Part 2: Did Mangawhai need a wastewater scheme?

Inquiry into the Mangawhai community wastewater scheme.

In our discussions with community members, many people asked whether it had really been necessary for Mangawhai to have a reticulated wastewater scheme. Several people suggested that stricter rules on the quality and maintenance of septic tanks would have been able to manage any environmental concerns. Some people suggested that KDC imposed the scheme on the community as part of a "push" for development in the area. Uncertainty about whether the scheme was really needed exacerbated concerns about the cost ratepayers were expected to bear.

Therefore, we spent some time reviewing the information and events leading to the initial decisions to begin work on a scheme. In this Part, we set out:

  • what studies showed about the water quality in the Mangawhai area by 1997;
  • how KDC went about considering options for improving water quality;
  • water quality results in 2001;
  • why septic tanks were not a suitable solution for Mangawhai; and
  • our comments on the steps KDC took.

In summary, we conclude that:

  • There was clear evidence that there was poor water quality in the Mangawhai area and that septic tank effluent was the likely cause.
  • Septic tanks were not an appropriate way to manage sewage disposal in the Mangawhai area.
  • The work KDC carried out in 1998 and 1999 to address the water quality problem was appropriate, and included good public consultation.
  • KDC's conclusion that Mangawhai needed a centralised reticulated wastewater scheme was soundly based and reached through a good process.

Water quality at Mangawhai by 1997

Mangawhai is a small harbour on the east coast of the Kaipara district, south of Whangarei. There are two parts to the community: a small village at the southern end of the Mangawhai Harbour and a much larger area of housing at Mangawhai Heads, which is five kilometres north towards the estuary mouth. The two areas are connected by a causeway. The area is a popular holiday destination. Its population can grow significantly during the summer months. In 1999, it was estimated that the permanently resident population was 1290 and that the peak summer population was around 4000.

As a small and remote community, Mangawhai had no centralised reticulated wastewater scheme. Individual properties used septic tanks, package plants, or long-drop toilets. KDC proposed communally based wastewater schemes in 1981 and 1988. The community rejected both proposals on the basis that KDC had not adequately demonstrated the need and that costs were too high.

In 1996, KDC publicly notified its draft District Plan. The Minister of Conservation made a submission asking for more restrictive controls on the use of land because of concerns about the environmental effects of the urban area on the Mangawhai Harbour. KDC accepted only part of the submission, and the Minister appealed to the Environment Court. To settle the appeal, KDC agreed to commission the Mangawhai Planning Study to look at the current and likely future growth of Mangawhai and its infrastructure needs, with a specific emphasis on wastewater requirements.

This study was carried out in 1997. It identified a potential health risk from using septic tanks in urban areas and highlighted evidence of contamination of the groundwater and the Harbour.

Around the same time, the Northland Regional Council (NRC) commissioned the Mangawhai Water and Shellfish Quality Study. This study surveyed the surface water quality in the Mangawhai area for levels of faecal coliforms and enterococci.1 Shellfish were also sampled for enterococci levels. The study showed that, when this work was carried out, there was:

… ongoing and at times significant faecal contamination of the drains and streams sampled in the Mangawhai Heads and Mangawhai Village areas. This was related to seepage from on-site sewage treatment and disposal system.

The study also found that:

The results of shellfish testing indicate that faecal coliform contamination of streams and drains in the Mangawhai Heads and Mangawhai Village areas is having significant adverse effects on shellfish water quality in Mangawhai Harbour.

The NRC study showed that nearly half of the sites had levels of faecal coliform or enterococci concentrations that exceeded the guidelines set by the NRC and the Ministries of Environment and Health.

Kaipara District Council considers options for improving water quality

In 1998, KDC commissioned the Mangawhai Infrastructural Assets Study. The aim of this study was to work out what infrastructure would be needed to support continuing growth in Mangawhai, while ensuring that development did not create significant adverse effects for the community, environment, or district. The study was to include options for wastewater treatment and disposal.

Beca was awarded the contract to conduct the study in October 1998. In November 1998, Beca prepared a scoping report setting out how it would carry out the study and outlining expected outcomes. As part of the study, it would prepare an Issues and Options report outlining specific infrastructural issues for water supply, wastewater, stormwater, roading, footpaths, and reserves. This report would then form the basis of extensive community discussion and debate.

Issues and Options report

The Issues and Options report assessed the existing methods for treating and disposing of wastewater (on-site disposal or small treatment facilities). It noted there was "evidence of significant failure of the on-site disposal systems particularly on the poorer soils".

The report set out three main options:

  • a centralised wastewater scheme with a single reticulation network, treatment plant, and discharge location;
  • a decentralised wastewater scheme with a reticulation network and several small treatment plants and disposal sites for different areas within Mangawhai; and
  • improved on-site sewage treatment and disposal, where each property was responsible for treating and disposing of its own wastewater.

Within those three options, the report explained the different possibilities for treating and disposing of wastewater. As well as improved on-site sewage treatment and disposal, the report discussed collecting sewage to one location followed by treatment:

  • in oxidation ponds or aerated lagoons, and disposal of the treated effluent to land;
  • in a treatment plant, ultraviolet (UV) disinfection, and discharge at the Mangawhai Harbour entrance; or
  • in oxidation ponds, then discharge to wetlands for further treatment, UV disinfection, and discharge at the Harbour entrance.

The report assessed the costs, environmental effects, and the potential effects on average lot size of each of the options and disposal possibilities.

The report was presented to the community through a series of workshops and a public meeting on 23 January 1999. About 170 people attended this meeting. A newsletter was also sent to all ratepayers that briefly summarised the issues and options, and sought feedback. Included with the newsletter was a demographic questionnaire. Beca received 309 responses to the newsletter and 324 responses to the questionnaire.

The NRC also provided comments on the report. It advised that, given the problems with water quality, the status quo was not considered desirable. The NRC also indicated that it did not support discharge of treated effluent to the Harbour entrance and preferred sustainable discharge to land.

Preferred Options report

In April 1999, Beca prepared the Preferred Options report for a further round of consultation. It also conducted another public meeting in May 1999. The report noted that the community had identified that the current treatment of wastewater by on-site disposal was inadequate and that action was needed. Feedback also revealed strong opposition to disposing of wastewater to water and a preference for land-based disposal:

Consultation and feedback to Newsletter #3 revealed that disposal of sewage to water is unacceptable to the community of Mangawhai. Feedback also indicated that Iwi find the disposal of sewage to water culturally offensive. In addition, the Regional council voiced a strong preference for land based disposal of sewage effluent. It is noted that the Proposed Regional Policy Statement and Proposed Coastal Plan both provide a clear policy approach away from disposal to any coastal water body. Accordingly as most parties and the relevant consent authority oppose disposal to water and as there is no apparent reason why land based disposal of treated effluent is not feasible, with several potential disposal sites having been identified, sewage disposal to water has thus not been considered further.

The report evaluated the three options in the Issues and Options report in light of technical investigations and the consultation feedback. It set out the environmental, economic, and community costs and benefits associated with each of the options. It identified the conventional wastewater treatment system (which was stated as involving collecting wastewater through a reticulated network, transferring it to a wastewater treatment plant, then treating it in oxidation ponds and disposing of the treated effluent to land) as the preferred option for the following reasons:

1. Provides the greatest degree of environmental protection

2. Provides the greatest degree of reliability

3. Provides the longest term sustainable sewage management

4. Easiest and most convenient to operate

5. Most affordable option for property owners on a community basis

6. Potentially the cheapest option.

Although the report included information about the likely capital cost for the preferred option, we note that it provided two different figures for that cost. On page 99, a table on the summary costs and charges showed that the estimated capital cost would be $10.84 million. However, on page 46, the report stated that the capital cost of the preferred scheme would likely be about $7.7 million. The annual operating and maintenance costs of the then-estimated current flows were estimated to be $61,000. This was made up of treatment costs of $32,000 and reticulation costs of $29,000.

The report recommended equitably distributing the cost of a new scheme between existing and future residents. Beca sent out a newsletter in May 1999 seeking feedback from the community on what it regarded as an equitable split. Beca received around 150 responses to the newsletter.

Beca then prepared a summary of the feedback, noting that:

While the majority of respondents supported a centralised wastewater treatment system, a number of submissions raised concerns about the affordability of this development and it is clear that careful costing and implementation of infrastructural development will be required.

The summary report

In August 1999, Beca prepared a draft summary report setting out the preferred options for the various types of infrastructural assets (including for treating wastewater) and outlining implementation plans for their development. The option Beca recommended for treating wastewater was a conventional drainage system to one wastewater treatment plant. Beca proposed to treat wastewater in oxidation ponds, followed by possible disinfection by either UV or chlorination, and then disposal to land. The report states that "It is assumed that the disposal site is arable, well-drained productive farm land some 1km or more distant from the coastline." This report confirmed that the estimated capital cost would be $10.8 million.

The Council considered a report from the Regulatory Support Officer about the draft summary report at its August meeting. The Council resolved to consider the draft report at a Council workshop and agreed to make copies of the summary available for the public to comment on.

A public meeting was held on 25 September 1999, where the final report was presented and the next steps discussed.

Beca was asked in October 1999 to prepare a strategic plan for implementing the infrastructure identified in the report, including options for funding, for the Council to consider.

Northland Regional Council confirms water quality problems in 2001

The NRC carried out some further studies on water quality that were reported in 2001. In those reports, the NRC concluded that:

Overall historical data from streams and drains flowing into the Mangawhai Harbour shows significant microbiological contamination. The majority of samples show levels of contamination well in exceedance of recommended guidelines by both the Northland Regional Council and the Ministries of Environment and Health. This contamination can be related to seepage from on-site sewage treatment and disposal systems.

A February 2001 report on water quality monitoring found that the:

Median concentrations of faecal coliforms and enterococci recorded in water samples from most sites within the Harbour are relatively high. This indicates that faecal contamination of streams in the Mangawhai area is widespread rather than being confined to a few "hot spots". As these areas are largely unsewered and residential, the likely cause is to be seepage from on-site sewage treatment systems.

Shellfish testing also indicated that contamination of the streams and drains in the Mangawhai area was having significant adverse effects on shellfish quality in the Harbour. The report also noted that more recent monitoring of the state of the environment had shown that several sites within the Harbour were unsuitable for swimming.

The need to replace septic tanks

We asked one of our engineers to help us review the use of septic tanks, package plants, and long-drop toilets for disposing of sewage in Mangawhai.

What makes a good septic tank system?

Septic tank systems generally consist of a tank and a disposal field that allows the effluent to soak into the ground. Package plants also require disposal fields, although the wastewater is treated to a higher standard before disposal.

Septic tanks provide a collection point for solid matter but do not treat raw sewage enough for it to be discharged into the environment. Effluent is disposed of from the septic tank through pipes into an area of ground known as an "absorption field". If an absorption field is not working effectively, the effluent will not be absorbed to land appropriately and might come through to the surface of the land or into surface waterways. This exposes the community to the risk of direct contact with the effluent and its pathogens.

The success of a septic tank system also relies on the owner regularly de-sludging their septic tank. This ensures that the build-up of solid matter does not result in overflow into the effluent absorption fields. This would block the absorption field, creating a risk of effluent ponding on the surface, running-off to neighbouring properties, and contaminating the groundwater system.

In addition to regular de-sludging, an effective septic system depends on several different factors, including:

  • soil of medium permeability;
  • rainfall of less than 900mm annually;
  • lot sizes greater than 4000m2;
  • individual properties having enough unpaved area for adequate absorption drains to be installed;
  • terrain not being excessively steep; and
  • avoiding areas with a high water table or a lot of rock.

The absence of any one of the above factors can cause the absorption fields to fail. The absence of multiple factors increases the likelihood of failure. The most significant factor leading to failure is that an individual lot size is too small.

Septic tanks in Mangawhai

Most lots in Mangawhai and Mangawhai Heads are around 1000m2. Septic tanks do not perform well on lots of this size, because each site is too small for an adequately sized absorption field. The density of development in Mangawhai exacerbates this problem. The cumulative effect of minor inadequacies in multiple neighbouring properties results in contaminated effluent being discharged to surface water and groundwater.

There are a variety of other problems with on-site disposal systems at Mangawhai Heads:

  • Some properties have steeply sloping terrain, so effluent drains through the soil into neighbouring properties.
  • Some properties are in areas with a high water table, which leads to effluent entering the groundwater system directly.
  • Sandy soils throughout much of the town enable effluent to leach directly into the groundwater system without pathogens dying off.
  • Many properties do not have enough unpaved area to allow enough length of drain to be installed to enable effluent to percolate into the soil without water-logging.

Within Mangawhai Village, there is a high water table, the soil has shallow cemented sand layers, and the lot sizes are small. These factors all compromise the capability of the soils to absorb and contain septic effluent.

Our engineer concluded that absorption fields will not be effective or reliable for disposing of effluent in Mangawhai, because many properties have factors that will limit effective absorption. The area's limited ability to retain effluent on site is exacerbated during the summer holiday season, when the population is at its peak. He agreed with the conclusions of the previous assessments of wastewater issues in Mangawhai that the previous systems, largely comprising septic tanks, were not sustainable. He also agreed that the indications were that these systems were having a detrimental effect on the water table and posed risks to people.

Our comments

In our view, there was reliable scientific evidence that there was poor water quality in the Mangawhai area and that the likely cause of that poor water quality was the sewage disposal methods used in the Mangawhai area at the time. It is also clear that, because of a variety of factors, such as soil type and lot size, on-site sewage disposal methods were unlikely to work effectively and were likely to result in adverse environmental effects.

KDC appropriately recognised that it needed to deal with this issue and carried out a study to determine how best to do so. It considered a range of options and consulted on those options with the community. As a result of this process, KDC decided it needed a centralised reticulated wastewater scheme. In our view, KDC's decision was soundly based and reached through a robust process.

The community and the NRC told KDC they did not support discharging the treated effluent to water, and KDC accepted this. This early decision to limit the disposal options to land-based solutions was reasonable at the time but had a significant effect on the way the scheme developed and on its costs.

1: Enterococci are bacteria that are found in the gut and faeces of animals. The NRC's 2001 Monitoring Results report notes that "They are used to indicate potential health risks to people using surface waters."

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