Our water supply comes from three sources:
- Hutt River
- combined flow of the Wainuiomata and Orongorongo rivers
- Waiwhetu Aquifer - a natural underground reservoir beneath the Hutt Valley that is fed by river-water seeping down into the ground
These water sources are carefully managed to protect their long term health.
Providing high quality treated water is easier if the water that we start with is relatively clean. Our river water collection areas are in mountainous land upstream of human settlement. Public access to them is limited, so the possibility of contamination from human sources is very low.
River water will naturally have some contamination in it, from dirt, decaying vegetation and animal waste. Microbiological contaimination- such as cryptosporidium and giardia - from animals has the potential to cause serious illnesses in people, so we monitor animal numbers closely in the water collection areas. Pest animal control is undertaken when needed.
River and aquifer water also absorbs chemical elements from the land that it flows over and through. Some of these elements can be harmful to humans if consumed at high levels. We monitor the levels of a variety of naturally occuring chemical elements in our water to ensure that it will be safe to drink.
Hutt Water Collection Area
The Hutt Water Collection Area covers almost 9,000 hectares of bush-clad mountains and valleys at the southern edge of the Rimutaka Ranges. The collection area is about equal in size to a square with the length of each side the same as the distance between Wellington's railway station and Petone. Rainwater collects in tributary streams that flow into the Hutt River. We have a weir (low dam) at Kaitoke, just north of Upper Hutt, where water is taken from the river, strained to remove sticks, leaves and silt, and piped through tunnels to the Te Marua Water Treatment Plant. We are allowed to take up to 150 million litres of water per day from the Hutt River, provided an adequate flow is maintained downstream of the weir. The Hutt Water Collection Area provides about 40 percent of the water we supply each year.
We have two large storage lakes at Te Marua - the Macaskill Lakes - as back-up to the Hutt River supply. The lakes are filled from the river when it is clean and there is plenty of water available. Stored water is pumped back to the treatment plant when there is not enough water in the river to meet public demand, when the river is too dirty - after heavy rainfall - or when it is in flood and the intake is closed to prevent rocks and gravel from entering the intake pipes.
The Macaskill Lakes are 17.3 metres deep and have a combined useable capacity of approximately 3,350 million litres, or enough to meet average water use for around 23 days.
Wainuiomata/Orongorongo Water Collection Area
The Wainuiomata/Orongorongo Water Collection Area is part of the Rimutaka Ranges to the east of Wainuiomata. The collection area covers 7,600 hectares. Five low dams (weirs) with intake pipes provide water to the Wainuiomata Water Treatment Plant. Weirs on the Wainuiomata River and George Creek provide around 15 percent of our annual water supply. In the Orongorongo Valley, weirs on the Orongorongo River, Big Huia Creek and Little Huia Creek provide around five percent of annual water supply. A 3.2 kilometre long access and pipeline tunnel links the Orongorongo Valley intakes to the Wainuiomata Valley.
We must leave a minimum water flow equivalent to 8.6 million litres per day in the Wainuiomata and Orongorongo Rivers, downstream of our weirs. Unlike Te Marua, there is no untreated water storage at Wainuiomata. If river levels are very low or the rivers are in flood the treatment plant is turned off temporarily.
Waiwhetu artesian aquifer
The Waiwhetu artesian aquifer is a zone of water-holding sand, gravel and boulders beneath the Hutt Valley. Water from the Hutt River starts to flow underground around Taita Gorge. From Melling southwards, the water becomes naturally pressurised beneath a layer of hard clay. This pressurised zone, the Waiwhetu artesian aquifer, stretches as far south as the harbour. It is estimated to be up to 70 metres thick at its western edge against the Wellington fault line, and 20 metres thick at the eastern edge of the harbour. The pressure in the aquifer has resulted in several fresh water springs in the harbour floor.
Water takes more than 12 months to pass through the aquifer to our wells and is naturally filtered while underground, making it free from disease causing micro-organisms. We monitor the aquifer closely to ensure we leave enough water in it to maintain pressure, so that seawater cannot enter via the harbour springs. The Waiwhetu aquifer provides around 40 percent of the annual water supply.