This zone comprises three water resource units in the southwestern part of the region that are neighbours but hydrologically discrete from one another.
The first area comprises the collective catchment of streams that drain the hills around Wellington city into the harbour as well as the open sea. There are no significant groundwater resources associated with these streams and demand for water is low. Consents for abstractions are limited to minor takes for golf courses and short term abstractions for large earth works projects (e.g. windfarm) and allocation is managed on a case by case basis. The greatest pressures on streams in this zone relate to runoff from the urban and low intensity farming areas as well as the heavy modification of stream channels in the city suburbs.
On the opposite side of Wellington Harbour are the Wainuiomata and Orongorongo river catchments. These neighbouring catchments drain the Rimutaka and Orongorongo ranges, respectively in a north-south direction and discharge to the Cook Strait. Both are subject to significant river abstractions in their headwaters for municipal supply. The Wainuiomata River also has a modest groundwater resource in its lower reaches associated with alluvial gravels and sands in the valley floor. The Orongorongo River catchment lies entirely within Department of Conservation (DoC) estate and is therefore not subject to any further abstraction pressure beyond the municipal take.
The most prominent water resource in this zone is the Hutt River and its associated aquifer system underlying the Hutt Valley. The Hutt River is the third largest in the region (after the Ruamāhanga and Ōtaki rivers) and is subject to the largest run-of-river individual abstraction. Once the Hutt River emerges onto the valley floor in Upper Hutt it begins to interact with groundwater. The Hutt aquifer comprises shallow alluvial gravels in direct connection with the river as well as deeper semi-confined water bearing layers. The most productive deep aquifer unit is the Waiwhetu Aquifer (30-100 metres) and this unit is tapped by a bore field in Lower Hutt to supplement the municipal supply. The Waiwhetu Aquifer is recharged primarily by the Hutt River and heavy, persistent pumping of the groundwater can induce significant river losses. A further risk with groundwater abstraction is the onset of saltwater intrusion.
Management of water resources in the Hutt Valley is focussed on treating surface and groundwater as a single, connected system. While the zone is split into two groundwater sub-zones (Upper and Lower Hutt) to help manage allocation limits, the linkage between these zones and the Hutt River is explicitly considered at the whole catchment scale.
{{item.displayText}}
