Managed aquifer recharge gives hope to Mid Canterbury's declining water quality

The MAR project at HInds: a "very leaky pond" that's opened up the aquifer and raised percolation rates.
Golder Associates

The MAR project at HInds: a "very leaky pond" that's opened up the aquifer and raised percolation rates.

 Mid Canterbury's low aquifer levels are being topped up by managing river flows to improve water quality, writes Pat Deavoll.

A ground breaking initiative to improve declining water quality in New Zealand rivers was launched in Mid Canterbury last month.

The Hinds/Hekeao managed aquifer recharge (MAR) project takes water from the Rangitata River and percolates it back into the aquifer. The water comes from the Ashburton District Council's unused stock water allocation via the Rangitata Diversion Race (RDR) and Valetta Irrigation Scheme. It is hoped the project will solve the poor quality water issues in the Hinds region, as well as improve stream flows.

Irrigation NZ chief executive Andrew Curtis says the project is an example of where irrigation schemes can use stored or ...

Irrigation NZ chief executive Andrew Curtis says the project is an example of where irrigation schemes can use stored or seasonal surplus water to improve the environment when drier months roll around.

Irrigation, leaky stock water races, increased groundwater pumping and drought have contributed to declining aquifer levels around Hinds, which also has some of the highest concentrations of nitrate pollution in New Zealand.

"Water resources in the region are showing signs of stress," Environment Canterbury (ECan) commissioner Peter Skelton says. "Nitrogen levels are high and increasing and water availability is decreasing."

ECan monitoring shows the waterways in the area have been getting more contaminated since 2004. Data from 2012 shows they were carrying 10mg of nitrogen per litre of water, a level toxic to aquatic life. Nitrate levels in groundwater have also been rising and, at almost 12mg per litre, are higher than World Health Organisation drinking water guidelines.

Mid Canterbury's productive dairy cows have led to excessive nutrient discharge; the cows defecation, and in particular urination, leading to high nitrate levels polluting the waterways, according to agri-ecology consultant Alison Dewes.

She says 30 per cent of the Hinds area's shallow wells have experienced an increase in nitrogen and pathogen levels after 10-15 years of irrigation and dairying on shallow, light soils.

"Communities in the Selwyn and Hinds areas have some of the highest rates of ecoli diseases in the world, and the highest rate of campylobacter, cryptosporidia and giardia. We have the highest rates of zoonoses (disease spread from animals to humans) in the world in some of the irrigated/dairy catchments such as these."

ECan deputy chairman of commissioners David Caygill says the MAR project is taking place in an area where groundwater nitrate concentrations are well above the national bottom-line.

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"The project will use clean Rangitata River water to soak into the aquifer in an area of high nitrate concentrations, diluting the nitrate, providing better reliability for groundwater takes, as well as allowing natural ecosystems to regenerate."

Caygill says the MAR project is a good example of where irrigation schemes can use stored or seasonal surplus water to improve environmental outcomes when the drier months roll around. The project will test whether managed aquifer recharge has a future role in the management of increasing irrigation in Canterbury, alongside efficient water distribution, and water and nutrient use.

The project  is currently commissioned for up to five years of pilot operations under consents granted by ECan and Ashburton District Council. For the first phase, up to 500 litres a second of un-utilised stock water from the Rangitata River will be diverted  for the project. 

The site is comprised of two infiltration basins. The first is designed to retain any natural sediments to manage clogging. The water then flows into the second (main) infiltration basin where the water percolates into the underlying shallow aquifer. Monitoring bores track both the increase in levels and the clean water 'plume' as it moves out across the aquifer.

"We have built a very leaky pond,"  says hydrologist Bob Bower of Golder Associates, the company that built the project. "We stripped the top soil to get rid of the pesticide history. Then we dug trenches and filled them with clean gravel. We've opened up the aquifer and raised percolation rates." 

"The project should reveal a 'sweet spot' where there is enough water to feed springs, streams and wells but not enough to cause flooding of low-lying areas."

Irrigation NZ chief executive Andrew Curtis says the recharge project in combination with better farm environmental performance will allow waterways in the zone to regenerate and thrive.

 "Managed aquifer recharge is used very successfully in the United States and Europe to replenish aquifers for domestic and agriculture water supply, as well as for ecological purposes.

 "Recently there have been calls by politicians for a national approach to managing water quality in New Zealand. The reality is there are many variables and complex issues. We need to be creative, innovative and work collaboratively to ensure the solution meets the needs established by the community.

National MP for Rangitata Jo Goodhew, says she welcomes the project which will test the feasibility of recharging groundwater sources. She says irrigation has "enormous growth potential" and it is important farmers have reliable sources of water.

"This project is part of unlocking that economic growth potential for our region."

However, opposition parties say the project is an indictment of the Government's water policies.

Labour Party water and environment spokesman David Parker says the project would not have been necessary if nitrate levels had not been allowed to rise.

"It's tragic that we've got to this point," Parker says, adding nitrate levels in the area were a serious problem. He believes government policy set freshwater standards too low.

"Effectively the National Party is legitimising previously illegal levels of pollution."

Parker believes the Government needs to set "appropriate limits on fertiliser application and stock levels".

A 2013 ECan report on the issue suggests the extent of the pilot's success or failure could affect expectations of new irrigation in future, and require more on-farm efforts to reduce nitrate leaching. The plan change rules apply to the Hinds/ Hekeao area to address the over-allocation of water and meeting environmental bottom lines, Parker says.

Catchment-level rules for the Hinds area stipulate that by 2017 farmers who are leaching more than 20 kilograms of nitrogen per hectare will need to have a resource consent and a farm environment plan.

Green Party spokeswoman Eugenie Sage says although she believes the MAR project might provide useful information, it "should not be part of the further large-scale manipulation of rivers and aquifers to enable more irrigation and intensive land use."

Sage says ECan should force the Ashburton District Council to surrender any unused stockwater take so the water could flow down the Rangitata River instead, and the Government should spend more on programmes to encourage "sustainable farming".

The MAR  project will collect a range of scientific information, including groundwater and flow levels, rainfall, temperature and water quality parameters. It will be influenced by factors such as water availability, storm events and flows in the coastal drains, the effect of clogging and the ability of the water to percolate into the aquifer.

Things won't always run smoothly. The Commonwealth Scientific and Industrial Research Organisation (CSIRO)  of Australia claims MAR  schemes can disturb ecosystems dependent on water table levels. They invariably experience clogging of some type during their operational life. To avoid clogging of the infiltration basins or trenches, the water needs to be of an adequate quality. Typically clogging can be caused by minerals on and in the soil, gases trapped in the soil and the accumulation of suspended algae and sediment.To recognise the potential for clogging and remedy it requires specialist knowledge and skills. What works in one location may not always work in another. 

However, it is clear the New Zealand public has an appetite for improved water quality for our lakes and rivers," Curtis says.

"Success requires local issues to be owned by local people who then implement local solutions. This scheme is a great example of the local community coming together to make positive change happen."


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