Participate Palmy

Option 5: Discharge to groundwater via infiltration, with land application in the drier months of the year

Treated wastewater would be discharged by high rate infiltration in specifically designed infiltration basins or trenches located over highly porous soils.

Photo shows illustration of treated wastewater discharged to groundwater, combined with land application in drier months.

Under this option, there’d be a significant upgrade to our treatment plant to achieve wastewater quality appropriate to discharge to groundwater and which meets environmental regulations.

The permeable nature of the basin or trenches and the underlying soil enables the treated wastewater to reasonably quickly drain to the groundwater and not cause surface ponding or flooding.

Groundwater is water held underground in the soil or in crevices in rock. The groundwater that would be accessed by the rapid infiltration approach is relatively shallow and will likely be directly connected to nearby streams and rivers. Therefore, the treated wastewater would likely reach nearby water in a matter of days.

A portion of the treated wastewater would be applied to land in the drier months of the year (October to May). Approximately 1,000 to 1,600 ha of land would be needed for the land application site.

Under this option we may also need to discharge small amounts, infrequently, to the Manawatū River. That’s because when it’s been raining heavily for a number of days the groundwater level may be higher. If the levels are too high then the land cannot absorb any more and that water would runoff elsewhere. That would have the potential to adversely affect the environment.

The construction and operation of this option is well understood.

What we'd need to install or build

  • pipes to move the treated wastewater from the treatment plant to the rapid infiltration and land application site
  • a storage basin to buffer flows
  • irrigation infrastructure
  • a wetland would likely be constructed prior to the rapid infiltration facility
  • significant upgrades to the treatment plant at Tōtara Road

We’d also need to purchase or lease the land application site.

Some landowners would be directly impacted if Council sought to acquire land for the rapid infiltration system and land application site. We also expect concerns about the risk to drinking water supplies.

Cost

  • Lifetime cost: $401-528 million
  • Potential rate increase per year: $450-600
  • Total wastewater charge per ratable unit per year: $750-860