Reports on Darfield and Kirwee wastewater released

Wednesday December 17, 2014

A report on wastewater treatment systems in Darfield and Kirwee has found that many septic tanks are poorly operated and maintained.

The survey of more than 100 septic tanks by Canterbury District Health Board found fewer than 10 percent of residents did regular service or maintenance of their septic tanks. Many had a poor knowledge of their on-site systems and about a third had not had their tank emptied in the last five years. Nearly one in three septic tanks had some kind of failure.

Dr Alistair Humphrey, Canterbury Medical Officer of Health, says if septic tanks are not properly maintained, they can cause health and environmental problems.

"Septic systems can work well in rural areas especially if they're regularly maintained and tank owners are careful with what they put down their sink and use in their home," Dr Humphrey says.

The report recommends regular reminders are provided to residents of the need for maintenance of onsite systems to minimise the likelihood of onsite system failure.

The survey was one of three reports released today by Canterbury District Health Board and the Institute of Environmental Science and Research Limited on the treatment of wastewater in Darfield and Kirwee. 

Dr Humphrey says the reports will enable the community to make evidence based decisions about future wastewater management for the area.

"The Selwyn District is the fastest growing district in New Zealand, so it's important that sewerage systems are able to keep up with population growth."

"It's good news the reports show septic tanks currently do not present a risk to health because groundwater is very deep. However, if tanks are not adequately maintained health issues could emerge, especially if population numbers keep climbing."

Another important recommendation is to ensure wastewater discharge is an adequate distance away from the sources of community drinking-water supplies – that is the influence zones of the bores – in these growing townships.

"This is particularly important for Kirwee, where the drinking water capture zone is relatively close to its wastewater discharge area," Dr Humphrey says. 

"Moreover, if the density of the Darfield and Kirwee populations keep increasing it will become very difficult for the community to avoid reticulating their sewerage as there will simply not be enough room to ensure adequate separation of septic tanks from property boundaries," Dr Humphrey says.

A working party comprising representatives from Selwyn District Council, Malvern Community Board, Environment Canterbury, Canterbury District Health Board and community representatives has been formed to look at issues and options for the future management of wastewater in Darfield and Kirwee.

Selwyn District Council Assets Manager Murray Washington says it is pleasing that the reports have found that existing systems have had no adverse public health or environmental impacts to date.

"We welcome the survey and reports which will provide useful information for the working party which will be looking at the issues they raise, as well as considering future demand for residential and commercial development and affordability issues," he says.


Three reports were produced as part of the investigation:

  1. An assessment by the Institute of Environmental Science and Research of the ability of current groundwater monitoring undertaken by SDC to detect contaminant plumes from the septic tank disposal fields.

  2. A sanitary survey undertaken by Community and Public Health which investigated the operation and maintenance of the septic tanks in the Darfield community.

  3. Drawing on the findings of the above two projects, ESR assessed the public health risk of sewage treatment and disposal systems used in Darfield and Kirwee.

The reports are available on the Community and Public Health website.

Page last reviewed: 17 December 2014