The Awarua Groundwater Management Zone (GMZ) covers approximately 44,000 ha, and encompasses the lowland area that drains to Awarua Bay, the Waituna Lagoon, and a small area of the New River Estuary. This zone contains the majority of the Awarua Wetland complex, which is hydrogeologically distinct from the rest of the Southland plains.
Topography: broad, flat to undulating alluvial terrace, recent marine terrace along the coastal margin
Main surface water catchments: Waituna Creek, Carran Creek, Waimatua Creek, Mokotua Stream
The Awarua GMZ comprises a number of small catchments that drain to the south coast, Waituna Lagoon and New River Estuary. The northern part of this zone comprises a flat to rolling alluvial terrace. Coastal lagoons and extensive wetlands occur across a low-lying marine terrace along the south coast.
Aquifer type: Lowland
The near-surface geology of the Awarua GMZ consists of a relatively thin layer of poorly sorted Quaternary clay-bound gravels. Fluctuating sea levels during the late Quaternary resulted in the alluvial materials along the south coast being reworked in a shallow marine environment forming extensive deposits of quartz-rich gravel and sand. The Waituna Lagoon, Awarua Lagoon and surrounding peat wetland areas formed on the flat-lying, relatively low permeability marine gravel deposits.
The Quaternary gravels are generally less than 20 metres thick and overlie a thick sequence of lignite measure sediments of the East Southland Group that comprise alternating layers of mudstone and lignite interspersed with sand and gravel. Mesozoic basement rocks of the Murihiku and Brook Street Terranes occur at depth (see diagram below).
A majority of soils in the Awarua GMZ are typically imperfectly to poorly drained with a high organic matter content. Impeded soil drainage increases surface run-off and results in widespread use of artificial drainage to maintain agricultural productivity.
A shallow unconfined aquifer is hosted in remnants of the original Quaternary gravel outwash surface, which is dissected by small first and second-order streams across the northern portion of the Awarua GMZ. The flat-lying, reworked sand and gravel deposits along the coastal margin also host a shallow unconfined aquifer system that is connected to waterbodies such as Awarua Lagoon and Waituna Lagoon, as well as the surface drainage network.
The depth to groundwater decreases from north to south. Along the coastal margin and around Waituna Lagoon, groundwater levels become so shallow that major wetlands have formed.
Alluvial deposits throughout the Awarua GMZ are typically low yielding, reflecting the weathering of the gravel materials and accumulation of fine-grained materials in the gravel matrix. Although containing some areas of higher yielding quartz gravels, the unconfined aquifer underlying the marine terrace along the south coast is typically low yielding due to the accumulation of substantial organic (peat) deposits.
Tertiary lignite measure sediments occur at depths ranging between 10 to 30 metres below ground level (bgl) across the zone and comprise an alternating sequence of estuarine (sand), marginal marine (mudstone and lignite) and terrestrial (gravel) materials. Sand and gravel layers within the lignite measures form locally significant, but relatively low yielding semi-confined to confined aquifers that are restricted to thin, laterally discontinuous layers.
The diagram below depicts a generalised conceptual hydrogeological understanding for the Awarua GMZ. Bore yields are generally low across the whole zone.
- Approximate residence time: 1.5 to 25 years (variable)
Depth to groundwater
- Alluvial terrace (north): generally 4 to 6 metres below ground level
- Marine terrace (south): <2 metres below ground level
Seasonal groundwater variation
- 1 to 3 metres, reducing closer to south coast
Recharge and discharge
The movement of water into (recharge) and out of (discharge) the shallow unconfined aquifer resource for this zone is depicted below.
The majority of recharge comes from infiltration of local rainfall. Throughflow from neighbouring aquifers is likely to be negligible.
- Rainfall recharge: 304 mm per year
- Average annual rainfall recharge volume: 130 million m3 per year
Discharge predominantly occurs via baseflow into first and second order streams and artificial drainage channels. Some discharge is also likely to occur to local springs, Waituna Lagoon, and directly to the sea.
Groundwater in this zone generally flows from north to south, following the topographic gradient. At a finer scale, groundwater flow is likely to coincide with surface catchment drainage patterns.
Abstraction and water use
Groundwater is utilised for domestic and farm water supplies across the Awarua GMZ. Rate of abstraction are generally restricted by the low-yielding nature of the aquifers. The suitability of groundwater for potable supply may be restricted due to naturally occurring water quality issues.
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 Awarua GMZ is variable. Groundwater generally contains low concentrations of dissolved ions. Hardness is typically low to moderate but elevated concentrations of iron (and manganese in places) may affect its suitability for use, particularly from aquifers within the lignite measure sediments. Nutrient concentrations are generally low due to the reducing nature of soils and groundwater.
Soils in the Awarua 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 run off or artificial drainage.
Nitrate concentrations are generally low to very low across the Awarua GMZ due to elevated denitrification potential associated with:
- reducing conditions in imperfectly to poorly drained soils,
- reducing conditions in aquifers containing elevated levels of organic carbon.
The acidic nature of peat soils means phosphorus is poorly retained and may occur in elevated concentrations in groundwater and hydraulically connected surface waters.
Microbial contamination of groundwater is typically limited by natural attenuation in the soil zone and underlying aquifers. Elevated potential for microbial contamination may be associated with the occurrence of fibrous peat soils at the land surface.
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: reducing
- Nitrate: generally low to very low
- Phosphorus: low to moderate, can be elevated in peat soils
- Microbial contamination: low, but risk can be elevated close to source
- Major ions: moderate hardness; iron and manganese can be elevated
Water quality - human health
Main issues in this zone
- 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.