Wendon

Boundaries – zone extends across the lower alluvial terrace running along the margins of the Waikaia River onto remnant alluvial terraces preserved along the base of the Round Downs to the east. South of Freshford the zone is limited to alluvial terraces on the true left (eastern) bank of the Waikaia River.

Physical setting

The Wendon GMZ occupies a series of elongate alluvial terraces extending along the margin of the Waikaia River between Piano Flat and the Mataura River confluence. The terraces include the flat-lying floodplain of the Waikaia River as well as gently undulating remnant alluvial terraces along the base of the eastern hills.

Aquifer type: Riparian

Geological setting

North of Waiparu, the Waikaia River occupies a narrow valley between the hills of the Round Downs to the east and an uplifted ridge of schist basement to the west. South of this point the valley continues southward between the eastern hills and the Wendonside Terrace to the west.

The Waikaia River valley comprises a broad, flat-lying late Quaternary (Q2) alluvial terrace, which rises onto older (Q6 and Q8) alluvial terraces along the base of the eastern hills. Recent (Q1) alluvial deposits are restricted to along the riparian margin of the Waikaia River (see diagram below).

The Q2 alluvial deposits comprise a thin (<20 metre thick) layer of poorly sorted gravels containing a high proportion of fine sediment (fine sand and silt). This gravel composition reflects the proximal origin of the alluvial deposits from schist terrain of the Garvie Mountains and Old Man Range. River morphology may also influence the nature of the alluvial sediments with numerous oxbows evident across the Q2 terrace suggestive of a lower energy dendritic channel form, compared to the higher energy semi-braided form of the Mataura River further downstream.

Remnants of older (Q6 and Q8) alluvial terraces are preserved along the eastern margin of the Waikaia River valley. These deposits extend beneath to Q2 alluvium and form the Wendonside Terrace to the west. The older alluvial deposits have a similar composition to the Q2 alluvium but have been subject to greater weathering, resulting in a more compact gravel matrix containing a higher proportion of clay minerals. Recent and historical mining operations in the Freshford and Waikaia areas have extracted gold from these basal gravel deposits.

The Quaternary alluvium is underlain by Tertiary lignite measure sediments of the East Southland group. These sediments comprise carbonaceous mudstone containing irregular layers of coarser sand and gravel.

Basement rocks of the Caples Terrace underlie the Wendon GMZ north of Wendon, while Dun Mountain-Maitai for the basement to the south. These rocks have been subject to significant structural deformation associated with a series of north-east to south-west trending faults that also influences the thickness and surface exposure of the overlying Tertiary sediments.

Hydrogeology

The Q2 alluvial terrace is overlain by free draining alluvial soils formed on the active floodplain of the Waikaia River. Older alluvial terraces are generally overlain by poorly drained soils formed in loess deposits.

The shallow Q2 alluvial deposits of the Wendon GMZ host a low yielding unconfined aquifer system. The hydraulic connection between the Waikaia River and the unconfined aquifer in the Wendon GMZ is significantly lower than that observed in other areas of the mid-Mataura catchment. It is noted that the Q2 terrace on the true right (western) bank of the Waikaia River is included in the neighbouring Wendonside GMZ, reflecting significant throughflow from the Wendonside Terrace to the Waikaia River in this area.

Groundwater levels in the Wendon GMZ range from 2 metres below ground level along the margins of the Waikaia River to upwards of 5 metres below ground level under higher terraces. Temporal groundwater level variations are generally less than 1 metre and typically track the seasonal distribution of rainfall recharge, with highest groundwater levels typically occurring after large rainfall events.

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

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

Depth to groundwater

• between 2 to 5 metres below ground level

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 Wendon GMZ is largely derived from infiltration of local rainfall and runoff from the eastern hills.

• Average annual rainfall recharge: 165 mm per year
• Average annual rainfall recharge volume: 16 million m3 per year

Discharge

A majority of groundwater discharge occurs via baseflow to the Waikaia River. Most tributary streams are perched across higher alluvial terraces and may have limited hydraulic connection to the unconfined aquifer underlying the Q2 terrace.

Groundwater flow

Groundwater flow in the Wendon GMZ generally occurs in a southerly direction obliquely toward the Waikaia River, particularly over the reach upstream of the Mataura River confluence.

Abstraction and water use

Groundwater is utilised for domestic and farm water supply in the Wendon GMZ. Some groundwater abstraction also occurs for irrigation.

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: