At a field day on Jack Sheres’ farm outside Putaruru in April, Wightman said pond failures could cause pollution of waterways and groundwater as well as a loss of valuable nutrients to the farming operation. There were often no warning signs, but the risk of failure could be minimised if the pond had been constructed and maintained correctly.
Scheres’ effluent pond leaked without warning in 2012. The original pond was constructed in a friable rhyolite soil and the effluent had leaked through piping in the soil causing a significant discharge of effluent and contaminating a nearby water supply.
“The old oxidation system was thought to be well-sealed, but it disappeared overnight,” Scheres said.
The incident highlighted how farmers needed to take special care when constructing holding facilities on rhyolite, which could be found throughout South Waikato and Taupo, Wightman said.
“The risk of pond failure can be minimised during the design and construction phase by ensuring the pond is constructed of an appropriate material, that it’s lined correctly, and that the location is structurally stable.”
A thorough site investigation to assess the soil profile and slope stability was needed, he said.
The best location for the farm operation might not be the best environmentally or geographically for an effluent pond, Opus principal engineer Rex Corlett said.
Testing the site for compatibility and permeability was a priority. The water table was also a significant factor in deciding on pond location or whether to install a tank.
If the water level was above the liner of a pond there was the possibility it would lift the base of the liner, so drainage was needed under the liner to take water away, as well as around the outside of the pond.
“The old style pond where you dug a hole in the ground, those days are in the past,” he said.
“I think there will come a time when councils will require an engineer to sign off the pond.”
Farmers should use the drainage system to monitor any possible leakage as well as checking regularly around their ponds. Signs of leakage could be crumbling soil, trees growing around the edge, slumping, unstable banks, visible soil horizons and cracking walls.
* Practice Note 21 and Farm Dairy Effluent Systems: Planning the right system for your farm, both available online at www.dairynz.co.nz.
