Waimea Plains

Boundaries – follow the flat-lying areas of the Waimea Plains, which form part of the Waimea Stream catchment

Physical setting

The Waimea Plains GMZ occupies a broad sequence of alluvial terraces that comprise the Waimea Plains, which are east of the Oreti River, between the Mataura Range to the north and the Hokonui Hills to the south. The terrace surface is drained by an extensive network of lower-order streams that drain into the Waimea Stream.

Aquifer type: Lowland

Geological setting

The Waimea Plains occupy a broad alluvial valley extending eastward from the current alignment of the Oreti River south of Lumsden. The valley is defined to the north by the foothills of the Mataura Range and to the south by the Hokonui Hills. East of Balfour the boundary of the Waimea Plains GMZ follows a bedrock ridge that crosses the eastern end of the Waimea Plains following a north-west/south-east alignment towards Mandeville.

The Waimea Plains comprise a sequence of Quaternary alluvial terraces formed along the historical alignment of the Oreti River before it was diverted to its current alignment through the Hokonui Hills south of Josephville during the late Quaternary.

West of Glenure the alluvial deposits comprise a broad Q6 terrace that has been reworked along late Quaternary overflow channels from the Oreti River. These overflow channels comprise flat-lying elongate (Q4) terraces extending to the Oreti River. East of Glenure, the Waimea Plains GMZ narrows along the active channel of the Waimea Stream, which is incised into the older Q6 terrace surface. Alluvial fan deposits extend northwards from the base of the Hokonui Hills and interfinger with the Quaternary alluvium. The thickness of the Quaternary deposits generally increases toward the northern side of the valley from less than 20 metres along the base of the Hokonui Hills to over 40 metres in the St Patricks area (see diagram below).

The Quaternary alluvium is underlain by Tertiary sediments of the East Southland Group. West of Balfour the alluvial deposits overlie sediments of the Gore Lignite Measures, which comprise mudstone interspersed with isolated layers of lignite, sand and gravel. Limestone and sandstone sediments of the Chatton and Forest Hill formations are exposed in the Sandstone area.

Basement rocks underlying the Waimea Plains GNZ comprise sandstone of the Dun Mountain-Maitai Terrace. The basement rocks exhibit significant structure due to structural deformation along a complex fault system. Basement rocks crop out (or occur close to the land surface) along a ridge running from Balfour to Mandeville as well as at various locations along the base of the Hokonui Hills and towards the western end of the Waimea Plains.

Hydrogeology

Soils toward the western end of the Waimea Plains and along the margins of the Waimea Stream tend to be poorly drained while large areas of the remnant Q6 terrace are overlain by moderately well to well drained soils formed in loess deposits.

The primary groundwater in the Waimea Plains GMZ is hosted in the Quaternary gravel alluvial deposits. These materials host a shallow, unconfined aquifer system that typically exhibits moderate to low permeability reflecting the amount of silt and clay present in the gravel matrix. A deeper moderate yielding semi-confined aquifer is also present along the central axis of the Waimea Plains to the west of St Patricks.

Groundwater levels in the Waimea Plains GMZ typically range between 1 and 5 metres below ground level. Groundwater levels are generally shallow (<2 m below ground level) towards the western end of the Waimea Plains, becoming deeper under the Q6 terrace surface south of Balfour. Seasonal groundwater level variation is typically between 1.5 to 2.5 metres following temporal variations in rainfall recharge. Groundwater levels may also vary on an inter-annual basis reflecting longer-term rainfall patterns.

A limited confined aquifer resource is hosted in sand and gravel lenses of the Tertiary Gore Lignite Measures.

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

Water age

• Mean residence time ranges from 4 to 12 years

Depth to groundwater

• Between 1 to 5 metres below ground level, becoming shallower toward the western end of the Waimea Plains

Seasonal groundwater variation

• 1.5 to 2.5 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 Waimea Plains GMZ is derived from infiltration of local rainfall and infiltration of runoff from the surrounding hills.

• Average annual rainfall recharge: 180 mm per year
• Average annual rainfall recharge volume: 35 million m3 per year

Discharge

A majority of groundwater discharge occurs via baseflow to the surface drainage network. This drainage is augmented by artificial drainage networks on poorly drained soils, particularly toward the western end of the Waimea Plains, which historically comprised extensive wetland areas.

Groundwater flow

Groundwater flow in the Waimea Plains GMZ generally occurs in a south-easterly direction following the overall drainage pattern of the Waimea Plains. At a local scale, groundwater flow may occur obliquely toward reaches of the surface water drainage network that are gaining, particularly toward the Glenure area.

Abstraction and water use

Groundwater is utilised for domestic and farm water supply in the Waimea Plains GMZ. Some abstraction 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: