The Central Plains Groundwater Management Zone (GMZ) covers approximately 35,900 ha. It encompasses a broad alluvial terrace that extends between the Waimatuku catchment and the Oreti River. The zone contains numerous first- and second order streams of the Bog Burn and Terrace Creek catchments.
Topography: broad, flat to gently undulating alluvial terrace
Main surface water catchments: Bog Burn, Terrace Creek, Waianiwa Creek
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
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.
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.
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.
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
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.
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.
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
Groundwater quality in the Central Plains GMZ is variable. Groundwater generally contains low concentrations of dissolved ions. Water Hardness is typically low to moderate. Slightly elevated concentrations of iron (and, in places, manganese) may affect its suitability for use in some locations. Nitrogen concentrations vary from low to high throughout the zone and are in general more elevated at shallow depths and areas associated with bypass flow or well drained soils.
Soils in the Central Plains GMZ are typically imperfectly to poorly drained and often contain significant amounts of organic material. This increases the potential for denitrification to occur but also increases the risk of contaminants being lost to surface water via direct runoff or artificial drainage.
Nitrate concentrations are generally low to moderate across the Central Plains GMZ due to elevated denitrification potential associated with reducing conditions in imperfectly to poorly drained soils. However, underlying aquifers typically exhibit a mixed to oxidising redox state. Therefore nutrients that bypass the soil matrix may result in elevated nitrate concentrations in groundwater.
- Phosphorus concentrations are typically low in the Central Plains GMZ.
Microbial contamination of groundwater is typically limited by natural attenuation in the soil zone and underlying aquifers. However, extensive artificial drainage and the occurrence of natural bypass flow in this zone increases the potential for losses of microbial contaminants to surface water and groundwater resources.
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.
Water quality state summary
- Redox state: mixed
- Nitrate: variable - low to high
- Phosphorus: low
- Microbial contamination: low, but risk can be elevated close to source
- Major ions: low to moderate hardness; iron and manganese can be slightly elevated in some 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.
- 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.