Securing a safe and reliable water supply for Greytown

You can view an interactive StoryMap on the history of the development of South Wairarapa’s water supply and the background behind the existing Memorial Park treatment plant here 

Supply of safe and healthy drinking water to the public on behalf of the South Wairarapa District Council (SWDC), and all our client councils, is Wellington Water’s number one service goal.

Upgrade or replacement of the Memorial Park treatment plant to provide a multi-barrier approach and a reliable supply of safe and healthy drinking water for the community is therefore currently Wellington Water’s highest priority project for SWDC.

The Waiohine plant alone is not currently capable of supplying both Featherston and Greytown during high demand periods due to the constraints and hydraulic limitations with the existing supply network.

In more detail, let’s examine how the supply network currently works. Two treatment plants serve Greytown: the Waiohine plant, which also supplies Featherston, and the Memorial Park plant that supplies Greytown only.

During high demand periods, Greytown relies on the Memorial Park plant operating to provide sufficient water supply pressure. When demand drops below a certain level, such as at night, the Memorial Park plant cannot operate due to its minimum flowrate constraints and it shuts down.  This in turn causes the network pressure in Greytown to drop below the set point of a control valve in Humphries St and it opens to supply Greytown from the Waiohine treatment plant.

When demand for water increases again, such as in the morning, the valve closes and the Memorial Park bore restarts to meet the higher demand in Greytown.

Ultimately, without Memorial Park operating during the day, network pressures would drop below that required for firefighting and supply of safe drinking water to areas on the edge of Greytown’s water supply network.

Hydraulic modelling shows that around 8km of watermains would need to be replaced with a high-level cost estimate of the order of $18million to resolve these issues, together with further work to improve the hydraulic connectivity between the Waiohine water treatment plant and the Greytown reservoir that is located close to the treatment plant.

Even after completing all this work, operational resilience would be reduced as there would be no alternative supply to Greytown in the event of disruption of the supply from the Waiohine water treatment plant.

The Memorial Park bore and treatment plant, or alternatively a new bore supply and treatment plant elsewhere in Greytown will provide the necessary hydraulic pressure to resolve these issues, and provide increased operational resilience compared to supply from the Waiohine water treatment plant only.

Yes.  A fourth bore was installed at the Waiohine plant, and an 8 million litre treated water storage reservoir (bladder) has recently been commissioned that both allow the treatment plant to operate as efficiently as possible. 

As noted in the earlier question, the Waiohine treatment plant operating alone is not sufficient to supply Greytown during high demand periods due to network hydraulic constraints.  It is also important to build and maintain network operational resilience by having multiple sources of drinking water available when required due to any unforeseen circumstances that could occur at either treatment plant or with the supply pipelines between the treatment plants and Greytown.

In 2012 Groundwater Investigations adjacent on the Featherston side of the Tauwharenikau River were completed and concluded that 'the groundwater potential in this area is poor'.  In addition to this, the Abbots Creek, Boar Bush and Taits Creek surface water supplies are no longer suitable for Featherston’s needs.  To see more details view our South Wairarapa District Council water supply story map here.  

Options for supply of water to Greytown are limited.  Most bores near Greytown have been installed into the shallow and highly permeable aged gravel layer that is fed from rainfall and the Waiohine river.  Few wells have been installed into the deeper gravels, which suggest they are less productive.

Finding an alternative source of drinking water would require exploratory drilling with no guarantee of sufficient quantity or improved source quality and take significant time and investment to complete.

Provided appropriate barriers to contamination are in place, the shallow ground water can provide a reliable source of safe and healthy drinking water to the community.

The August 2016 Havelock North incident was traced to contamination of the drinking water supplied by two bores in Brookvale Road, on the outskirts of Havelock North, drawing water from an aquifer. That aquifer was thought to be a ‘confined’ aquifer and the water was thought to be secure from contaminants and, as such, the District Council did not treat water drawn from it (i.e. there were no barriers to contamination in place).

In the early 1900’s Featherston sourced its drinking water from Abotts Creek to the west of the township.  In 1954 the Boar Bush storage reservoir (concrete tank) was constructed to store water sourced from the Boar Bush Gully stream, and this tank also allowed the water to be treated with chlorine.  The Boar Bush Gully storage dam was constructed in the 1960’s to supplement this source, until the 1980’s when a constructed weir and pipeline from Taits Creek (located off Underhill Road on the other side of the Tauherinikau River) connected this new source to Featherston.

The Taits Creek supply could not support the needs of the growing Featherston township and water quality and quantity issues resulted in a pipeline being laid to connect Featherston to the Waiohine water treatment plant adjacent to the Waiohine river.  Taits Creek was last used to supply Featherston in about 1999.

Ultimately, the increasing demand of the growing township and need to source better quality untreated water means that the Abbots Creek, Boar Bush Gully and Taits Creek sources are no longer adequate for the needs of Featherston.

The water treatment plant, with a temporary containerised Ultra Violet (UV) system in place, is supplying safe and compliant water.

However, we need to maintain ‘multiple barriers against contamination’. This is one of the six principles of safe drinking water, stated in the Report of the Havelock North Drinking Water Inquiry: Stage 2: ‘Principle 3: Maintain multiple barriers against contamination: Any drinking water system must have, and continuously maintain, robust multiple barriers against contamination appropriate to the level of potential contamination. This is because no single barrier is effective against all sources of contamination and any barrier can fail at any time. Barriers with appropriate capabilities are needed at each of the following levels: source protection; effective treatment; secure distribution; effective monitoring; and effective responses to adverse signals. A “source to tap” approach is required.’

While the UV system in place at the Memorial Park provides a barrier and the treatment plant is monitored closely, there remains a risk that the single barrier against protozoa such as Giardia and Cryptosporidium that is in place might fail.

The current bore still meets demand but there is no backup and this creates supply risks. The pump and treatment facilities are very old and need replacing. The older they get, the more risk there is of failure. Without backup, we cannot switch off the pump and treatment facilities to allow for repairs, maintenance, or replacement without compromising supply pressure and fire flow availability to areas of Greytown for extended periods  -particularly at times of high demand.

It also means we don’t have the ability to commission the planned new treatment facilities ‘offline’, without causing an extended outage to complete the project, with similar impacts to those noted above for a very long period of time.

The proposed new bore will provide much-needed backup or ‘redundancy’ in the network, and combined with the new plant and treatment facilities, ensure a resilient and reliable supply of safe and healthy drinking water to the Greytown community.

An additional benefit is that the proposed work would remove the existing unsightly temporary container for the plant and treatment facilities and replace it with another that is discretely located away from the main car parking area.

Any new water supply bores will be designed near the water treatment plant taking into account the relevant constraints of the site. The new bore location will consider factors such as impacts during construction of the bore (which can include vibration or ground settlement) as well as operational impacts in addition to the bore yield.

These factors will be checked by completing ground investigations. If the site constraints lead to a bore being placed close to an existing structure then additional assessments will be completed to inform the design, and the construction methodology developed to mitigate these impacts and avoid adverse effects.

However, in the first instance the designers are trying to avoid such risks by placing the bore in a location relatively away from existing structures.

The proposed work would remove the existing temporary container in the main carparking area and replace it with a new containerised water treatment plant, discretely located away from main parking area to the right side of the pool complex.

Once the treatment plant was up and running the proposal would be to remove the existing containerised treatment systems enabling the car parking to be returned to the community.

This involved three screening stages. At each stage a possible site had to meet a set of requirements to move to the next stage. We also had information from different sources at each stage, including discussions with knowledgeable locals.

It was a detailed process, but here’s a simple summary:

1. Initial long list – we identified a long list of 11 possible options. To make this list the site must have access to the aquifer. We also had input from geographic maps, SWDC information and local knowledge.
2. Long listed sites – We long listed 10 sites. To make this list, the must haves include reasonable access to utilities, vehicle (chemical truck deliveries) accessible. We had input from water treatment and civil engineering technical specialists and  Wellington Water.
3. Two sites short listed – To make this list, the site must have connection to 11kV electrical network, be truck accessible and be near to water and wastewater infrastructure. The site must also have capacity to accommodate key factors like community health and safety risk, flooding and contamination risk and be near critical services. Our input sources included  stakeholders and technical engineers.

Having met all requirements for an alternative site, the two shortlisted sites were assessed using a Multi Criteria Analysis (MCA) process. This involved scoring each site against critical criteria (such as consentability, social and cultural impacts, flood risk, soil contamination risk). Through this process cost factors and ranges were also estimated.

Greytown has sourced its drinking water from the Waiohine river since the 1940’s, with a weir being constructed at Bassets Creek.  A storage pond allowed water from the river to be treated with chlorine.  The Waiohine water treatment plant was constructed in 1999 allowing treated drinking water to be supplied to both Featherston and Greytown. 

The increasing demand for drinking water and need to supply sufficient water for firefighting from Greytown’s growing community resulted in an additional bore being drilled at Memorial Park to supplement supply from the Waiohine treatment plant in 2005. 

Alternative sources of water for Greytown are very limited. Finding a reliable alternative source not already in use would require exploratory drilling with no guarantee of locating an improved quality or suitable quantity of water to meet Greytown’s needs, and will require significant investment and time to implement.