Underground water reservoirs at risk from seawater contamination
Friday, 1 February 2019
The pōhutukawa fringing Petone's McEwan Park bloom crimson, signalling summer has arrived in Wellington. That's also the cue for the park's other beacon – tucked in flax bushes just back from the beach – to be on high alert.
Unceremoniously named Bore R27/0122, this green watchtower – a metal box atop a pipe extending into the aquifer below – is charged with ensuring the underground reservoir supplying 40 per cent of Wellington's water remains safe to drink.
The threat is not polluted rivers or too much cow pee or poo. It comes from a silent source beyond the beach, about 600m into Wellington Harbour, where the water in the Waiwhetu Aquifer breaks the hard clay that acts like a cap. The pressure pushes the water out into the sea through submarine vents – dark spots on depth charts, dotted around the harbour.
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When the water pressure inside the aquifer remains high, they're vents for fresh water to get out. But if the aquifer water level falls too low, or the sea level rises, they can become vents to let seawater in.
The Waiwhetu Aquifer is one of 200 mapped aquifers around New Zealand, which feed the country's rivers and springs and provide about a third of Kiwis' daily supply. About 150 of those are on the coast. As climate change brings predicted sea level rises and greater demand for water, those underground tanks will become increasingly vulnerable to saltwater contamination.
The gravels that make up Waiwhetu Aquifer have been laid down by the Hutt River over 400,000 years. Every day, water percolates through from a 5km stretch of river downstream of Taita, into the aquifer below. It takes 12-18 months to travel the length of the underground reservoir, being filtered clean as it goes.
But the natural balance that keeps the aquifer water moving out to sea is altered when you start pumping from the deep. Last year, 23.2 billion litres of water – equivalent to 9300 Olympic swimming pools – was sucked from the aquifer to supply Wellingtonians.
In summer, demand for water soars, lowering the aquifer level and increasing the risk of the submarine springs drying up and seawater seeping in. Salinity as low as 2 per cent could render the water undrinkable, and because aquifer water moves slowly, some reports have suggested the contamination could take two years to flush out.
Which is where the McEwan Park monitor bore comes in. Greater Wellington Regional Council has set three levels to trigger action to prevent saltwater contamination. When the water level falls into the danger zone, the monitor bore alerts Wellington Water, to reduce pumping.
Records show the first tier 'warning level' of 2.5 metres above mean sea level (mamsl) is regularly breached during high summer demand. The water has also fallen below the second-tier 'critical' threshold of 2.3 metres above mean sea level several times since 2002, but only for brief periods. Records show one recent breach of the third-tier 'minimum' level of 2 metres above mean sea level, in March 2016, but not for a full 24-hour monitoring period.
Seawater contamination is not just a theoretical risk. The aquifer hit an all-time low of 1.19m above mean sea level in February 1973, at the end of a long dry period.
The aquifer pressure fell so much that water started flowing back from the sea. It's not known whether that meant Wellingtonians had to drink briny water.
Since then, the main extraction bore has been moved inland, to Waterloo, allowing the aquifer level underneath the foreshore to bounce back. But population growth, sea level rise and a hotter, drier climate pose a new risk. And to make things worse, Wellington is sinking.
Assuming a sea level rise of 1m by 2100, water pumping from the aquifer would have to be cut by one-fifth to keep seawater out. By the same year – based on population projections – demand for water is expected to increase by more than 60 per cent.
There are options to hold back the tide – researcher Gregory De Costa suggested managed aquifer recharge, which involves pumping water in to push seawater out. (See sidebar)
'The risk of sea water intrusion cannot be reduced only by controlling the level of abstraction, particularly if demand continues to increase and recharge decreases,' De Costa concluded in a 2009 report. But Wellington Water says the only solution considered so far is reducing extraction, which leaves a looming hole in Wellington's water supply. GWRC planning documents show the council plans to develop a new water source from 2032, at a cost of about $320 million.
Wellington Water says it's working on a sustainable water supply study, due in 2019/20, which will consider the long-term impact of sea level rise on the aquifer.
'This will consider water holistically, including sources, treatment systems and infrastructure, as well as human aspects such as behaviour and use of water,' it said in a statement.
But the Waiwhetu Aquifer is just one of hundreds potentially at risk from sea-level rise and higher demand for water. GNS head of hydrogeology Stewart Cameron says New Zealand's high rainfall flowing into rivers and recharging aquifers has traditionally kept seawater out. But that is changing. Records show seawater has already seeped into many coastal aquifers nationwide (see below).
'With increased extraction and water use, there have been incidences of seawater intrusion and climate change will push that seawater wedge further inland.'
One of the biggest problems, says Cameron, is that we don't know enough about which aquifers are vulnerable, and how they could be contaminated. While the Waiwhetu Aquifer discharges directly into the sea, some coastal aquifers are completely closed systems.
'The aquifer systems in New Zealand are relatively poorly understood, because New Zealand hasn't invested enough in fundamental knowledge acquisition for a long, long time … The first thing we need to do is understand our coastal groundwater systems better – their structure and how the groundwater is connected to the sea.'
Back at Bore R27/0122, dog walkers and cyclists trundle along the foreshore oblivious to the precious resource beneath. But they will certainly notice if the tap at home takes on a salty tang.
HISTORICAL INCIDENTS OF SEAWATER SEEPING INTO COASTAL AQUIFERS
Northland: Salty groundwater has been recorded at Ruawai, Pataua North and Cable Bay. Another 21 locations are at risk and being closely monitored. Saltwater contamination was also raised by submitters opposing a resource consent application by 17 Far North avocado growers to collectively take two million cubic metres of water a year from the Aupōuri aquifer.
Auckland: Waiwera and Whangaparaoa Peninsula have had seawater intrusion and Karaka, Middlemore, Omaha and Parakai are subject to special monitoring.
Waikato: In 2001, seawater seep forced the Opoutere motor camp, in the Coromandel, to abandon their well and switch to roof water. In Whangamatā, a water supply well was shut down because of salty groundwater. Cooks Beach, Hahei and Pauanui are considered at risk.
Bay of Plenty: Seawater intrusion has affected a Papamoa Beach well and Tauranga has special monitoring.
Hawke's Bay: No documented saltwater contamination, but Esk/Bayview and Wairoa-Mahia are considered at risk.
Taranaki: An investigation by South Taranaki District Council found the Whenuakura Aquifer near Pātea is at high risk from potential saltwater intrusion. Two 1930s shallow bores at the Pātea freezing works were decommissioned because of seawater seep. The Waitara freezing works also had a 125m deep bore 1.5 kilometres from the coast that in 1980 became too salty to drink.
Manawatu-Whanganui: The western coast, between the Rangitikei and Manawatū Rivers, is at risk, but no incidents have been recorded so far.
Wellington: The Waiwhetu Aquifer hit an all-time low in February 1973, causing saltwater to flow back from the sea. The extraction point has now been moved inland and levels are monitored by a bore with an automatic alarm. The Kāpiti Coast is also considered at risk.
Tasman: In 1990, seawater contaminated a 1.5km by 0.4km on the Motueka Plains, disrupting orchard irrigation. The irrigation wells were moved 2km inland. In 2001, on the Waimea Plains, drought and heavy groundwater pumping caused seawater to move inland, forcing water-use cutbacks of up to 60 per cent. Seawater also seeped into a shallow coastal aquifer in Takaka, Golden Bay, in the late 1990s, due to irrigation pumping.
Marlborough: Seawater has entered the shallow aquifer at Havelock, due to pumping from bores for mussel processing. The coastal edge of the Wairau Plains is considered at risk.
West Coast: No known incidents of seawater intrusion and no areas of special monitoring.
Canterbury: Groundwater pumping caused the Woolston/Heathcote aquifer levels to drop below sea level for prolonged periods, causing seawater seep. Since extraction limits were set in the 1990s, levels have improved. Salty water has also been found in bores at St Andrew's Golf Course, Red Ruth near Timaru and at the Amberley Golf Course.The Eastern Canterbury Plains coastline from Waipara River to Waitaki River is subject to monitoring.
Otago: South Dunedin monitoring wells have shown sea water intrusion around 400m inland.
Southland: Had no monitoring data in 2011.