Upper Mataura

Boundaries – lateral boundaries generally follow the boundary between Quaternary alluvium and bedrock of the surrounding hills. The northern boundary follows the Southland Region boundary across Trotters Plain. The southern basin is truncated at an arbitrary location across the Mataura River where the alluvial deposits thin through a narrow valley upstream of Parawa.

Physical setting

The Upper Mataura GMZ encompasses the Mataura River Valley between Parawa and the Southland Region boundary, which crosses Trotters Plain approximately 2 kilometres south of Kingston. This area includes the extensive glacial outwash deposits of Trotters Plain that comprise glacial outwash materials deposited during the Late Quaternary when Lake Wakatipu drained into the Mataura catchment. South of Trotters Plain, the Mataura Valley narrows and the zone comprises the recent floodplain of the Mataura River, which is flanked by remnant alluvial terraces along the valley margins.

Aquifer type: Riparian

Geological setting

The geology of Upper Mataura GMZ comprises a sequence of Quaternary alluvial and glacial outwash terraces that flank the Mataura River (see diagram below).

Trotters Plain comprises extensive deposits of Q4 glacial outwash deposits formed during the Late Quaternary when Lake Wakatipu drained south into the Mataura River catchment. These glacial outwash deposits are underlain by a thick (>100 metres) succession of mid to late Quaternary moraine and glacial outwash deposits that contain layers of fine sediment. Layers of fine sediment were deposited during periods when the spatial extent of Lake Wakatipu extended further south than its current location at Kingston.

South of Fairlight, the Mataura Valley is infilled with relatively thin (<30 metres) sequence of reworked alluvium (Q1/Q2) on the active floodplain of the Mataura River. Small remnants of older alluvial terraces (Q4 to Q12+) are preserved along the margins of valley. Recent alluvial deposits typically comprise relatively coarse, poorly sorted alluvial gravel and sand, characteristic of a deposition in a relatively high-energy fluvial environment. In contrast, the alluvial deposits comprising older, elevated terraces typically comprise highly weathered, tightly packed alluvium that contains a high percental of fine material (silt and clay) in the gravel matrix.

Basement rock consists of semi-schist of the Caples Terrane. The effects of faulting in the basement rock are evident at the southern end of the valley where the Mataura River is diverted through a narrow gorge at Parawa.

Hydrogeology

Soils types in the Upper Mataura GMZ range from shallow, stony, well-drained soils along the margins of the Mataura River through to moderately-well to imperfectly drained soils on elevated terraces along the valley margins.

The Upper Mataura GMZ hosts a shallow, moderate to high yielding unconfined aquifer in reworked (Q1) alluvium along the margins of the Mataura River. This aquifer system is hydraulically connected to the Mataura River and tributaries. Deeper glacial outwash deposits underlying Trotters Plain host moderate to high yielding semi-confined aquifers. Higher terraces along the margins of the Mataura Valley contain a low yielding groundwater resource.

Depth to groundwater in the Upper Mataura GWZ is typically less than 3 metres below ground along the margins of the Mataura River, increasing with distance from the river channel. Groundwater levels may be up to 20 metres below ground level under higher terraces and across Trotters Plain. Groundwater levels along the margin of the Mataura River generally vary in response to river flow with a seasonal variation of less than 1 metre. Groundwater levels under higher terraces and across Trotters Plain reflect seasonal variations in land surface recharge and may range up to 3 metres.

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

Water age

• Mean residence time ranges from 2 to 9 years

Depth to groundwater

• 1-3 metres below ground level along the margins of the Mataura River increasing up to 10 metres below ground level under higher terraces and across Trotters Plain

Seasonal groundwater variation

• <1 metre along the margins of the Mataura River

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 Upper Mataura GMZ is derived from infiltration of local rainfall and runoff from the surrounding hills.

Many streams draining the surrounding hills lose water as they flow across alluvial terraces along the valley margins. Substantial flow losses and gains are observed over alternating reaches of the Mataura River and major tributaries such as Eyre Creek.

• Average annual rainfall recharge: 300 mm per year
• Average annual rainfall recharge volume: 29.7 million m3 per year

Discharge

Groundwater discharge occurs to the Mataura River and spring-fed streams crossing the Q1 terrace. The Brightwater Spring near Garston is the largest spring-fed stream in Southland and gains appreciable flow, which is lost from the upstream reach of the Mataura River. Smaller spring-fed streams occur along the base of terrace risers and gain flow as they drain toward the Mataura River.

Groundwater flow

Groundwater flow in the Upper Mataura GMZ generally occurs from the margins of the valley towards the Mataura River.

Abstraction and water use

Groundwater is utilised for domestic and farm water supply in the Upper Mataura GMZ. 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: