Castlerock

Physical setting

The Castlerock GMZ encompasses an elevated alluvial terrace (the ‘Castlerock Terrace’) that extends along the true right (southern) bank of the Oreti River between Mossburn and Castlerock. This terrace grades onto a large alluvial fan extending along the base of the North Range.

Aquifer type: Terrace

Geological setting

The Castlerock GMZ encompasses the Q2 to Q6 alluvial deposits and associated alluvial fans that extend along the foot slopes of the North Range between Ram Hill and Mossburn, forming the Castlerock Terrace. These materials typically comprise loose, poorly sorted sand and gravel deposits containing varying amounts on fine sediment (silt) in the gravel matrix. The Quaternary alluvial deposits decrease in thickness from over 60 metres to the north, to less than 15 metres on the lower slopes of the North Range.

The alluvial deposits overlie a layer of Tertiary sediments comprising of mudstone and lignite of the East Southland Group.

Basement rock consists of sandstone and siltstone of the Dun Mountain-Maitai Terrane, and greywacke basement of the Murihiku Terrane. The structure of the basement rocks is disrupted by several faults that cross the area. The effects of this structural deformation are evident at Castle Rock, a localised basement outcrop approximately 80 metres high, which occurs toward the eastern margin of the Castlerock Terrace. Geological setting is depicted in the diagram below.

Hydrogeology

Soils overlying lower elevation parts of the Castlerock Terrace are generally well drained, enabling water to readily infiltrate from the land surface into underlying groundwater. Imperfectly to poorly-drained soils occur on the lower slopes of the North Range where the proportion of surface run-off occurring increases.

The Quaternary alluvium and alluvial fan deposits of the Castlerock terrace host a spatially extensive low yielding unconfined aquifer. This aquifer system typically has a saturated thickness of less than 20 metres.

Deeper alluvial sediments host a confined aquifer system (the ‘North Range Aquifer’), which is separated from the unconfined aquifer by a thick layer of compact claybound gravel. Water levels in the North Range Aquifer differ appreciably from those in the unconfined aquifer and the deeper groundwater resource is managed as a separate confined aquifer system.

Depth to groundwater ranges from 2 to 6 metres below ground level, becoming shallower toward the northern and eastern terrace margins. Seasonal groundwater level variation is typically less than 2 metres.

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

Water age

• Approximate residence time ranges from 1.5 to 6 years

Depth to groundwater

• 2 to 6 metres below ground level, increasing toward the base of the North Range

Seasonal groundwater variation

• Less than 2 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

The majority of recharge comes from the infiltration of local rainfall and runoff from the lower slopes of the alluvial fan fronting the North Range. Throughflow from neighbouring aquifers is likely to be negligible.

• Rainfall recharge: 244 mm per year
• Average annual rainfall recharge volume: 16 million m3 per year

Discharge

A significant proportion of discharge from the Castlerock GMZ occurs via spring discharge. Waterways such as Murray Creek and the Roe Burn, are perched above the water table across the Castlerock Terrace and gain appreciable discharge along the base of the terrace riser marking eastern margin of the Castlerock Terrace. A number of small, un-named springs also occur along the eastern terrace margin between Frisco Road and Keown Road.

Discharge also occurs as throughflow to the alluvial gravels of the lower lying Oreti GMZ, which extends along the riparian margin of the Oreti River.

Groundwater flow

Groundwater flow in the Castlerock GMZ occurs in a north-easterly direction at the base of the North Range, moving to a more east north-easterly direction toward the northern terrace margin.

Abstraction and water use

Groundwater is utilised for domestic and farm water supplies across the Castlerock GMZ. Rate of abstraction are generally restricted by the relatively low-yielding nature of the alluvial materials.

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: