Avoiding effluent pond failure

"Such failures can cause pollution of waterways and groundwater, and mean that valuable nutrients are lost to farming systems," said the council's proactive monitoring manager, Ross Wightman.

The education day Putaruru will offer practical advice about how to avoid pond failures, he says.

South Waikato pond failures will be discussed, as will the design considerations to take into account when constructing new effluent storage.

"There are often no warning signs when an effluent pond fails but the risk of failure can be minimised if the pond has been constructed and maintained correctly," says Wightman.

In the South Waikato a pond failure occurred without warning in mid-2012. The pond was constructed in a friable rhyolite soil. Effluent '"tunnelled" through the pond wall and caused a significant discharge of effluent into water and contaminated a nearby water supply.

This pond failure highlights how farmers need to take special care when constructing holding facilities on rhyolite.

"Volcanic rhyolite soil can be found throughout the South Waikato and Taupo as a non-welded, gravelly and sandy pumice soil, or as a dense welded rock. Rhyolite soils are common in the Waikato through the Taupo volcanic zone that stretches from Mount Ruapehu to White Island," says Wightman.

"The risk of pond failure can be minimised during the design and construction phase by ensuring that the pond is constructed of an appropriate material, that it is lined correctly, and that the location is structurally stable.

"Also, when constructing a new effluent pond or assessing an existing one, it is very important to undertake a thorough site investigation to assess the soil profile and slope stability."

The effluent education field day will run from 10.15am-1.30 pm on Wednesday, April 30 at Jack and Elizabeth Scheres' property on 169 Leslie Rd, Putaruru, Fonterra, supply number 77783.

For further information and to register call 0800 800 401.