Lower Mataura

The Lower Mataura Groundwater Management Zone (GMZ) covers an area of approximately 35,000 ha in the lower reaches of the Mataura River catchment, downstream of Gore.

Topography: Flat to rolling alluvial terraces in the lower reaches of the Mataura River catchment.
Main surface water catchments: Mataura River, Charleton Stream, Waimumu Stream, Mimihau Stream, Mokoreta River, Oteramika Stream, Titiroa Stream

Boundaries – the western boundary follows the terrace marking the lateral extent of the Mataura River valley. The eastern boundary follows the approximate contact between Quaternary alluvial sediments and basement rocks of the Catlins Hills.

Physical setting

The Lower Mataura GMZ comprises of a series of alluvial terraces of various ages that formed during entrenchment of the Mataura River during the mid to late Quaternary period. The Lower Mataura GMZ excludes the Edendale Terrace, which is managed as a separate groundwater resource.

Aquifer type: Terrace

Geological setting

The Lower Mataura GMZ encompasses a series of alluvial terraces formed during entrenchment of the Mataura River during the mid to late Quaternary. The alluvial terrace deposits increase in age and elevation towards the margins of the river valley and generally comprise weathered, poorly sorted, silty, sand gravel. The thickness of the alluvial deposits varies across the zone from <5 metres adjacent to the Mataura River, to over 40 metres under higher terraces.

The Quaternary gravel deposits overlie Tertiary sediments of the East Southland Group (Gore Lignite Measures and Chatton Formation). These deposits are in excess of 200 metres thick in places and are comprised of thick layers of mudstone interspersed with layers of lignite sand and gravel.

Basement rock consists of greywacke of the Murihiku Terrane. The depth to basement rock varies creating a series of sedimentary basins (Croydon Basin, Mataura Basin, Edendale Basin) into which the Tertiary sediments are deposited. Basement rocks also crop out in areas along the current alignment of the Mataura River.

Hydrogeology

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

The Quaternary alluvial deposits of the Lower Mataura GMZ form a spatially extensive unconfined aquifer system that could potentially be subdivided into several discrete but interconnected aquifer systems. However, for the purposes of regional groundwater resource management these aquifers are grouped as a single management unit.

Alluvial aquifers in the Lower Mataura GMZ are typically low yielding, reflecting a high percentage of fine-grained material (predominantly silt) in the weathered gravel matrix. Higher groundwater yields occur in Q2 alluvium along the margin of the Mataura River to the north of Mataura township. The aquifer system is perched above the Mataura River for a majority of its extent, particularly north of Wyndham.

Depth to groundwater generally ranges from 2 to 3 metres below ground level, other than under higher alluvial terraces where it may increase to between 5 and 10 metres below ground level. Temporal groundwater level variations reflect seasonal variations in rainfall recharge with levels typically varying by 1 -2 metres from a peak in winter to a minimum in early autumn.

Tertiary lignite measure sediments underlying the Quaternary alluvium host a limited confined groundwater resource in discontinuous layers of sand and gravel. Artesian discharge from lignite measure aquifers was recorded by historical investigations in the Waimumu area.

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

Water age

Approximate residence time between 2 to 4 years

Depth to groundwater

<2 to 10 metres below ground level, increasing under high alluvial terraces

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 Mataura GMZ is primarily derived from infiltration of local rainfall.

Rainfall recharge: 286 mm per year
Average annual rainfall recharge volume: 100 million m3 per year

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

Discharge

Springs occur frequently in the Lower Mataura GMZ, particularly along the base of larger terraces.

Groundwater discharge occurs to the Mataura River downstream of Mataura. Groundwater discharge also provides baseflow to the lower reaches of many of the larger tributary streams (e.g. Charleton Stream, Titipua Stream).

Some discharge may occur to the coast at Toetoes Harbour.

Groundwater flow

Groundwater in this zone generally flows obliquely towards the Mataura River, following the general topographic gradient of individual alluvial terraces.

Abstraction and water use

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

Groundwater quality in the Lower Mataura GMZ is variable. Groundwater generally contains low concentrations of most dissolved ions. Hardness is generally low. Deeper groundwaters associated with Tertiary deposits typically contain elevated iron concentrations. Elevated iron concentrations may also occur in alluvial aquifers. Nitrate concentrations are variable and but commonly high in shallow groundwater associated with areas of oxidising soils and intensive land use.

Soils

Soils in the Lower Mataura GMZ are typically poorly drained, increasing the potential for reduction nutrient concentrations via denitrification.

Nutrients

The redox state of groundwater in alluvial aquifers is generally mixed to reducing, reflecting reducing conditions in the soil zone and the shallow depth of organic-rich lignite measure sediments. Confined aquifers typically contain elevated iron concentrations. These factors increase the potential for denitrification to occur in shallow groundwater, although localised areas of elevated nitrate concentrations may occur in response to intensive land use where groundwater is more oxic.

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 Mataura 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 pathways for contaminants to reach groundwater in areas of this zone that are not peat are via deep drainage (top left) and artificial drainage (top right).

The main pathways for contaminants to reach groundwater in areas of peat are via deep drainage (bottom left) and artificial drainage (bottom right).

Water quality state summary

Redox state: mixed to reducing

Nitrate: variable

Phosphorus: low but can be elevated where reducing conditions exist in shallow groundwater

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

Major ions: hardness generally low, elevated iron and manganese concentrations in many areas

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 from intensive land use in areas where groundwater is oxic.
Groundwater quality in this zone may be compromised by elevated iron and manganese concentrations that occur naturally in reducing aquifers.

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.