The Castlerock Groundwater Management Zone (GMZ) covers an area of approximately 6,600 ha, encompassing an elevated alluvial terrace that runs along the foot slopes of the North Range in the middle reaches of the Oreti River catchment.
Topography: broad, flat alluvial terrace grading to an alluvial fan along the northern slopes of the North Range.
Main surface water catchments: Murray Creek, Roe Burn
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
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.
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.
- 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.
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
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 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.
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:
Water quality pressures
Natural groundwater quality in the Castlerock GMZ is generally good. Groundwater generally contains low concentrations of dissolved ions. Hardness is typically low to moderate and concentrations of iron and manganese is generally low. Elevated nitrate concentrations may be associated with intensive land use.
Soils in this zone are typically well drained, with elevated potential for bypass flow. This reduces the potential for natural attenuation of contaminants contained in recharge from the land surface infiltrating to groundwater. Greater attenuation of contaminants may occur in imperfectly drained soils along the base of the North Range, although these soils have a greater potential for contaminant loss via overland flow.
Oxidising conditions are prevalent in the thin, unconfined aquifers. Combined with limited dilution from external river sources and moderate rates of groundwater throughflow, this increases the potential for elevated groundwater nitrate concentrations to occur as a result of intensive land use.
- Phosphorus is typically strongly bound to soils.
Microbial contamination of groundwater is typically limited by natural attenuation in the soil zone and underlying aquifers. Microbial contamination may occur in shallow groundwater where soils have an elevated potential for bypass flow and the water table is shallow.
The potential for microbial contamination of groundwater supplies can be reduced by locating wells and bores away from local sources of pollution and ensuring good wellhead protection.
Water quality state summary
- Redox state: oxidising
- Nitrate: moderate to high
- Phosphorus: low
- Microbial contamination: low, but risk can be elevated close to source
- Major ions: moderate hardness; iron and manganese can be elevated
Water quality - human health
Main issues in this zone
- Groundwater quality in this zone may be compromised by elevated nitrate and microbial contamination levels in some locations.
- The susceptibility of groundwater to nitrate contamination reflects the predominance of oxidising conditions in the thin unconfined gravel aquifers.
Disclaimer: This Information Sheet describes the typical average properties of the specified groundwater zone. It is essentially a summary of information obtained from drilling records, consent applications and investigation surveys. It has been prepared in good faith by trained staff within time and budgetary limits. However, no responsibility or liability can be taken for the accuracy of the information and interpretations. Advice should be sought from Environment Southland, drilling companies or other experts before making decisions on individual sites. The characteristics of the groundwater at a specific location may differ in some details from those described here.