Central Plains

Physical setting

The Central Plains GMZ comprises an extensive remnant alluvial terrace on the true right (western) side of the Oreti River, downstream of Centre Bush. The zone consists of a shallow unconfined aquifer system hosted in remnants of the original Quaternary gravel outwash surface, which is dissected by a dense network of first- and second-order streams. This network of small, partially incised streams forms the gently undulating to gently rolling topography characteristic of lowland aquifer settings.

Aquifer type: Lowland

Geological setting

The geology of the Central Plains GMZ consists largely of poorly sorted, moderately weathered alluvial gravels in a fine-grained matrix.

The southern portion of the Central Plains GMZ is a remnant of an historically extensive, moderately to highly weathered glacial outwash (Q8) gravel terrace. North of Drain Road, a marked terrace denotes a lower erosional surface formed by a previous course of the Aparima River, which flowed into the Lower Oreti catchment during the Late Quaternary period. Much of the northern portion of the Central Plains GMZ is comprised of thin (<10 metres) surficial gravel deposits reworked to varying degrees by the Aparima River (see diagram below). At depth, the gravel deposits retain the characteristic weathered silty clay matrix of the older terrace surface to the south.

The thickness of the gravel deposits appears to vary across the Central Plains GMZ, from 20 metres near Waianiwa to in excess of 50 metres near Hundred Line Road. The Quaternary gravels are underlain by a thick layer of sediments of the Winton Hill Formation, many hundreds of metres in depth. These Tertiary sediments are composed of mudstone and sandstone with some lignite in the central and south-eastern parts of the basin. A small exposure of limestone occurs at Dunearn and may be present at depth elsewhere in the zone.

Basement rocks underlying the Tertiary sediments comprise of greywacke of the Murihiku Terrane.

Hydrogeology

Soils in the Central Plains GMZ are mostly poorly drained heavy silt loam to silty clay soils. Impeded soil drainage increases the potential for surface run-off and results in widespread use of artificial (mole and tile) drainage to maintain agricultural productivity.

The alluvial deposits of the Central Plains GMZ host spatially extensive unconfined and semi-confined to confined aquifers.

A relatively thin unconfined aquifer system is hosted in the upper 10 to 20 metres of the Q2 to Q4 alluvial deposits across the northern portion of the Central Plains GMZ. This low-yielding aquifer system is hydraulically connected to surface water bodies, and drainage of groundwater provides baseflow in the extensive network of first and second order streams.

Low yielding semi-confined aquifers are hosted at depth in the Q8 alluvial deposits. Lenses of silt, clay and silty gravel form aquitards, partially separating these water-bearing intervals from the overlying unconfined aquifer above. These aquifers become increasingly well confined with depth.

The depth to groundwater generally reflects surface topography, increasing from around 1 metre below ground adjacent to surface waterways to 3 or 4 metres below ground under higher elevation areas between the surface drainage network. Wetland areas occur along the transition from the Q2 to Q4 alluvium to the north and the Q8 alluvium to the south.

Seasonal groundwater level variation typically ranges between 1 and 2 metres. Groundwater level increase rapidly to a maximum in early winter then exhibit a relatively constant decline through to a minimum in early autumn.

A limited low yielding groundwater resource may occur in isolated sand and gravel layers in the Tertiary sediments. Limestone deposits may also contain localised groundwater resources.

The diagram below depicts a generalised conceptual hydrogeological understanding for the Central Plains GMZ.

Water age

Mean residence time is about 1 year at a depth of 12 metres Mean residence time increases to:

• about 60 years at depths of 12 - 15 metres
• 110 years at depths of 15 - 18 metres
• 190 years at depths of approximately 31 metres

Depth to groundwater

• 1 to 4 metres below ground level, less adjacent to surface waterways

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

Virtually all recharge to the Central Plains GMZ is derived from infiltration of local rainfall. Infiltration of runoff from the Taringatura Hills to the north may also provide a minor recharge contribution. Throughflow from neighbouring aquifers is likely to be negligible.

• Rainfall recharge: 241 mm per year
• Average annual rainfall recharge volume: 86.5 million m3 per year

Discharge

Groundwater discharge provides baseflow to a network of small streams. Temporal groundwater level variations typically reflect the hydraulic connection to small streams. For example, groundwater levels rise rapidly in early winter or in response to large recharge events. Groundwater levels then decline in a relatively linear manner, reflecting progressive water flow to hydraulically connected aquifers. Significant discharge of groundwater also occurs via the extensive artificial drainage network, particularly during the winter months when groundwater levels are high.

Groundwater flow

Recharge to the aquifer system typically drains toward via the shallow unconfined aquifer system to the nearest surface waterway, reflecting the characteristic drainage pattern of lowland aquifers.

A small component of recharge may infiltrate through deeper water-bearing layers following the overall south-easterly topographic gradient.

Abstraction and water use

Groundwater is extensively utilised for domestic and farm water supplies across the Central Plains GMZ. Rate of abstraction is generally restricted by the low-yielding nature of the aquifers in this zone. In some locations the suitability of groundwater for potable supply may be restricted due to naturally occurring concentrations of iron and manganese.

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: