Aquifer recharge project extended
Mid Canterbury farmers are no longer just relying on rain to recharge the aquifer, writes Heather Chalmers.
AMid Canterbury project aimed at giving Mother Nature a helping hand to replenish groundwater reserves is to be extended this year after positive early results.
In addition to the initial pilot trial site at Lagmhor near Ashburton, where water has seeped from a purpose-built leaky pond since mid-2016, another 16 smaller infiltration test sites are to be developed this year. Further expansion is likely as more scientific information is collected.
Known as the Hinds managed aquifer recharge (MAR) project, it aims to enhance the recharge which occurs from rainfall, rivers, unlined water races and canals and irrigation.
Encouraging early results show that in Hinds MAR’s first year, from June 2016 to July 2017, groundwater volumes increased and nitrate-nitrogen concentrations reduced.
While aquifers beneath the Canterbury Plains provide what once seemed an unlimited source of clean water for domestic supplies and agricultural use, the system is now under serious pressure.
A combination of the move to more efficient irrigation systems, the decommissioning of leaky stockwater races, an increase in annual groundwater pumping and changing weather patterns leading to extended droughts have all contributed to declining aquifer levels and drying coastal waterways. Groundwater health has also degraded, with areas recording higher nitrate concentrations.
To rebalance this, water is being deliberately fed back into the aquifer to help the natural recharge process and improve groundwater quality.
Hinds MAR technical lead Bob Bower says earlier use of borderdyke (flood) irrigation, meant the community had a history of accidental recharge, as surplus water seeped into the ground and flowed to the lower plains.
‘‘MAR is now replacing this, but doing it purposely and scientifically. However, considerably more sites need to go in to redress the balance.’’
Of the 58,000 hectares irrigated by MHV (Mayfield Hinds Valetta) Water, the community scheme between the Rangitata River and the south branch of the Ashburton River, less than 5 per cent is still watered by borderdyke, with the remainder converted to more efficient spray irrigation. In addition to 320km of open race, it now also distributes water via 100km of pipe.
MHV Water chief executive Mel Brooks says that MAR is a community-led project and a way of dealing with historic issues. ‘‘Good management practices for irrigators have changed from even five to 10 years ago. We need to be part of the solution.’’
The 16 new sites spread across the Hinds plains will be smaller infiltration basins, each consented to take up to 50 litres a second. These will be developed and tested during the next six months. Each will be a 6.5-metre clamshell-shaped hole, backfilled with clean gravel, with a bund around the outside ensuring only clean water is absorbed.
‘‘It provides an additional tool to achieve the water quality outcomes we are seeking. This is in addition to what farmers are doing on-farm to irrigate more efficiently and reduce nitrate leaching.
‘‘We have multiple generational shareholders in the scheme, so environmental stewardship is very important to us,’’ says Brooks.
Results from the first year of testing at the Lagmhor MAR trial site show that recharge infiltration rates were about half a metre per day (5000 cubic metres per day per hectare of ponded area) at a peak rate of 113 litres per second. The total volume in year one was more than 2.4 million m3 of water over 260 days.
Increases in groundwater levels are estimated to have reached a distance of about 7km from the pilot trial site. Measured average nitratenitrogen concentrations in groundwater influenced by MAR reduced from an average 14 grams per cubic metre (g/m3) to less than 4g/m3, with the catchment goal being an average 6.9g/m3.
In one example, one of the monitoring bores showed groundwater rising about 18 metres and nitrate-nitrogen concentrations reducing from about 7g to 2g/m3.
Hinds Hekeao MAR Governance Group chairman Peter Lowe says that while farmers are behind the scheme, the wider community is also realising its benefits.
Lowe, a sheep, deer and arable farmer near Hinds, who relies on mainly groundwater and some surface water for irrigation, says the project is exciting. ‘‘The results are very positive, but it’s got to be combined with on-farm changes needed to reduce nutrient loss. MAR alone is not going to solve the issues.
‘‘MAR is not about increasing farming intensification, but addressing problems and to restore and enhance the environment.’’ As the Hinds Plains are in an Environment Canterbury red zone, groundwater and nutrient allocations are capped.
Information from the new trial sites will be fed into a business plan, expected to be complete midyear. As well as expanding the smaller clamshell sites, other options were a 20m dry bore to feed water into a deep aquifer and a recharge site in the Hinds riverbed.
The governance group includes representatives from iwi, Fish and Game, local landowners and three Mid Canterbury professional and business leaders.
Bower says that while it has already lifted aquifer volumes and water health, it will take longer before springs and seeps return along drains and with it fish and insect life. ‘‘But we anticipate that clean water will move out from the aquifer to those springs.
‘‘Aquifer recharge does a really good job of slowly putting water into an aquifer. It’s low-tech and it’s cheap, but unlike a storage dam which can grab great chunks of storm flow water and store it, it’s sort of a more steady background thing.
‘‘Politically, Green lobbyists argue that it’s a big dilution project when it is much more complicated than that. It has a lot to do with how much water is going into an aquifer versus how much is coming out.’’
‘‘It is being more astute about helping Mother Nature get clean water into the groundwater rather than hoping it rains.’’
The results are very positive, but it's got to be combined with onfarm changes needed to reduce nutrient loss ...
Peter Lowe