Lower Oreti

The Lower Oreti Groundwater Management Zone (GMZ) covers an area of approximately 22,500 ha in the lower reaches of the Oreti River catchment, downstream of the Hokonui Hills.

Topography: flat to gently undulating alluvial terraces in the lower reaches of the Oreti River catchment, rising onto limestone outcrops at Winton Hill, Centre Bush and Kauana.
Main surface water catchments: Oreti River, Winton Stream, Tomoporakau Creek

Boundaries – The western boundary follows the Oreti River channel south of the Hokonui Hills. The eastern boundary follows the boundary between the Oreti catchment to the west and the Makarewa catchment to the east.

Physical setting

The Lower Oreti GMZ encompasses a late Quaternary (Q2) alluvial terrace on the true left (eastern) bank of the Oreti River downstream of the Hokonui Hills.

Aquifer type: Lowland/Terrace

Geological setting

The subsurface geology of the Lower Oreti GMZ consists of a relatively thin layer of Quaternary gravels (Q1 and Q2) overlying Tertiary deposits of the Forest and Winton Hill Formations north of Forest Hill, and the Gore Lignite Measures to the south (see diagram below).

The thickness of the alluvial deposits appears to decrease towards the south, with 20-30 metres of Quaternary alluvium recorded in the Centre Bush-Winton area, decreasing to less than 10 metres in the Wallacetown area. The gravel deposits generally comprise generally poorly sorted silty sandy gravels.

In the vicinity of Winton Hill and from Centre Hill to Kauana, limestone deposits of the Forest Hill Formation underlie the Quaternary alluvium at relatively shallow depths. The deposits comprise of sandy to massive limestone deposits, which are jointed and fractured in places. Some evidence of karst development is also observed in limestone outcrops at Centre Hill and Kauana.

Elsewhere the Quaternary deposits are underlain by lignite measure sediments of the East Southland Group (Gore Lignite Measures). These sediments comprise a thick sequence of carbonaceous mudstone interspersed with discontinuous layers of land and gravel.

The Tertiary sediments are typically 200 to 300 metres thick and are underlain by greywacke basement rocks of the Murihiku Terrane.

Hydrogeology

Most soils in the Lower Oreti GMZ are deep, fine-grained and poorly drained. This increases the potential for runoff of sloping areas and has resulted in the use of artificial drainage on flatter-lying areas.

The Lower Oreti GMZ hosts a spatially extensive unconfined aquifer system in the thin layer of Quaternary alluvium. Bore yields are generally low but increase along the margins of the Oreti River where the alluvial materials have been reworked.

Depth to groundwater generally ranges from 1 to 4 metres below ground level, becoming shallower closer to the Oreti River. Seasonal groundwater level variations in the Q2 alluvium exhibit the peaked response characteristic of Lowland aquifers, with groundwater levels increasing rapidly to a maximum in early winter then declining gradually through to a minimum in early autumn.

Groundwater levels in the Q1 alluvium indicate interaction between the Oreti River and adjacent unconfined aquifer along the riparian margin to the west and north of Winton. South of Lochiel the bed of the Oreti is sufficiently incised into the Quaternary alluvium and underlying Tertiary sediments to result in the surrounding unconfined aquifer being perched above river level.

Tertiary limestone sediments in the northern and eastern areas of the Lower Oreti GNZ host a poorly defined unconfined to semi-confined groundwater resource. Lignite measure sediments underlying the southern section of the zone contain a limited groundwater resource in localised confined sand and gravel aquifers.

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

Water age

Mean residence time is approximately 9 years

Depth to groundwater

<1 to 4 metres below ground, decreasing toward the Oreti River

Seasonal groundwater variation

<2 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

Recharge to the Lower Oreti GMZ is primarily derived from infiltration of local rainfall. Some interaction may occur with the Oreti River along the riparian margin but is likely to comprise a minor component of the water balance. Throughflow from neighbouring zones is unlikely.

Rainfall recharge: 245 mm per year
Average annual rainfall recharge volume: 55.2 million m3 per year

Diffuse recharge to (and discharge from) deeper Tertiary aquifers is likely to occur at a slow rate from overlying alluvial aquifers.

Discharge

Groundwater discharge occurs to the Oreti River (north of Lochiel) and numerous lower order tributary streams that originate across the Q2 terrace. This discharge may be augmented by extensive artificial drainage in some areas. Some discharge is also likely to occur to the coast via New River Estuary.

Groundwater flow

Groundwater in this zone generally flows from north to south following the general topographic gradient. At a local scale groundwater flow may occur obliquely towards individual surface waterways.

Abstraction and water use

Groundwater is utilised for domestic and farm water supplies across the Lower Oreti 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:

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

Water quality pressures

Natural groundwater quality in the Lower Oreti GMZ is generally good, however, many areas now show moderate to high levels of contamination from land use activities. Groundwater generally contains low concentrations of most dissolved ions. Hardness is typically low but can be elevated in limestone deposits and adjacent alluvial aquifers. Confined lignite measure aquifers typically contain elevated iron concentrations. Elevated iron concentrations may also occur in alluvial aquifers, particularly south of Lochiel where soils are poorly drained and lignite measure sediments occur close to the land surface. Moderate to high nitrate concentrations are observed in some parts of the Lower Oreti GMZ.

Soils

Soils across the Q2 terrace are generally poorly drained, increasing the potential for reduction of nutrient concentrations via denitrification.

Nutrients

The redox state of groundwater in alluvial aquifers is generally oxidizing. This means there is limited potential for denitrification in groundwater once it has passed/infiltrated through the soil zone. Combined with the relatively slow rate of groundwater throughflow this increases the potential for elevated nitrate concentrations to occur where denitrification in the soil zone is limited.

Microbial contamination

Microbial contamination of groundwater is typically limited by natural attenuation in the soil zone and underlying aquifers. The potential for microbial contamination of groundwater in the Lower Oreti GMZ is typically low due to the poorly drained nature of soils and the relatively slow rate of groundwater flow.

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 pathway for contamination to reach groundwater in this zone is via artificial drainage (left). Deep drainage is the main pathway for contaminants in riparian areas (right).

Water quality state summary

Redox state: oxic

Nitrate: variable but some areas of high concentrations

Phosphorus: generally low

Microbial contamination: low, but risk can be elevated close to source

Major ions: hardness generally low but may be elevated where limestone deposits are close to the land surface, elevated iron and manganese concentrations occur in confined aquifers and alluvial aquifers where lignite measure sediments occur at shallow depths.

Water quality - human health

Main issues in this zone

Groundwater quality in this zone may be compromised by elevated nitrate and microbial contamination levels in some locations.
There is potential for elevated nitrate concentrations in areas of intensive land use.
Groundwater quality in this zone may be compromised by elevated iron and manganese concentrations where lignite measure sediments occur at shallow depths.

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.