The Five Rivers Groundwater Management Zone (GMZ) covers an area of approximately 13,800 ha. It extends across the central and northern areas of the Oreti Basin to the foothills of the surrounding Eyre Mountains and Mataura Range.
Topography: broad, flat to undulating alluvial terrace
Main surface water catchments: Oreti River, Irthing Stream, Cromel Stream, Acton Stream, Oswald Stream
Boundaries – the eastern and western boundaries follow the contact between Quaternary alluvium and basement rocks of the Eyre Mountains and Mataura Range. The southern boundary follows the northern boundary of the Castlerock Terrace, immediately north of the Mossburn-Lumsden Highway (SH94).
The Five Rivers GMZ is an extensive, flat-lying to undulating, late Quaternary alluvial terrace that has been locally reworked along a series of tributary streams draining into the middle reaches of the Oreti River. The alluvial deposits grade into extensive alluvial fans along the foothills of the Eyre Mountains to the west and the Mataura Range to the east.
Aquifer type: Riparian
The geology of the Five Rivers GMZ consists of a thin layer of Quaternary predominantly (Q2) alluvium deposited by the Oreti River and tributaries draining the Eyre Mountains and Mataura Range. These Q2 alluvial deposits overlie layers of older Q4 to Q8+ alluvium and have been locally reworked along several major tributaries that cross the Five Rivers GMZ.
The alluvial deposits range in thickness from 20 metres around the margins to the basin to upwards of 60 metres closer to the current alignment of the Oreti River. The alluvial deposits may also increase where basement rocks have been deformed along folds and faults that cross the area.
Quaternary gravels overlie a thin sequence of Tertiary Gore Lignite Measure sediments, comprising layers of mudstone interspersed with isolated deposits of sand and gravel.
Underlying basement rock consists of semi-schist of the Caples Terrane to the north and Dun Mountain-Maitai Terrane sandstone and volcaniclastic sediments to the south (see diagram below).
Soils overlying the Five Rivers GMZ are generally well drained, enabling water to readily infiltrate from the land surface into underlying groundwater. Areas of imperfectly to poorly drained soils occur around the margins of the Oreti Basin.
The Five Rivers GMZ contains an extensive shallow unconfined aquifer system. This aquifer exhibits low to moderate yields, which increase in recent Q1 alluvium along the Oreti River and major tributaries.
Groundwater generally occurs within 2 to 3 metres of the ground surface across much of the Five Rivers GMZ. Seasonal groundwater level variation is typically of the order of 1 to 2 metres. Groundwater levels in many areas respond to periods of high river flow as well as seasonal variations in land surface recharge.
The Lumsden Aquifer is a spatially extensive confined aquifer that occurs in deeper alluvial sediments across central parts of the Oreti Basin. This aquifer system underlies the southern portion of the Five Rivers GMZ and is separated from the shallow unconfined aquifer by a layer of compact, low permeability silty gravel. The Lumsden Aquifer is a significant groundwater resource and is managed separately to the unconfined aquifer.
Tertiary sediments and basement rocks underlying the alluvial deposits likely contain a limited groundwater resource.
The diagram below depicts a generalised conceptual hydrogeological understanding of the Five Rivers GMZ.
- Mean residence time is about 1 year
Depth to groundwater
- Generally 2 to 3 metres below ground level across much of the zone
Seasonal groundwater variation
- 1 to 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 to the Five Rivers GMZ is derived from infiltration of local rainfall and runoff from the surrounding hills.
- Rainfall recharge: 278 mm per year
- Average annual rainfall recharge volume: 48.7 million m3 per year
The Oreti River and its tributaries exhibit a high degree of hydraulic connection to shallow groundwater in adjacent Q1 alluvial deposits. Many of these waterways lose water as they flow from headwaters on surrounding hills onto the alluvial plain, then gain flow from groundwater discharge in their lower reaches.
Many waterways, such as the Acton and Cromel streams, gain flow from groundwater discharge upstream of their confluence with the Irthing Stream.
Numerous spring-fed streams also occur on the Oreti River floodplain, particularly in the vicinity of the Irthing Stream confluence.
Groundwater in this zone generally flows in a south-easterly direction toward the confluence of the Irthing Stream and the Oreti River. At a finer scale, groundwater flow is likely to coincide with the surface drainage patterns of tributary streams.
Abstraction and water use
Groundwater is utilised for domestic and farm water supplies across the Five Rivers GMZ. However, bore yields from the shallow unconfined aquifer are generally modest.
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:
Water quality pressures
Natural groundwater quality in the Five Rivers 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 dissolved ions. Hardness is typically low to moderate and concentrations of iron and manganese are generally low. Very high nitrate concentrations are observed in some parts of the Five Rivers GMZ.
Soils in the Five Rivers zone are typically well drained, with elevated potential for bypass flow. This reduces the potential for natural attenuation of contaminants contained in the land surface recharge that infiltrates to groundwater.
Greater attenuation of contaminants may occur in imperfectly drained soils around the eastern and western margins of the Five Rivers GMZ, although these soils have a greater potential for contaminant loss via overland flow.
The oxidising conditions present in soils and groundwater in the Five Rivers GMZ increase the potential for elevated groundwater nitrate concentrations to occur as a result of intensive land use, particularly in areas where there is limited recharge from low nutrient surface water.
Some areas of the Five Rivers GMZ exhibit moderate to very high nitrate concentrations.
Microbial contamination of groundwater is typically limited by natural attenuation in the soil zone and underlying aquifers. Microbial contamination may occur in shallow groundwater where soils have an elevated potential for bypass flow and 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 pathways for contamination to reach groundwater in this zone are via deep drainage (left) and artificial drainage (right).
Water quality state summary
Redox state: oxidising
Nitrate: moderate to very high
Microbial contamination: low, but risk can be elevated close to source
Major ions: low to moderate hardness; iron and manganese generally low
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.
- Elevated nitrate concentrations are associated with intensive land use.
- Contaminants infiltrating to groundwater along the riparian margins of waterways can be rapidly exported to surface water.
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.