Awarua

Physical setting

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

Geological setting

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).

Hydrogeology

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.

Water age

• 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.

Recharge

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

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 flow

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.

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: