The Waimatuku Groundwater Management Zone (GMZ) covers an area of approximately 23,700 ha. The zone encompasses the Waimatuku Stream catchment, which drains to the south coast approximately 9 kilometres east of Riverton.
Topography: extensive flat to undulating alluvial terrace.
Main surface water catchments: Waimatuku Stream, Middle Creek
Boundaries – follow the surface water catchment boundary of the Waimatuku Stream.
The Waimatuku GMZ occupies a flat-lying alluvial terrace that extends along a former course of the Aparima River south of Wreys Bush. The terrace surface is drained by an extensive lower-order stream network and includes a large wetland in the Bayswater area.
Aquifer type: Lowland
The Waimatuku GMZ occupies a historic channel of the Aparima River, which formed when the river flowed southwards from Wreys Bush during the last interglacial period (130,000 to 70,000 years before present), before subsequently diverting westwards to its present course. During this period the Aparima River entrenched into a sequence of older (Q8+) weathered alluvium, depositing a layer of partially reworked gravel deposits (Q2).
The overall thickness of the Quaternary alluvium generally ranges between 30 to 50 metres, reaching a thickness of 80 metres in the Drummond area. The upper 5 to 15 metres consist of younger, partially reworked gravel deposits (Q2), and are underlain by older Q8+ gravels. Alluvial deposits at the southern end of the zone include remnants of a marine terrace formed during a late Quaternary sea level highstand.
The Q2 alluvium generally comprises poorly sorted silty, sandy gravels. Deeper alluvial deposits are generally highly weathered and comprise poorly sorted tight claybound alluvium interspersed with layers of coarser sand and gravel.
These alluvial materials overlie limestone of the Forest Hill Formation in the Isla Bank area, and mudstone of the Gore Lignite Measures elsewhere across the Waimatuku GMZ. The Forest Hill Formation limestone crop out along the western margin of the Waimatuku GMZ forms the prominent terrace extending from Otautau to Isla Bank.
Mesozoic basement rocks of the Murihiku Terrane occur at depth (see diagram below).
Soils in the Waimatuku GMZ are generally poorly drained, although some areas of well drained soils occur along remnants of late Quaternary alluvial channels. A distinguishing characteristic of the poorly drained soils in the Waimatuku GMZ is their propensity to exhibit shrink-swell behaviour in response to varying soil moisture reflecting their provenance from mafic rocks in the Takitumu Mountains. This behaviour increases the potential for bypass flow to occur when soils are dry.
The Q2 alluvium hosts a low to moderate yielding unconfined aquifer system that is hydraulically connected to the surface water drainage network. Deeper water-bearing gravel layers generally become increasingly well confined at depth due to the occurrence of multiple layers of lower permeability materials, such as silt and compact claybound gravels. In some areas such deeper aquifers may be relatively well confined. Well yields from the deeper alluvium are generally low, although moderate yielding water-bearing layers have been intercepted at depth in the Drummond area where the alluvial deposits are thicker.
Groundwater levels in the Q2 alluvium generally occur within 2 to 4 metres of the ground surface. Groundwater levels respond relatively rapidly to rainfall events then recede relatively quickly as water drains to the surface drainage network. Seasonal groundwater levels generally vary by less than 1 metre across the Waimatuku GMZ.
The diagram below depicts a generalised conceptual hydrogeological understanding of the Waimatuku GMZ.
- Average saturated thickness = 40 metres
- Approximate volume of groundwater storage = 1,660 million m3
- Mean residence time 5 to 12 years
Depth to groundwater
- typically 2-4 metres below ground level
Seasonal groundwater variation
- >1 metre
Recharge and discharge
The movement of water into (recharge) and out of (discharge) the shallow unconfined aquifer resource for this zone is depicted below.
Groundwater recharge in the Waimatuku GMZ is derived from infiltration of local rainfall.
- Average annual rainfall recharge: 269 mm per year
- Average annual rainfall recharge volume: 64 million m3 per year
A majority of groundwater discharge occurs via baseflow to the surface drainage network. This drainage is augmented by artificial drainage networks on poorly drained soils. A small component of the overall water balance may discharge to the south coast.
Shallow groundwater flow in the Waimatuku GMZ generally occurs obliquely toward the surface drainage network. A small component of groundwater flow through deeper semi-confined to confined aquifers may occur to the south following the overall surface water drainage pattern.
Abstraction and water use
Groundwater is utilised for domestic and farm water supply in the Waimatuku GMZ. Some abstraction occurs for irrigation.
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
Natural groundwater quality in the Waimatuku GMZ is variable. Groundwater generally contains low concentrations of dissolved ions. Hardness and iron concentrations are also typically low, but can be slightly elevated in some areas, particularly around wetlands. Nitrate concentrations are moderate to very high in many parts of the zone.
Much of the soils overlying the Waimatuku GMZ are poorly drained with an elevated potential to attenuate nitrate concentrations via denitrification. However, the shrink-swell behaviour of these soils increases the potential for bypass flow to occur. This has a major influence on water quality in the Waimatuku GMZ.
Bypass flow occurring in poorly drained soils allows nutrients built up in the soil zone during the summer months to rapidly drain into underlying groundwater as soil moisture levels increase in autumn. This has a major influence on nutrient concentrations in underlying groundwater. The hydraulic connection between shallow groundwater and surface water allows elevated nitrate concentrations to be exported to surface water relatively rapidly. Nitrate concentrations are very high in many areas of the zone. Particularly in the middle and northern portions.
Phosphorus is typically strongly bound to soils.
Microbial contamination of groundwater is typically limited by natural attenuation in the soil zone and underlying aquifers. The potential for microbial contaminants to be attenuated in the soil zone in the Waimatuku GMZ is reduced due to the potential for bypass flow to transport contaminants rapidly from the ground surface to underlying groundwater.
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 contamination to reach shallow groundwater in areas of this zone that are not peat are via deep drainage (left) and artificial drainage (right).
The main pathways for contamination to reach groundwater in areas of peat are via deep drainage (left) and artificial drainage (right).
Water quality state summary
Redox state: oxidising
Nitrate: moderate to very high
Microbial contamination: elevated risk where bypass flow occurs
Major ions: hardness is low, iron concentrations are generally low but can be slightly elevated in areas where bypass flow is limited and in current or historical wetland areas with organic soils
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: Localised microbial contamination results from bypass flow through the soil zone.
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.