The Te Anau Groundwater Management Zone (GMZ) covers an area of approximately 79,000 ha, and encompasses the Te Anau Basin and the Waiau River valley upstream of Sunnyside.
Topography: extensive flat to rolling glacial moraine and outwash terraces
Main surface water catchments: Waiau River, Mararoa River, Upukerora River, Whitestone River, Home Creek, Princhester Creek
Boundaries – generally follow the contact between glacial or alluvial sediments and bedrock of the surrounding hills.
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
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.
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.
- 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.
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.
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 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.
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:
Water quality pressures
Natural groundwater quality in the Te Anau GMZ is generally good. Groundwater generally contains low concentrations of dissolved ions. Hardness is typically low but may become elevated in the vicinity of limestone sediments of the Waiau Group around the margins of the Te Anau Basin. Iron concentrations are typically below detection but in places may be elevated in deeper water-bearing layers within moraine deposits and in shallow groundwater near wetland areas. Nitrate concentrations are generally low but will likely be elevated in shallow groundwater associated with intensive land use.
Soils across the Te Anau GMZ are well drained, limiting the potential for attenuation of nutrient concentrations via denitrification. However, particularly on higher terraces, loess deposits and lower permeability layers within the moraine deposits increase the potential for lateral flow through the soil zone.
Shallow groundwater in the Te Anau GMZ is typically oxidising, potentially becoming more reduced with depth. This means there is limited potential for denitrification in groundwater once it has passed/infiltrated through the soil zone. However, the potential for accumulation of elevated nitrate concentrations is generally offset by the high volume of land surface recharge (relative to other areas of Southland) and the relatively low intensity land use.
Phosphorus is typically strongly bound to soils.
Microbial contamination of groundwater is typically limited by natural attenuation in the soil zone and underlying aquifers. In many areas of the Te Anau GMZ the potential for microbial contaminants to be transported through the soil zone into groundwater is limited by the deep water table and low permeability of near-surface loess layers.
The potential for microbial contamination of groundwater supplies can be reduced by locating wells and bores away from local sources of pollution and ensuring good wellhead protection.
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).
The main pathways for contamination to reach groundwater in areas of peat are via deep drainage (top left) and artificial drainage (top right).
The main pathway for contamination to reach groundwater in hill country areas is via overland flow and subsequent deep drainage (below)
Water quality state summary
Redox state: oxidising
Nitrate: generally low
Microbial contamination: low, but risk can be elevated close to source, particularly on well drained soils where the water table is shallow
Major ions: hardness is generally low, iron concentrations are generally low but can be elevated in deeper moraine deposits and around wetland areas
Water quality - human health
Main issues in this zone
- Nitrate: Elevated nitrate concentrations are associated with intensive land use.
- Microbial contamination: Groundwater quality in this zone may be compromised by elevated nitrate and microbial contamination levels in some locations.
Disclaimer: This Information Sheet describes the typical average properties of the specified groundwater zone. It is essentially a summary of information obtained from drilling records, consent applications and investigation surveys. It has been prepared in good faith by trained staff within time and budgetary limits. However, no responsibility or liability can be taken for the accuracy of the information and interpretations. Advice should be sought from Environment Southland, drilling companies or other experts before making decisions on individual sites. The characteristics of the groundwater at a specific location may differ in some details from those described here.