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.
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.
This situation has been investigated a number of times since 1990, with various actions initiated.
An investigation was carried out in 2012 to determine 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 (during dry weather, the flows are much lower and can be diverted into the wastewater network).
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
- Carrying out fortnightly inspections of the diversion structure
We have also installed a depth sensor in the dry weather leachate diversion manhole. Linked to our telemetry system to receive alerts, this gives us an early warning that maintenance of the diversion structure is required. This reduces 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.
In 2013 we carried out an investigation to identify an engineering solution for the odour issues that is being experienced around the Houghton Bay stormwater outfall. This investigation recommended installing an odour control chamber. We are currently working our way through the consenting process and we expect that the odour control chamber will be installed by the end of 2017.
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 Council. Our report on behalf of Wellington City Council on the Houghton Bay stormwater outfall consent to Greater Wellington Regional Council in August 2016 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.