Local Focus: Vertical flow wetland to solve legacy pollution at Hastings' Lowes Pit

Lake to be filled in with a gravel filter. Made with funding from NZ On Air.

By Patrick O'Sullivan

A small lake taking roadside and industrial stormwater will be filled in, allaying fears it could pollute Hastings' drinking water.

Lowes Pit was a gravel quarry from the 1940s, used to construct roads.

Originally surrounded by farms, the council-owned land was used to drain industrial properties that were too low to easily link into the council stormwater system.

As a result, pollutant-carrying stormwater was sent to the pit, along with stormwater from a section of Omahu Rd.

Hastings District Council Water Service Manager Brett Chapman said the pit was an anachronism, a legacy inconsistent with modern environmental standards.

"I know concerns were raised around the potential for its connectivity with groundwater beneath," he said.

"So we had independent experts Tonkin & Taylor do an assessment, to understand whether there was connectivity, which would mean that contaminants could potentially pass from the stormwater system into Lowes Pit, and then with connectivity, into the groundwater beneath and then flowing into bores.

"In particular the Frimley Bore," he said "which is where we source some of our drinking water supplies. We went through quite detailed analysis to understand what the risk of that was."

"With these sorts of things you can't categorically say there is absolutely no connection, but the investigation said that the risk was very, very low on the basis that there were a number of mitigating factors - particularly distance and low connectivity between itself and the groundwater beneath."

Chapman said the cost of diverting the stormwater into the wider council stormwater network was prohibitive – 15 years ago it was estimated at $5 million.

Councillors recently opted to fill in the pit, which is 10,800sq m and 6.5m deep, with specialised gravels to act as a filter.

"What we are proposing is a vertical-flow wetland," Chapman said.

"What that means is an area is set aside that has filtration material in it. It has properly-structured filtered material and you have plantings on top.

"When it rains stormwater discharges on to that land area and it then filters through the ground. So it is not a wetland as such, there is no permanent body of water.

"It is dry, unless it rains and there is stormwater ponding, which will soak away. That is typical of what happens in the surrounding areas around the Heretaunga Plains."

The vertical wetland will use a lot less space, leaving the council with more than a quarter hectare of new land. Hastings Mayor Sandra Hazlehurst said parkland was being considered.

"We just can't wait to get this water body filled and have a dry space," she said.

"Stormwater has to go somewhere and we want clean water to go into our streams and waterways."

The vertical wetland will cost $3.1 million to develop but just $2 million has been set aside for the project, leaving sale of the land a possible solution to the funding shortfall.

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