Te Anau

Boundaries – generally follow the contact between glacial or alluvial sediments and bedrock of the surrounding hills.

Physical setting

The Te Anau GMZ encompasses the extensive flat to rolling glacial moraine ridges and intervening alluvial terraces that occur across the wider Te Anau Basin.

Aquifer type: Terrace

Geological setting

The subsurface geology of the of Te Anau GMZ is dominated by glacial moraine ridges and terraces of associated fluvioglacial outwash materials, deposited during successive glacial and interglacial cycles during the late Quaternary Period.

The Quaternary deposits are generally comprised of layers of poorly sorted gravels in a tight, weathered silt and clay matrix, interspersed by lenses of more well sorted gravels and sand. Large alluvial terraces along the major river systems comprise deposits of coarser reworked alluvium. The Quaternary moraine and alluvial deposits range in thickness from around 50 metres under lower alluvial terraces to upwards of 100 metres under large moraine ridges (see diagram below).

The Quaternary moraine and outwash deposits are underlain by a thick Eocene to Pliocene sedimentary sequence, which is in turn underlain by Cretaceous to Eocene coal measures with interbedded oil shales and carbonaceous mudstones. In central areas of the Te Anau Basin these deposits can be up to 2 to 3 kilometres thick.

Hydrogeology

The majority of soil types in the Te Anau GMZ are alluvial, shallow, and well drained. Water is able to readily infiltrate from the land surface into underlying sediments.

The glacial moraine and alluvial outwash materials form a thick stratified unconfined aquifer system. Gravel materials comprising large moraine terraces are generally low to very low yielding, although in places isolated layers of higher permeability gravel and sand may be encountered, reflecting local reworking of the glacial materials by nearby rivers and streams. These deeper water-bearing layers are typically semi-confined.

Bore yields increase under lower terraces comprising Q1 alluvium along the riparian margins of the major rivers (Mararoa, Whitestone and Upukerora rivers), where the gravel materials have been reworked.

Localised areas with water tables perched on low permeability silt layers (often comprising buried loess layers) occur under higher moraine terraces. Where these features intersect the land surface, they form springs and seeps on terrace risers.

Depth to groundwater ranges from 2 to 4 metres below ground level under lower (Q1) alluvial terraces, increasing up to 40 metres below ground under higher moraine ridges. Seasonal groundwater level variation is generally between 1 to 2 metres, decreasing in alluvial aquifer along the riparian margin of the major rivers.

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

Water age

• Mean residence time for shallow groundwater ranges from 3 to 4 years

Depth to groundwater

• 1 to 3 metres below ground level under lower alluvial (Q1) terraces, increasing up to 40 metres under higher moraine ridges.

Seasonal groundwater variation

• <1 metre

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

The majority of recharge in the Te Anau GMZ is derived from infiltration of local rainfall and infiltration of slope runoff. Some recharge is sourced from riverine recharge in riparian aquifers along the margins of the major rivers.

• Average annual rainfall recharge: 430 mm per year
• Average annual rainfall recharge volume: 340 million m3 per year

The Riversdale GMZ also receives throughflow from the Longridge GMZ to the west.

Discharge

Springs are a common feature in many parts of the Te Anau GMZ. A significant number of these features occur at the base of large moraine or alluvial terraces where the groundwater table intersects the land surface. The largest spring fed streams are Home Creek, which drains the Kepler Swamp, and the Kakapo swamp outlet, which drains across the alluvial terrace adjacent to the Whitestone River. Springs are also commonly observed on terrace faces where perched groundwater moves laterally along lower permeability layers within the moraine deposits.

Groundwater discharge is also inferred to occur to Lake Te Anau and Lake Manapōuri.

Groundwater flow

Groundwater flow in this zone follows the local topographic gradient draining to both the larger river systems and Lake Te Anau and Lake Manapōuri.

Abstraction and water use

Groundwater is extensively utilised for domestic and farm water supplies across the Te Anau GMZ.

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:

The main pathways for contamination to reach groundwater in areas of this zone that are not peat or hill country are via deep drainage (left) and artificial drainage (right).