Wendonside

Boundaries – zone encompasses the Wendonside Terrace. The southern boundary of the Wendonside GMZ extends onto the lower alluvial terrace adjacent to the Mataura River to reflect the spatial extent of deeper water bearing layers. The south-eastern margin extends across the lower alluvial terrace adjacent to the Waikaia River reflecting throughflow of groundwater from the Wendonside Terrace immediately upstream of the Mataura River confluence.

Physical setting

The Wendonside GMZ occupies a spatially extensive, flat-lying, elevated alluvial terrace that occupies a roughly triangular area south of the Garvie Mountain foothills, upstream of the confluence of the Mataura and Waikaia Rivers. In places the zone boundaries extend down onto the lower alluvial terraces along the margins of the Mataura and Waikaia rivers.

Aquifer type: Terrace

Geological setting

The largely flat-lying topography of the Wendonside Terrace obscures a complex geological setting (see diagram below).

The Wendonside Terrace is bounded to the north by the foothills of the Garvie Mountains and along the eastern boundary north of Freshford by an uplifted ridge of schist basement rock. Areas of uplifted schist basement also occur along the western margin of the Wendonside Terrace.

The Wendonside Terrace comprises deposits of Quaternary alluvium. The upper 30 to 40 metres of these materials comprise weathered claybound alluvium that form a broad Q6 alluvial terrace. The thickness of the alluvial materials increases across central and southern areas of the Wendonside GMZ, where the Q6 deposits are underlain by layers of coarse sand gravel. These deeper alluvial deposits are inferred to represent gravel outwash materials deposited in a historical channel of the Mataura River, which is incised into the underlying Tertiary sediments. These deeper gravel layers extend out from the southern boundary of the Wendonside Terrace under the northern portion of the lower Q2 alluvial terraces along the margins of the Mataura and Waikaia Rivers.

The Quaternary alluvium is underlain by lignite measure sediments of the East Southland Group. These sediments are relatively shallow (<30 m below ground level) along around the northern margin of the Wendonside Terrace but may be up to 70 metres below ground level along the southern boundary where the Quaternary alluvium thickens. Depth to these sediments decreases between the Wendonside Terrace and the Mataura River, reflecting the southern extent of the Mataura River paleochannel. The lignite measure sediments largely comprise carbonaceous mudstone interspersed with irregular layers of lignite sand and gravel.

Basement rock comprises semi-schist of the Caples Terrace , which has been extensively deformed along a series of north-east to south-west trending faults that run under the eastern side of the Wendonside Terrace.

Hydrogeology

Reflecting the geological setting, the hydrogeology of the Wendonside Terrace is relatively complex.

Quaternary alluvium hosts a complex, stratified aquifer system that varies appreciably in hydraulic characteristics across the Wendonside Terrace. Northern areas host a low yielding, shallow unconfined aquifer system. This aquifer system is recharged by local rainfall and runoff from the foothills of the Garvie Mountains and may, in places, be perched on deeper layers of tightly claybound alluvium extending out from the hills to the north. South of Freshford Plains Station Road depth to groundwater increases rapidly in a low yielding aquifer system that typically occurs between 25 and 40 metres below ground level. Deeper gravel layers across central and southern parts of the Wendonside GNZ host a highly permeable semi-confined aquifer system.

Depth to groundwater ranges from 3 to 5 metres below ground level around the northern margin of the Wendonside Terrace, to 15 to 20 metres below ground level in central areas, and up to 30 metres below ground level in deeper water-bearing layers. Differences in static water level between intermediate and deeper water-bearing layers reflect the semi-confined nature of deeper groundwater.

In the shallow unconfined aquifer, seasonal groundwater levels of 2 to 3 metres generally follow seasonal variations in land surface recharge. Groundwater levels at intermediate depths also exhibit fluctuations that follow seasonal rainfall recharge but also vary on an inter-annual basis, reflecting longer-term rainfall departure. Groundwater levels in deeper semi-confined water-bearing layers exhibit limited response to seasonal variations in rainfall recharge, instead following longer-term rainfall variation.

A limited confined aquifer resource is hosted in sand and gravel lenses of the East Southland Group lignite measure sediments.

The diagram below depicts a generalised conceptual hydrogeological understanding of the Wendonside GMZ.

Water age

• Mean residence time ranges from approximately 16 years in unconfined areas to 34 to 50 years in deeper semi-confined layers

Depth to groundwater

• between 2 to 5 metres below ground level in the unconfined aquifer around the northern margin of the Wendonside and south of the Wendonside Terrace margin
• up to 30 metres below ground level in deeper water-bearing layers along the southern margin of the Wendonside Terrace

Seasonal groundwater variation

• 1.0 to 1.5 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

Groundwater recharge in the Wendonside GMZ is largely derived from infiltration of local rainfall and runoff from the southern flanks of the Garvie Mountain foothills.

• Average annual rainfall recharge: 211 mm per year
• Average annual rainfall recharge volume: 27 million m3 per year

Discharge

A majority of groundwater discharged from the Wendonside GMZ is inferred to occur via baseflow to the Waikaia River downstream of Waiparu. Some groundwater discharge may also occur via the lower reaches of the Garvie Burn along the southern margin of the bedrock ridge, which defined the north-eastern boundary of the Wendonside GMZ.

Groundwater flow

Groundwater flow in the Wendonside GMZ generally occurs in a south-easterly direction toward the lower reaches of the Waikaia River. Due to the presence of a basement ridge along the eastern terrace margin, groundwater flow occurs in a southerly direction north of Freshford Plains Station Road.

Abstraction and water use

Groundwater is utilised for domestic and farm water supply in the Wendonside GMZ. Significant abstraction of groundwater for irrigation occurs from deeper, semi-confined water-bearing layers.

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: