Waipounamu

Boundaries – extend between the channel of the Mataura River to the south, and a northern boundary that approximates a transition in the geology and hydraulic properties of the Quaternary alluvial deposits.

Physical setting

The Waipounamu GMZ occupies a narrow, elongate section of the lower (Q2) alluvial terrace extending along the riparian margin of the Mataura River, between Ardlussa and the Waikaia River confluence.

Aquifer type: Riparian

Geological setting

The Waipounamu GMZ occupies a narrow, elongate section of the lower (Q2) alluvial terrace on the true left (northern) bank of the Mataura River downstream of Ardlussa. The subsurface geology of this area comprises a thin (generally <15 metres thick) layer of alluvial materials reworked by the Mataura River over the late Quaternary Period. These alluvial materials comprise coarse, poorly sorted sandy gravels that are underlain by older Quaternary alluvium containing a higher percentage of fine material (fine sand and silt) in the gravel matrix (see diagram below).

The Waipounamu GMZ is distinguished from the Wendonside GMZ, which covers the northern half of the Q2 terrace. The shallow alluvial deposits (10 to 15 metres thick) of the Wendonside GMZ comprise claybound alluvium that is underlain by a layer of coarse sandy gravels. This is almost the reverse of the geology occurring the in Waipounamu GMZ.

The Quaternary alluvial deposits are underlain by lignite measure sediments of the East Southland Group. These sediments comprise thick layers of carbonaceous mudstone interspersed with irregular layers of sand, gravel and lignite. The thickness of the alluvial deposits varies across the zone, possibly reflecting paleo-topography developed on the upper surface of the underlying Tertiary sediments. In particular, the gravel deposits thin-out over underlying Tertiary lignite measure sediments toward the Waikaia River confluence.

Basement rocks underlying the Waipounamu GNZ comprise semi-schist of the Caples Terrane and volcaniclastic sediments of the Dun Mountain-Maitai Terrace, which are separated by the Livingstone Fault.

Hydrogeology

The Waipounamu GMZ is overlain by thin, free draining alluvial soils formed along the margins of the Mataura River .

The shallow alluvial deposits of the Waipounamu GMZ host a high yielding alluvial unconfined aquifer system that is hydraulically connected to the Mataura River. The deeper alluvial deposits are typically lower yielding.

Groundwater levels in the Waipounamu GMZ typically range between 2 and 3 metres below ground level. Seasonal groundwater level variation in typically between 1.0 and 1.5 metres following temporal variations in rainfall recharge, but also showing some response to high flow events in the Mataura River.

A limited confined aquifer resource is hosted in sand and gravel lenses of the Tertiary Gore Lignite Measures.

The diagram below depicts a generalised conceptual hydrogeological understanding of the Waipounamu GMZ.

Water age

• Mean residence time is likely to be less than 2 years close to the Mataura River

Depth to groundwater

• 2 to 3 metres below ground level

Seasonal groundwater variation

• 1.0 to 1.5 metres

Recharge and discharge

The movement of water into (recharge) and out of (discharge) the shallow unconfined aquifer resource for this zone is depicted below.

Recharge

Groundwater recharge in the Waipounamu GMZ is derived from infiltration of local rainfall and throughflow of groundwater from the Wendonside GMZ to the north. The unconfined aquifer system is also hydraulically connected to the Mataura River, with flow exchange occurring with the unconfined aquifer depending on relative river and groundwater levels.

• Average annual rainfall recharge: 190 mm per year
• Average annual rainfall recharge volume: 3.3 million m3 per year

Discharge

A majority of groundwater discharge occurs via baseflow to the Mataura River and the lower reaches of the Waikaia River, a majority of which may occur towards the Waikaia River confluence.

Groundwater flow

Groundwater flow in the Waipounamu GMZ generally occurs in a south-easterly direction, sub-parallel to the Mataura River. At a local scale, groundwater flow may occur obliquely toward the Mataura and Waikaia Rivers, depending on relative surface and groundwater levels.

Abstraction and water use

Groundwater is utilised for domestic and farm water supply in the Waipounamu GMZ. Groundwater abstraction also occurs for irrigation.

Groundwater quantity

Historically, Southland has had an abundance of water, with modest limits on use being appropriate. There has been increasing demand for the use of water for a variety of activities. Environment Southland has a framework for managing groundwater abstraction in Southland.

Potential effects of abstraction

There are a range of environmental effects that could result from the abstraction of groundwater in this management zone. Examples of potential effects are highlighted below: