In the middle of the Wellington suburb of Petone, people line up every day to fill their water bottles from Te Puna Wai Ora, the Spring of Life. The fountain is one of several artesian wells along the Hutt River valley that taps the Waiwhetū aquifer, a vast freshwater lens that stretches underground and continues to run under much of Wellington Harbour.
Matiu/Somes Island, in the middle of the harbour, draws its entire drinking water supply from an offshore groundwater bore, and during dry years, when Wellington’s water reservoirs can run low, the capital city relies on this submarine aquifer.
A new detailed study of the harbour floor, pockmarked with freshwater seeps, has delivered a case study that could help scientists find untapped drinking water in other parts of New Zealand and around the world.
Joshu Mountjoy, a marine geoscientist at the National Institute of Water and Atmospheric Research (Niwa), says the study used several techniques − including acoustic surveys and chemical analyses − to map the natural springs where freshwater emerges from the deep. It closes a gap in knowledge about large coastal aquifers around New Zealand.
There were two main motivations for this work, says Mountjoy. “One is we might discover totally new water bodies that we didn’t know existed. This would add a whole new freshwater drinking resource … and even if it is a finite resource, there still could be a lot of drinking water there to tap into.
“The other aspect is better understanding of aquifers that are on land but have an offshore component.”
Take Canterbury, for example, where a huge aquifer supplies drinking water but is also used for irrigation. Mountjoy says it’s known that the aquifer continues under the ocean but “we don’t have any direct information beyond the coastline; there are no drill holes and nobody knows where there’s any seepage coming out.” A study is now underway.
The geology of the Canterbury Plains suggests there could be a vast submarine freshwater lens offshore. “If you go back in time some 20,000 years to the last ice age, the sea level was 120m lower and the Canterbury Plains extended tens of kilometres further out. The big rivers, the Rangitata and Waimakariri, continued well past the modern-day coastline and were putting down deposits of gravel and silt well out into what is now ocean. The aquifers were forming at that stage and there’s connectivity between the deposits on land and offshore.”
Accurate knowledge of the size of submarine aquifers is critical for modelling and setting the amount of sustainable water use, he says. “If you’re in Christchurch city and you’re extracting water from the aquifer, you’re right as long as you don’t take more than is recharging. If you start to draw too much, then you start bringing the transition between freshwater and saltwater closer towards the land. Once [the zone where freshwater meets seawater] reaches the coast, it could take decades or even hundreds of years to recover.”
Other areas for further exploration are Hawke’s Bay and parts of the East Coast. While these regions experienced extreme rainfall this summer, they are generally prone to long periods of drought. “They get very reliant on groundwater, and it’s probably going to get more so with climate change,” says Mountjoy.
At this point, even if we know that an aquifer continues under the sea, quantifying a water budget for the offshore part is “close to guesswork”. Wellington’s Waiwhetū aquifer is well understood, but even there, he says, “there’s a huge way to go to understand the volume of water that’s being discharged to the ocean, which can be up to 50% of the aquifer throughflow”.
Understanding where submarine aquifers are and eventually being able to measure their flow will help water managers to quantify the resource. “And that’s true in New Zealand and all around the world, where many cities are dependent on groundwater systems”.
