metals from sediment samples and water quality samples show that these are below the possible negative effects for recreational purposes or health issues (Annual Compliance Report, August 2012).

The investigation also noted that leachate-contaminated stormwater discharges to the receiving waters only during extreme wet weather conditions.

Based on that investigation, a detailed assessment was undertaken of management and treatment options to further “prevent or minimise” inputs of contaminants (leachate) being discharged into the stormwater culvert.  

Managing the problem

The following management and treatment options are now in place:

  • Diverting the dry weather base flow into the wastewater network and control of leachate flow in the stormwater pipe (diversion) by a weir arrangement.
  • Monthly monitoring of weir overflows and stormwater outlets.
  • Removing encrustation growth in the stormwater pipe – flushing the pipeline on an annual basis or more frequently if required.

In addition we are installing a depth sensor in the dry weather leachate diversion manhole. Linked to our telemetry system to receive alerts, this will enable an early warning to undertake maintenance where the diversion structure requires attention. This will further reduce the likelihood of leachate entering the outfall and allow speedier identification and resolution of any issues. 

We have inspected sumps in the catchment to identify whether sump boxes have baffles to trap debris. Preventing debris entering the stormwater main reduces potential blockages of the diversion chamber.

We have also recently commissioned an investigation to identify an engineering solution for the odour issues that is being experienced around the Houghton Bay stormwater outfall. The investigation is expected to be completed by the end of October 2013.

Details of our investigation were included in an annual compliance report (August 2012) to Greater Wellington Regional Council.

Collaborative effort

We've collaborated closely with stakeholders interested in public and environmental health issue relating to Houghton Bay, including Wellington City Council and Greater Wellington Regional Councils. Our report on behalf of Wellington City Council on the Houghton Bay stormwater outfall consent to Greater Wellington Regional Council in August 2012 showed that conditions at the outfall fully complied with the current resource consent.

We appreciate that even occasional discharges to Houghton Bay are not ideal. However, we and our colleagues are doing what we can to deal with an historical situation, and to minimise future incidents and impacts.