Te Waewae

The Te Waewae Groundwater Management Zone (GMZ) covers an area of approximately 12,000 ha in the lower reaches of the Waiau River catchment, downstream of Clifden.

Topography: series of flat-lying elevated alluvial terraces bordering the Waiau River
Main surface water catchments: Waiau River, Orauea River, Alton Burn, Boundary Creek, Kowhai Creek

Boundaries – generally follow the contact between glacial or alluvial sediments and bedrock of the surrounding hills. The northern boundary follows the exposure of Lower Clifden subgroup limestone in the Waiau River bed at Clifden.

Physical setting

The Te Waewae GMZ encompasses a series of broad flat-lying alluvial terraces that flank the lower reaches of the Waiau River.

Aquifer type: Terrace

Geological setting

The Waiau River valley follows a depression formed by faulting in the underlying bedrock (see diagram below).

The geology of Te Waewae GMZ consists of a series of elevated alluvial terraces that flank the Waiau River. The terrace age and elevation above river level decrease in stepwise fashion towards the active river channel of the Waiau River. For example, the relatively broad river flats around Clifden and Papatotara contain a sequence of up to six distinct alluvial terraces (ranging from Q1 to Q8) that step down towards the current river channel. The alluvial deposits generally increase in thickness to the north, from around 10 metres near Tuatapere to as much as 25 metres nearer Clifden.

The Quaternary alluvial deposits are underlain by Tertiary sediments of the Waiau Group that comprise mudstone, siltstone, conglomerate (and limestone in places). Sandstone, conglomerate and carbonaceous mudstone of the Beaumont Formation underlie the Quaternary alluvium near the Longwood range, while siltstone, sandstone and shellbeds of the Te Waewae formation form the low hills to the west.

Basement rock consists of granite and diorite of the Hollyburn Intrusives.

Hydrogeology

Soil types in the Te Waewae GMZ range from shallow, stony well drained soils along the margins of the Waiau River to moderately well to imperfectly drained soils on higher terraces along the valley margins.

The Quaternary alluvial deposits infilling the Lower Waiau valley typically host a thin, low yielding unconfined aquifer system. In the Tuatapere area this aquifer system is perched above the Waiau River, which is incised into the underlying Waiau Group sediments.

Groundwater and surface water resources are closely linked across the Te Waewae GMZ. Groundwater under the higher terraces feed spring-fed streams that originate along the base of terrace risers. These streams in turn lose flow, recharging underlying aquifers across intermediate terrace surfaces and ultimately discharge into springs and wetland areas adjacent to the river.

Groundwater levels are typically shallow (< 2 metres below ground level) under the lower (Q1) increasing to as much as 10 metres under elevated terraces along the valley margins. Seasonal groundwater levels may vary by up to 2 to 3 metres, with greater variability observed under higher alluvial terraces.

Tertiary siltstone and mudstone sediments of the Waiau Group and Beaumont Formation are generally not water-yielding. However, a karst aquifer system may occur in limestone deposits in the Clifden area.

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

Water age

Mean residence time for shallow groundwater ranges from 1 to 5 years

Depth to groundwater

1 to 2 metres below ground level under lower alluvial (Q1) terraces
increasing up to 10 metres under higher alluvial terraces

Seasonal groundwater variation

2 to 3 metres, decreasing on lower (Q1) terraces

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 this zone comes from the infiltration of local rainfall and infiltration of runoff from the surrounding hills.

Average annual rainfall recharge: 455 mm per year
Average annual rainfall recharge volume: 54 million m3 per year

Discharge

Discharge predominantly occurs to springs, spring-fed streams and wetlands along the base of terrace risers. Kowhai Creek is an example of a spring-fed stream that receives baseflow from groundwater underlying the Q4 alluvial terrace in the Papatotara area. The Waiau River also receives baseflow from the adjacent riparian aquifer in the north of the zone.

Some discharge is also inferred to occur to the ocean at Te Waewae Bay.

Groundwater flow

Groundwater flow in the Te Waewae GMZ occurs obliquely toward the Waiau River following the local topographic gradient.

Abstraction and water use

Groundwater is extensively utilised for domestic and farm water supplies across the Te Waewae 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:

More information about these effects is available in our guide to groundwater ecosystem health monitoring.

Water quality pressures

Groundwater quality in the Te Waewae GMZ is variable. 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. Iron concentrations are often elevated in groundwater underlying elevated terraces and where the saturated thickness of the unconfined aquifer is limited. Nitrate concentrations are generally elevated on terraces away from the Waiau River, especially where groundwater conditions are oxidising and land use intensity is high.

Soils

Soils across the Te Waewae GMZ are imperfectly to well drained, limiting the potential for attenuation of nutrient concentrations via denitrification.

Nutrients

Shallow groundwater in the Te Waewae GMZ exhibits a range of redox states. Elevated nitrate concentrations are common in areas where conditions are oxidising.

Phosphorus is typically strongly bound to soils.

Microbial contamination

Microbial contamination of groundwater is typically limited by natural attenuation in the soil zone and underlying aquifers. In areas of the Te Waewae GMZ where soils are well drained and the water table is shallow there may have an elevated potential for localised microbial contamination.

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 is via deep drainage (left). Areas of artificial drainage may occur in parts of the zone (right).

Water quality state summary

Redox state: variable

Nitrate: moderate

Phosphorus: 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 may be elevated under older alluvial terraces

Water quality - human health

Main issues in this zone

Nitrate: Groundwater quality in this zone may be compromised by elevated nitrate and microbial contamination levels in some locations.
Microbial contamination: Localised microbial contamination may occur where the water table is shallow.
Iron and manganese: Iron concentrations may be elevated under older alluvial terraces.

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.