Makarewa

Boundaries – follow the extent of the Makarewa River catchment upstream of its confluence with the Oreti River to the base of the Hokonui Hills.

Physical setting

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

Geological setting

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.

Hydrogeology

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

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.

Discharge

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.

Groundwater flow

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.

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: