EXECUTIVE SUMMARY

Maitai Reservoir summer minimum flow and temperature consent conditions require that the drinking water abstracted from the Maitai South Branch (upstream of the Maitai Reservoir), is replaced with cold water drawn from deep reservoir intakes. During summer this results in the periodic discharge of water from the Maitai Reservoir's deep anoxic layer (hypolimnion).

The discharge of anoxic reservoir water during summer low flows may be the cause of declining biological stream heath indicators observed during routine biomonitoring assessments below the Maitai Reservoir.

The resource consents (Resource Consent No. 960396 and RM025151/2) held for the operation of the Maitai Reservoir expire in 2017. In preparation for re–consenting the scheme, this report provides a high–level feasibility assessment of potential options for improving the water quality in the Maitai Reservoir and receiving river environment.

The potential options are:

Option 1: Mechanical aeration of the reservoir hypolimnetic zone.

Option 2: Source a greater proportion of drinking water from the reservoir.

Option 3: Modify the temperature consent conditions to permit the discharge of(warm) oxygenated water from above the hypolimnion.

Option 4: Modify the minimum flow consent conditions to reduce the amount of water discharged from the reservoir.

Option 5: Discharge hypolimnetic reservoir water during floods.

Aerating the reservoir (Option 1), is the preferred option. This should improve water quality in the reservoir and river. A combination of Option 2 and 3 could be pursued if Option 1 is considered too costly. We do not recommend Option 4. Further investigation is required to assess the feasibility of Option 5.