Oreti

The Oreti Groundwater Management Zone (GMZ) covers an area of approximately 3,300 ha in the middle reaches of the Oreti River, between Lumsden and Ram Hill.

Topography: flat alluvial terrace.
Main surface water catchments: Oreti River, Murray Creek, Roe Burn

Boundaries – the western boundary of follows the terrace riser marking the eastern extent of the Castlerock Terrace. The eastern boundary follows the approximate alignment of the Oreti River/Waimea Stream catchment divide.

Physical setting

The Oreti GMZ encompasses the active floodplain of the Oreti River (Q1) and adjacent late-Quaternary (Q2) alluvial terrace between Lumsden and Ram Hill.

Aquifer type: Riparian

Geological setting

Geology of Oreti GMZ consists of a thin sequence of heterogeneous late Quaternary (Q1/Q2) alluvial gravels deposited on the floodplain of the Oreti River. The gravels comprise layers of poorly sorted coarse sand and gravel. The gravel matrix contains variable amounts of silt, and extends to a depth of 10 to 15 metres. These sediments are underlain by layers of older alluvial deposits deposited and reworked by the ancestral Oreti River during successive late-Quaternary glacial and interglacial cycles. The total thickness of the alluvial deposits range from 40 to 60 metres (see diagram below).

The Quaternary alluvium overlies Tertiary sediments of the Gore Lignite Measures, which comprise mudstone interspersed with layers of sand and gravel. The Tertiary sediments are typically 200 to 300 metres thick and are underlain by basement rocks of the Dun Mountain-Maitai Terrane.

The geometry of the basement rocks is complicated by the effects of several faults that cross the Oreti Basin in the vicinity of the Oreti River. Displacement along these faults during the Late Quaternary Period may disrupt the lateral continuity of the overlying alluvial sediments.

Hydrogeology

Most soils in the Oreti GMZ are alluvial, shallow, and well drained, which results in water readily infiltrating from the land surface to underlying groundwater.

The Q1/Q2 alluvial deposits in the Oreti GMZ host a thin, unconfined aquifer system that is hydraulically connected to the Oreti River and tributaries. The aquifer system is generally low to moderate yielding, although permeability is likely to increase in recent alluvium along the margins of the Oreti River. Depth to groundwater is generally less than 2 metres. Seasonal groundwater level variation is typically less than 2 metres.

Deeper alluvial sediments in the Oreti GMZ may also host a significant groundwater resource. The Lumsden Aquifer extends under the north-western corner of the Oreti GMZ. This confined aquifer is hydraulically separated from the unconfined aquifer by a layer of compact claybound gravel that limits vertical groundwater flow. Available information suggests this deeper aquifer is not laterally continuous under the Oreti GMZ and may contribute throughflow to the overlying unconfined aquifer in the vicinity of the Oreti River. Elsewhere in the Oreti GMZ, deeper Q6 to Q8+ alluvial deposits host a low yielding unconfined aquifer.

Tertiary sediments underlying the Quaternary alluvium host low yielding confined aquifers in localised sand and gravel layers of the Gore Lignite Measures.

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

Water age

Mean residence time is approximately 2 years

Depth to groundwater

<1 to 3 metres below ground level, 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 Oreti GMZ is derived from infiltration of local rainfall and riverine recharge from the Oreti River.

Rainfall recharge: 237 mm per year
Average annual rainfall recharge volume: 7.8 million m3 per year

The Oreti GMZ also receives appreciable throughflow from the Castlerock GMZ to the west and may also be recharge via seepage from upward seepage along the down-gradient extent of the Lumsden Aquifer.

Discharge

Appreciable groundwater discharge occurs to the Oreti River between Lumsden and Ram Hill. This discharge is inferred to include direct infiltration into the bed of the Oreti River as well as discharge in spring-fed stream such as Murray Creek and the Roe Burn, which flow across the Q2 terrace.

Groundwater flow

Groundwater flow in the Oreti GMZ occurs toward the Oreti River.

Abstraction and water use

Limited utilization of groundwater for domestic and farm water supplies occurs in the Orepuki 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

Natural groundwater quality in the Lower Oreti GMZ is generally good, however some areas show moderate contamination from land use activities. Groundwater contains low concentrations of most dissolved ions. Hardness is low, iron and manganese concentrations are typically below detection. Nitrate concentrations are generally low to moderate reflecting the high rate of groundwater throughflow, although isolated ‘hotspots’ may be associated with intensive land use.

Soils

Soils across the Q2 terrace are generally well drained, limiting the potential for attenuation of nutrient concentrations via denitrification.

Nutrients

Groundwater in the Oreti GMZ is oxidising. This means there is limited potential for denitrification in groundwater once it has based through the soil zone. However, the potential for accumulation of elevated nitrate concentrations is mitigated to an extent by the high rate of groundwater throughflow.

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. Due to the well-drained nature of soils, there is the potential for microbial contaminants to be transported through the soil zone into groundwater where the water table is shallow.

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 is via deep drainage.

Water quality state summary

Redox state: oxidising

Nitrate: generally low but may be elevated due to intensive land use

Phosphorus: low

Microbial contamination: low, but risk can be elevated close to source, particularly on well drained soils

Major ions: hardness, iron and manganese concentrations are low

Water quality - human health

Main issues in this zone

Nitrate: There is potential for elevated nitrate concentrations in areas of intensive land use where groundwater is oxic.
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.