Croydon

Boundaries – the northern boundary follows the boundaries between flatter-lying Q2 alluvium on the Mataura River floodplain and higher, rolling alluvial terraces to the north to the west of Knapdale Road. East of Knapdale Road the northern boundary follows the approximate boundary between Q1 and Q2 alluvium. The southern boundary follows the Mataura River between Pyramid and Otamita and the approximate contact between alluvial sediments and underlying bedrock along the Hokonui Hills between Otamita and Gore.

Physical setting

The Croydon GMZ encompasses a lower alluvial terrace (Q1) that extends along the margin of the Mataura River between Pyramid and Gore. West of Otamita, the Croydon GMZ is separated from the Riversdale zone by the Mataura River, while to the east it follows the geological boundary between recent Q1 alluvium and older gravel deposits of the Knapdale GMZ.

Aquifer type: Riparian

Geological setting

The subsurface geology of the Croydon GMZ consists of a thin layer Quaternary alluvium located along the margins of the Mataura River. The upper portion of the alluvial deposits represent reworked Q1 alluvium deposited on the active floodplain of the Mataura River while deeper parts represent remnants of older Quaternary alluvial deposits.

The Quaternary alluvial deposits overlie a thick sequence of Tertiary lignite measure sediments of the East Southland Group. These sediments comprise thick layers of mudstone interspersed with thin, irregular deposits of lignite, sand and gravel. Basement rocks underlying the Tertiary deposits comprise lithified siltstone, sandstone and volcaniclastic sediments of the Dun Mountain-Maitai Terrane.

The Hillfoot Fault, which runs along the northern flanks of the Hokonui Hills, separates the Tertiary sediments and Dun Mountain-Maitai Terrane basement rocks from basement greywacke of the Murihiku Terrane.

The geological setting of the Croydon GMZ is depicted in the diagram below. Note that the Dun Mountain-Maitai Terrane is at depth beneath the Tertiary deposits and is therefore not depicted.

Hydrogeology

Soils overlying the Croydon GMZ mostly comprise thin, gravelly soils accumulated on the floodplain of the Mataura River. These free draining, permeable soils enable water to readily infiltrate from the land surface into underlying groundwater. The potential for surface runoff increases on areas of imperfectly drained soils along the lower slopes of the Hokonui Hills.

The Q1 alluvial deposits along the margins of the Mataura River typically comprise silty, sandy gravels that range in thickness from 10 to 40 metres. These alluvial deposits host a moderately permeable unconfined aquifer that is hydraulically connected to the Mataura River.

Depth to groundwater in the Croydon GMZ is generally less than 3 metres, increasing under higher elevation areas along the base of the Hokonui Hills. Seasonal groundwater levels variation is typically less than 1 metre, reflecting the hydraulic connection to the Mataura River. Groundwater levels along the riparian margin follow temporal variations in river stage.

The Tertiary lignite measure sediments underlying the alluvial deposits host a limited groundwater resource contained in layers of sand and gravel that have restricted spatial extent.

The diagram below depicts a generalised conceptual hydrogeological understanding for the Croydon GMZ.

Water age

• Mean residence time = 2 years

Depth to groundwater

• 2 to 5 metres below ground level, increasing toward the base of the Hokonui Hills

Seasonal groundwater variation

• Generally less than 1 metre

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

Recharge to the Croydon groundwater zone is sourced from rainfall and infiltration of runoff from the Hokonui Hills to the south. Some localised recharge may also occur along the riparian margin of the Mataura River during high flows. Throughflow to the Croydon GMZ is likely to occur from the Knapdale GMZ to the north.

• Rainfall recharge: 160 mm per year
• Average annual rainfall recharge volume: 7.4 million m3 per year

Discharge

A consistent increase in flow in the Mataura River is observed between Otamita and Gore. This flow gain is interpreted to largely reflect drainage of groundwater from the Croydon GMZ via a combination of direct discharge of groundwater to the river as well as flow in small spring-fed tributaries that originate in oxbows or along the base of small terrace risers along the margins of the river.

Groundwater flow

Groundwater flow in the Croydon GMZ occurs concentrically toward the Mataura River (i.e. to the north-east south of the river and to the south-east north of the river).

Abstraction and water use

Groundwater is utilised for domestic, farm water and irrigation water supply across the Croydon GMZ. Bores generally exhibit moderate yields, particularly along the margins of the Mataura River. Yields may be low toward the margins of the Q1 terrace.

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: