Infiltration trial could solve Silverstream’s nitrate issues

A three-year infiltration trial being carried out by Environment Canterbury and Waimakariri Irrigation Limited (WIL) aims to reduce Silverstream’s nitrate levels by introducing more water into the stream.

Silverstream currently has nitrate levels above 10 mg/L which exceed the maximum level of 6.9 mg/L set by national policy guidelines for freshwater.

The trial is operating on a WIL shareholder’s farm using a trench connected to an irrigation race which receives water from the Browns Rock intake of the Waimakariri River. The trench, which is located seven kilometres east of Silverstream, is 1.5 metres deep and 150 metres long.

Environment Canterbury senior hydrogeologist Zeb Etheridge says initial results from the first eight weeks of the trial are promising.

“We knew within the first week that it was a really good infiltration location as we’re getting at least 80 litres of water per second into the trench.

“We’ve also recorded a drop from 9-10 mg/L to 4.5 – 5mg/L of nitrate at the piezometer located next to the trench which is really positive.”

The trench is an old braid of the Waimakariri River which helps the water move towards Silverstream through porous gravel. Zeb says adding this clean water to Silverstream could help reduce nitrate issues, while providing a cost-effective solution.

“Water quantity is the key. Silverstream flows at about 400 litres per second in its upper reaches so if we could get 100 litres per second of additional low nitrate water flowing through there then in the best-case scenario we could reduce nitrate levels to 7mg/l, which would put us well on the path to reaching the guidelines.

“Infiltration is one of the lowest-cost and most practical solutions so if it does work then it would be a real win-win from an environmental and a cost point of view.”

WIL environmental manager Paul Reese says WIL shareholders reacted positively to idea right from the beginning because they saw the trial as an opportunity to use their resources and infrastructure to reduce their impact on the environment.

“The landowner has been exceptional. He has given up a significant area of productive land for this project as he and all our shareholders recognise that Silverstream has significant issues and we all need to work together to solve them.

“Everyone is willing to do whatever they can to be part of the solution for this nitrate issue.”

The initial trial has been designed to run outside of the regular irrigation season, but Paul says if water is available after irrigation demand is met, WIL will be happy to provide it for other periods of the year.

“We need to be mindful that when we are putting water into the trench, we are also taking away from the Waimakariri River flows, so we need to take that into consideration.”

The next stage of the trial will measure how much water is moving from the trench through to Silverstream. If the trial is successful, WIL and Environment Canterbury will consider replicating the trial at other sites.