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Waiwhetu Aquifer

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Waiwhetu Aquifer
Artesian aquifer
Map
LocationHutt Valley and Wellington Harbour, New Zealand
Offshore water bodiesWellington Harbour

The Waiwhetu artesian aquifer, sometimes referred to as the Hutt aquifer, is a pressurized zone of water-retaining sand, gravel and boulders beneath the Hutt Valley and Wellington Harbour which provides about 40% of the public fresh water supply for Lower Hutt and Wellington city. Water from the Hutt River begins to flow underground south from Taita Gorge, then becomes pressurized under a seal of clay. Water is extracted from the pressurized area for public use, but concerns about overuse and damage by earthquakes have lead to investigations of alternative sources of fresh water.

Formation and location

The Waiwhetu aquifer covers a wedge-shaped area of 75 km² under the Hutt Valley and Wellington Harbour, and by size is the fifth-largest in New Zealand.[1]

The harbour basin contains massive quantities of gravel washed down from the Hutt River, in some places hundreds of metres deep.[2]: 22, 24  Above the gravel is a layer of mud and silt which seals fresh water within the gravel, creating an artesian aquifer.[1] There are several aquifers in the area in different layers underground, but the Waiwhetu aquifer is the largest and most productive one.[3] Water flows down into the aquifer from a five-kilometre stretch of the Hutt River south of Taita Gorge, at the rate of 1000 litres per second.[1][3] Rainwater also contributes to the aquifer.[1] South of Melling the aquifer becomes pressurized by the layer of mud and silt above the gravel layer holding the water in, meaning that if a bore is sunk into the aquifer, water will rise up the pipe. Water from the aquifer also reaches the surface through natural springs at various places around the harbour.[1] Pressure within the aquifer stops sea water from getting into the aquifer.

Once in the aquifer, the water moves slowly. It takes several years for water in the underground aquifer to reach Waterloo from the Taita Gorge, about 10 years to reach the Petone foreshore and 20 years to get as far as Matiu / Somes Island.[1] The water level in Wellington Harbour was much lower 20,000 years ago, and the ancient Hutt River used to flow down a paleochannel to the east of Matiu / Somes Island as far as the present-day Miramar Peninsula.[4]: 28  [2]: 75  [1] Much of the water in the Waiwhetu aquifer moves under the sea bed from the direction of the Hutt River to the Falcon Shoals area (between Karaka Bay and Worser Bay) at the harbour mouth via the paleochannel. The characteristics of the aquifer between Matiu / Somes Island and the harbour mouth are not as well studied as the portion to the north of the island.[5]

Extraction

The Te Puna Wai Ora fountain flows with water from the Waiwhetu aquifer

The Waiwhetu aquifer provides water to 150,000 people in Wellington and Hutt City.[6] This is around 40 percent of the Wellington region's annual water supply, and up to 70 percent in summer months.[7]

From the 1880s, residents and businesses in the Hutt Valley and Petone area sank wells into the aquifer for fresh water,[8] and in 1908 Lower Hutt Borough introduced a public water supply fed from artesian bores.[9][10] In 1963 Petone Borough built a pumping station at Buick Street. This was decommissioned in 1999 when a new pumping station was built at Waterloo.[9] In the 1980s, eight bores drawing from the aquifer were put in along Knights Road and nearby streets in Lower Hutt (known as the 'Knights Road spine'), and these feed into the Waterloo pumping station for treatment and public distribution as drinking water.[6] Greater Wellington Regional Council manages the system. There is a bore at Hutt Hospital for use in emergencies. Other bores into the aquifer are used mainly for industrial purposes.[6]: 13  A 2014 report identified 13 private bores in the Hutt Valley and Petone area, five of which had consents to take 1,000 cubic metres or more of water from the aquifer each day.[3]: 12 At the time of its sale in 2014, the Unilever soap factory in Petone had consent to take 900,000 cubic metres of water from the aquifer each year.[11]

In 2017, Hutt City Council granted Waiwhetu Marae $150,000 to install a bore, and the bore was opened for use in 2020. This bore is unchlorinated, but treated with UV light.[12][13]

Two other bores provide free unchlorinated water from the aquifer direct to the public. One is at the Dowse Art Museum in Lower Hutt, where a small park with water feature was created in 2012,[14] and the other is Te Puna Wai Ora (Spring of Life) in Buick Street, Petone, built in 2003.[15][9]

Water treatment

Lime is added to aquifer water coming in to the Waterloo station, to adjust its pH level so that the pipes are not damaged.[6] Apart from this, prior to 2016, water from the aquifer was not treated because its long period filtering underground meant the water was free from bacteria and viruses.[6] Water from the aquifer going to Wellington (but not the Hutt) was treated with chlorine as it mixed with chlorinated water coming from Wainuiomata and passed through the Gear Island pumping station.[6] Water from the aquifer going to 74,000 people in Petone, Hutt Central, Naenae and Eastbourne was not chlorinated.[16]

In August 2016 there was an major outbreak of gastroenteritis in Havelock North caused by public water supply contamination.[17] As a precautionary measure, Wellington Water responded by changing its regime of water testing from the Waiwhetu aquifer. On 1 December 2016, routine testing of water from a bore on the Knights Road spine showed contamination with e. coli, the first time since 1980 that a positive test had occurred.[6] The water supply was temporarily chlorinated while further tests were carried out. It was suspected that the contamination was a result of ground disturbance caused by the Kaikōura earthquake on 14 November, but an investigation by Wellington Water did not reach a firm conclusion.[6] Tests at other bore locations in February and April 2017 also returned results positive for e. coli. After comprehensive testing and investigation, Greater Wellington Regional Council decided to continue chlorination permanently and install UV filters at the Waterloo plant.[16]

In April 2017 the unchlorinated bore supply available to the public at Te Puna Wai Ora and the Dowse Art Museum were shut down after returning positive results.[6] These were reopened to the public after a system of filtering the water and treating it with UV light was installed.[6][18]

A new pipe system for water diversion was also installed along Knights Road, through Lower Hutt to the harbour, completed by the end of 2017.[6][19] The project won an award from Civil Contractors NZ Wellington/Wairarapa.[20]

Protection

Photo of bore pipe protuding from ground
Water level monitoring bore at McEwan Park, Petone foreshore

Water pressure within the aquifer keeps sea water out, but the pressure can drop due to too much water being extracted, or due to less water in the aquifer in dry spells when the Hutt River and rainfall are low. If the pressure drops then sea water may get into the aquifer via vents on the harbour floor. This is known as saline intrusion, and could make water in the aquifer undrinkable. Greater Wellington Regional Council controls the amount of water extracted and monitors the level of water in the aquifer (and therefore the pressure) with three bores on the Petone foreshore, and others further inland in the Hutt Valley.[21][22]: 23  If the water level drops, extraction can be reduced. There are three warning levels. The first warning level (2.5 metres above mean sea level) is reached regularly during summer. Water has fallen to the second level (2.3 metres above mean sea level) for short periods several times since 2002, and in March 2016 water in the aquifer briefly got as low as the third level (2 metres above mean sea level).[21] Matiu / Somes Island gets its fresh water from a bore sunk into the aquifer just off shore at the main wharf. In February 2016 the Department of Conservation temporarily restricted visitor access to the island because the long dry summer had lowered the volume and pressure of water in the aquifer, necessitating strict water conservation.[23] The lowest recorded water level in the aquifer occurred in the summer of 1973, when the water level sank to 1.19m above mean sea level and sea water started to enter the aquifer.[21][22]

Apart from the risk that over-extraction could lower the pressure in the aquifer and cause saline intrusion, other activities which threaten the aquifer include accidental piercing of the aquifer, dredging and disturbances to the sea floor caused by shipping movements. For example, in 1929, workers driving piles for a wharf at Point Howard pierced the aquifer, causing a large spring to form. The spring was so strong that a diver sent to investigate had trouble approaching the hole.[24] Dredging activities at the mouth of the Hutt River in 1937 caused a severe drop in pressure in the aquifer throughout the Hutt Valley. Pressure was restored by filling the hole made by the dredging with excavated material, and with material carried down the river by flooding.[24][25] In 2000, a study of the aquifer raised the possibility that the Lynx, a fast ferry, was disturbing the sea bed near the Falcon Shoals and thus causing water to leak from the aquifer.[2]: 104 

Future proofing

Responding to the November 2016 Kaikōura earthquake, in 2017 Wellington Water began a project drilling into the harbour bed to look for fresh water that might supply the city in an emergency.[26]

References

  1. ^ a b c d e f g Ballance, Alison (30 June 2017). "The science of a water aquifer". RNZ. Retrieved 2023-04-14.
  2. ^ a b c Harding, Steven James (August 2000). The Characteristics of the Waiwhetu Artesian Aquifer beneath Wellington Harbour including the Spatial Distribution and Causes of Submarine Spring Discharge [thesis] (PDF). Victoria University, Wellington, New Zealand.{{cite book}}: CS1 maint: location missing publisher (link)
  3. ^ a b c Gyopari, Mark (June 2014). Lower Hutt Aquifer Model Revision (HAM3): Sustainable Management of the Waiwhetu Aquifer [report] (PDF). New Zealand: Institute of Environmental Science & Research Ltd.
  4. ^ McConchie, Jack; Winchester, David; Willis, Richard, eds. (2000). Dynamic Wellington: A contemporary synthesis and explanation of Wellington. Wellington, New Zealand: Institute of Geography, Victoria University of Wellington. ISBN 0475110560.
  5. ^ "Wellington Harbour Bores - Exploratory Drilling Findings : Water New Zealand". www.waternz.org.nz. Retrieved 2023-04-23.
  6. ^ a b c d e f g h i j k Edwards, Laurence; Blakemore, Rob (2018). The Battle of Waterloo [Conference paper]. Wellington, New Zealand: Wellington Water.
  7. ^ "The Battle Of Waterloo : Water New Zealand". www.waternz.org.nz. 26 September 2018. Retrieved 2023-04-17.
  8. ^ "[untitled]". Evening Post. 12 June 1888. Retrieved 17 April 2023 – via Papers Past. The Gear Company have been most fortunate in their efforts to obtain a water supply for their works at Petone by means of artesian wells. Another splendid flow from a two-inch pipe was obtained yesterday.
  9. ^ a b c "Hutt City – Petone's Artesian Water" (PDF). Hutt City Council. 13 May 2010. Archived from the original (PDF) on 13 May 2010.
  10. ^ "Hutt Water Supply". The Dominion. 22 February 1908. Retrieved 17 April 2023 – via Papers Past.
  11. ^ "Developer snaps up Unilever site". Dominion Post. 6 December 2014. ProQuest 1630428419.
  12. ^ Boyack, Nicholas (8 May 2017). "Marae advised not to use bore water after E.coli found in Lower Hutt water". Stuff. Retrieved 2023-04-28.
  13. ^ Hemi, Tema (26 February 2020). "Waiwhetu marae opens new water well for Hutt Valley". Māori Television. Retrieved 2023-04-28.
  14. ^ "Artesian Water". WildBay. 26 February 2013. Retrieved 2023-04-20.
  15. ^ "Te Puna Wai Ora (Spring of Life)". Jackson Street Program. Archived from the original on 9 April 2020. Retrieved 9 April 2020.
  16. ^ a b "Waiwhetu aquifer chlorination". Wellington Water. 20 August 2018. Retrieved 2023-04-26.
  17. ^ "Timeline: NZ's worst waterborne outbreak". RNZ. 2016-08-19. Retrieved 2023-04-26.
  18. ^ https://www.wellingtonwater.co.nz/assets/Resources/Drinking-Water/268No-chlorine-at-public-fountains.pdf
  19. ^ George, Damian (2017-10-09). "$11 million pipeline required to fix Lower Hutt drinking water". Stuff. Retrieved 2023-04-18.
  20. ^ "Waterloo UV Run to Waste, 2017 | E Carson & Sons Limited | Drainage & General Contractors". Retrieved 2023-04-18.
  21. ^ a b c Macdonald, Nikki (1 February 2019). "Underground water reservoirs at risk from seawater contamination". Stuff. Retrieved 2023-04-28.
  22. ^ a b Phreatos Ltd (2001). Waiwhetu Artesian Aquifer Saltwater Intrusion Risk Management Review [Technical report] (PDF). Wellington, New Zealand: Wellington Regional Council.
  23. ^ "Freshwater supply shortage triggers partial closure of Matiu/Somes Island". www.doc.govt.nz. 24 February 2016. Retrieved 2023-04-14.
  24. ^ a b "Valley Port opposed by city". Evening Post. 28 September 1944. Retrieved 28 April 2023 – via Papers Past.
  25. ^ "Trouble at Hutt: Supplies affected". Evening Post. 25 January 1937. Retrieved 28 April 2023 – via Papers Past.
  26. ^ Cropp, Michael (28 June 2017). "'Urgent' freshwater search under way in Wellington". RNZ. Retrieved 2023-04-15.
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