Akaroa wastewater scheme

Summary

The area of land that meets the criteria for spray irrigation in Robinsons Bay is 41 hectares and for drip irrigation is 64 hectares. Therefore, there is enough land in Robinsons Bay for a spray or drip irrigation scheme. A 5 kilometre pipeline from the treatment plant to Robinsons Bay would be required, with wastewater flowing by gravity.

Please see page 11 of the booklet [PDF, 15 MB] [PDF 16MB] for a map showing the areas that meet the criteria for spray and drip irrigation and spray irrigation to pasture in Robinsons Bay, and possible storage pond locations.

There are four locations where wastewater storage ponds could be constructed in the valley, and two other locations on the north and south sides of the valley. If the ponds were located in the valley, they would be able to be fed by gravity from the proposed wastewater treatment plant as it is at a higher elevation. A pump station would be needed next to the storage pond(s) to pump the reclaimed water to the irrigation area.

Below are images of the site currently and an artist’s impression of irrigation to trees in upper Robinsons Bay. This is indicative only and has not been prepared in accordance with the NZ Institute of Landscape Architects Best Practice Guide, Visual Simulations.

Actual view of Robinsons Bay viewed from the paper road between Robinsons Valley Road and Okains Bay Road

Artist’s impression of irrigation to native trees in upper Robinsons Bay viewed from the paper road between Robinsons Valley Road and Okains Bay Road

The current land use compromises of residential lifestyle blocks, tourist accommodation and pastoral farming developed from its rich pioneering history, with associated archaeological areas and several historic cottages. The current landscape reflects this history.

The estimated cost of a scheme for drip irrigation to trees in Robinsons Bay is $6.6 million, which is the least expensive standalone option. It is less expensive than a similar scheme in Takamātua Valley as there are better sites for storage ponds. The estimated cost for of a scheme for spray irrigation to pasture in Robinsons Bay is $7.7 million. This is slightly more expensive than the estimated cost of $7.4 million for a harbour outfall, so additional budget would be required to fund this option.

Advantages

• Opportunity for beneficial reuse
• Enough suitable land for irrigation, with several potential pond sites
• Fewer land owners with whom to negotiate land acquisition, compared with Takamātua Valley
• A known landowner willing to sell an area suitable for irrigation to trees
• Most favourable soil types of the three irrigation areas being considered
• Low groundwater level, compared with Takamātua Valley
• Suitable distance from treatment plant to allow conveyance by gravity
• Least expensive standalone option – lower capital and whole-life costs; irrigation to trees is within project budget

Disadvantages

• Reclaimed water from Akaroa is disposed in an area which is populated and does not have a reticulated wastewater system
• There would be an increased risk of instability of stream banks near the irrigation area
• The valley is prone to flooding; this would be taken into account in the design of the irrigation area and pond(s).

Summary

The area of land that meets the criteria for spray irrigation at Pompeys Pillar is 35 hectares and for drip irrigation is 41 hectares, so there is enough land for either option. This location has the benefit of being on one property which is remote, has no residential dwellings onsite and has few neighbouring properties.

However, Pompeys Pillar is much further from the treatment plant than Robinsons Bay or Takamātua Valley, and wastewater would need to be pumped through a 10 kilometre long pipeline. The pipeline would pass over a 620 metre high hill, so would need to be a high-pressure pipeline made of stainless steel. There are three potential pond sites on flatter areas,or a storage pond could be formed by damming a gully.

Below are images of the site currently and an artist’s impression of irrigation to pasture at Pompeys Pillar. This is indicative only and has not been prepared in accordance with the NZ Institute of Landscape Architects Best Practice Guide, Visual Simulations.

Actual view of Pompeys Pillar

Artist’s impression of irrigation to pasture at Pompeys Pillar

Please see page 12 of the booklet [PDF, 15 MB] [PDF 16MB] for a map showing the areas that meet the criteria for spray and drip irrigation, and possible storage pond locations.

The estimated cost of a scheme for drip irrigation to trees in Pompeys Pillar is $11.9 million and for spray irrigation to pasture $13.7 million. These are amongst the most expensive options due to the high pipeline cost. It is significantly more expensive than the estimated cost of $7.4 million for a harbour outfall, so additional budget would be required to fund this option. This site is in the takiwā of Te Rūnanga o Koukourārata. The rūnanga has advised that the area has cultural significance and a cultural impact assessment is being undertaken.

Advantages

• Opportunity for beneficial reuse
•  Enough suitable land
• Area within one land parcel
• Could provide additional stock drinking water in a dry area
• Lowest groundwater level
• Suitable for a fire storage pond
• Suitable for small-scale hydro electricity generation on the downhill pipeline

Disadvantages

• Potential negative effect on a sensitive landscape
• Distance from treatment plant; pump station and high-pressure pipeline needed
• May be a culturally significant site
• Expensive option; additional budget would be needed

Summary

The area of land that meets the criteria for spray irrigation in Takamātua Valley is 9 hectares and for drip irrigation is 19 hectares. A larger area is required for spray irrigation due to the larger buffer distances. There is not enough land in Takamātua Valley for a spray or drip irrigation scheme to be solely located here, so it would need to be in combination with another area. A 2.5 kilometre long pipeline from the treatment plant to Takamātua Valley would be required.

A Takamātua Valley irrigation scheme would be problematic due to land constraints and areas of high groundwater. The topography of the valley dictates that suitably sloping and potentially irrigable land in the valley floor is a long and thin zone close to the Takamātua Stream. It is divided into many small areas owned by many different landowners, making the scheme more difficult to construct and operate. In addition the path to groundwater is likely to be shortest for this location, with the highest risk of nutrients leaching to groundwater.

Ōnuku Rūnanga has advised Council that they support investigations to determine the feasibility of discharge of treated wastewater to land within the Silent File area. The Council will continue to work closely with Ōnuku Rūnanga to address concerns if they arise.

There is one location within Takamātua Valley that meets the criteria for a storage pond. Nearby locations are on Old Coach Road opposite the treatment plant site or on the ridge above Takamātua Valley Road. Please see page 13 for a map showing the areas that meet the criteria for spray and drip irrigation, and possible storage pond locations.

The estimated cost of a scheme for drip irrigation to trees in Takamātua Valley combined with some land in Robinsons Bay is $8.4 million, which is more than the cost of a harbour outfall. The estimated cost for of a scheme for spray irrigation to pasture in Takamātua Valley combined with some land in Robinsons Bay is $11.8 million. This is amongst the most expensive options due to the fragmented nature of the land that is potentially suitable for irrigation. Additional budget would be required to fund both options.

Advantages

• Opportunity for beneficial reuse
• Proximity to treatment plant; wastewater could be conveyed by gravity
• Reclaimed water used in more than one location

Disadvantages

• Small amount of land in fragmented parcels
• Negotiations needed with many landowners
•  Not enough land for a standalone irrigation scheme
• Single pond site; groundwater close to surface, requiring above ground pond, which increases the cost
• Reclaimed water from Akaroa is disposed of in a populated area that has no reticulated wastewater system
• Increased risk of instability for stream banks near the irrigation area
• The valley is prone to flooding; this would be taken into account in the design of the irrigation area and pond
• Expensive option; additional budget would be needed

Summary

Further work has been done which has found that non-potable reuse in Akaroa is a realistic option for around a quarter of the annual reclaimed water volume. A reclaimed water pipe (“purple pipe”) could be constructed at the same time as the upgrades to the wastewater network to bring reclaimed water to Akaroa for beneficial reuse. This could be used to keep parks and reserves lush and green all summer long (e.g. Akaroa Recreation Ground, Jubilee Park, L’Aube Hill Reserve, Stanley Park) and flush public toilets.

The estimated cost is $1.7 million and would need to be in combination with another option.

In future, a reticulated “purple pipe’’ residential supply, for flushing toilets, watering gardens and washing cars could be installed for all of Akaroa. While in theory it may be possible to use the annual volume of reclaimed water, it may not be possible or practical to build enough storage ponds to store the required volume over winter when water use is low. The Council also needs certainty that it can dispose of all of Akaroa’s reclaimed water all of the time, rather than relying on voluntary use so even in the long term this would need to be in combination with another disposal or reuse option.

Advantages

• Beneficial reuse
• Improved park amenity
• Reduced demand on drinking water supply
• Proximity to treatment plant; wastewater could be conveyed by gravity
• Reclaimed water used in more than one location
• Paves the way for a more extensive “purple pipe” scheme in Akaroa

Disadvantages

• Not a standalone solution

Summary

This is the option that the Council originally preferred and applied for resource consents to implement. Resource consents for the outfall and the discharge were declined. This consultation is assisting the Council to assess whether a land-based discharge option is now preferred.

The outfall pipe would reach from the new treatment plant, out into the middle of the harbour (around 2.5km out from Childrens Bay) with a diffuser at the end for the discharge. This compares with the current outfall pipe from the existing treatment plant at the Takapūneke site, which extends only 100 metres from Redhouse Bay. The outfall pipe would be fully buried over its entire length in Council land and roadway, and then out into the harbour in Childrens Bay. The potential location of the outfall has been discussed with the Harbourmaster to ensure there is no conflict with boat mooring sites.

The diffuser would be 9.5 metres below the water surface, and the wastewater would be diluted at least 78 times before it reaches the surface, and further dilution is achieved as the plume spreads out.

A public health risk assessment found that the illness risk to swimmers would be generally low when compared to tolerable risks inherent in the New Zealand water quality guidelines for recreational areas. It also found that the illness risk to people eating raw shellfish would be higher than for swimmers, but would be still low. This would be a significant improvement on the public health risks from the current discharge from Redhouse Bay. There would be no ecological effects from the harbour outfall, other than disturbance of seagrass beds at Childrens Bay during construction, which would be replanted afterwards.

Advantages

• No additional land area required
• Low operation and maintenance costs; wastewater would flow by gravity
• Relatively low cost, within the project budget

Disadvantages

• No beneficial reuse of reclaimed water
• Some risk to public health from contact during recreation or eating raw shellfish
• Offensive to the Ngāi Tahu parties
• Not supported by the RMA unless there has been reasonable consideration and discounting of other options