The Makarewa Groundwater Management Zone (GMZ) covers an area of approximately 66,000 ha in in the Makarewa River catchment.
Topography: Undulating alluvial terraces in the lower reaches of the Mataura River catchment.
Main surface water catchments: Makarewa River, Hedgehope Stream, Otapiri Stream, Titipua Stream, Tussock Creek
Boundaries – follow the extent of the Makarewa River catchment upstream of its confluence with the Oreti River to the base of the Hokonui Hills.
The Makarewa GMZ occupies remnants of a late Quaternary (Q8) alluvial terrace that has been partially dissected by the Makarewa River and tributaries. The topography of the zone largely comprises rolling downlands.
Aquifer type: Lowland
The Makarewa GMZ encompasses the thin alluvial gravel deposits of the Kamihi Terrace, which were deposited by the ancestral Mataura River during the early Quaternary Period. The zone includes a large area where these alluvial materials have been removed by post-glacial entrenchment of the Makarewa River, exposing Tertiary sediments of the East Southland Group at the land surface.
Numerous sedimentary units of the East Southland Group occur in different parts of the zone. Limestone of the Forest Hill Formation is found on the western slopes of Forest Hill. The Winton Hill Formation extends from the eastern side of Forest Hill where it is exposed at the surface, to the Makarewa River where it is overlain by Quaternary gravels. Gore Lignite Measures are found in the eastern part of the zone, and consist largely of mudstone, interspersed with lignite and sand and gravel lenses. Sandy quartz conglomerate of the Pebbly Hill Gravels are exposed at Pebbly Hills.
Greywacke of the Murihuku Terrane forms the basement rock underlying the Tertiary sediments.
Most soils in the Makarewa GMZ are deep, fine-grained, and imperfectly to poorly drained. This increases the potential for runoff from sloping areas and has resulted in the use of artificial drainage on flatter-lying areas.
The Quaternary alluvial deposits of the Makarewa GMZ form a thin, spatially extensive low yielding unconfined aquifer system. The alluvial deposits are generally thin (<10 metres) or absent across much of the Makarewa GMZ but may reach a thickness of 30 metres in the Hedgehope area and closer to the Oreti River. The alluvial deposits typically comprise highly weathered silty sandy gravels.
Across the undulating topography, groundwater levels in the unconfined aquifer are typically close to the land surface adjacent to rivers and streams but occur at some depth beneath intervening ridges, with the water table ranging between 2 to 10 metres below ground level. This pattern results in a piezometric surface that broadly follows the topographic surface, creating localised hydraulic gradients toward the surface drainage network.
Temporal groundwater level variations reflect seasonal variations in rainfall recharge with levels typically varying annually by 1 to 2 metres, from a peak in winter to a minimum in early autumn.
The Tertiary sediments contain a limited groundwater resource, primarily within isolated sand and gravel layers in the Gore Lignite Measure or Forest Hill Formation. A limited groundwater resource is hosted in limestone deposits of the Forest Hill Formation.
The diagram below depicts a generalised conceptual hydrogeological understanding of the Makarewa GMZ.
Depth to groundwater
- <2 to 10 metres below ground level, increasing under higher 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 to the Makarewa GMZ is primarily derived from infiltration of local rainfall.
- Rainfall recharge: 272 mm per year
- Average annual rainfall recharge volume: 180 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.
Groundwater discharge predominantly occurs via baseflow a network of small streams. Extensive areas of artificial drainage also divert water from the land surface to surface waterways.
At a sub-regional scale, groundwater flow in the Makarewa GMZ is interpreted to follow the general drainage pattern of the Makarewa catchment. At a finer scale, groundwater flow is likely to occur obliquely to individual hydraulically connected surface waterways.
Abstraction and water use
Groundwater is utilised for domestic and farm water supplies across the Makarewa GMZ.
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:
Water quality pressures
Groundwater quality in the Makarewa GMZ is variable. Groundwater generally contains low concentrations of most dissolved ions. Hardness is generally low but can be elevated in areas where groundwater is hosted in limestone sediments. Elevated iron concentrations also occur in both alluvial and Tertiary aquifers. Nitrate concentrations are generally low, reflecting the mixed to reducing redox conditions prevalent in this zone, although high concentrations are commonly associated with oxidised conditions and intensive land use.
Soils in the Makarewa GMZ are typically poorly drained, increasing the potential for reduction nutrient concentrations via denitrification.
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, particularly across the eastern portion of the zone. Tertiary 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 of groundwater is typically limited by natural attenuation in the soil zone and underlying aquifers. The potential for microbial contamination of groundwater in the Makarewa 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.
Top: The main pathways for contamination to reach groundwater in areas of this zone that are not peat are via deep drainage (left) and artificial drainage (right).
Bottom: The main pathways for contamination to reach groundwater in areas of this zone that are peat are via deep drainage (left) and artificial drainage (right).
Water quality state summary
Redox state: mixed to reducing
Nitrate: generally low but can be elevated due to intensive land use where groundwater is more oxic
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
- Nitrate: Groundwater quality in this zone may be compromised by elevated nitrate and microbial contamination levels in some locations.
- Microbial contamination: There is potential for elevated nitrate concentrations in areas of intensive land use where groundwater is oxic.
- Iron and manganese: Groundwater quality in this zone may be compromised by elevated iron and manganese concentrations.
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.