The Orepuki Groundwater Management Zone (GMZ) covers an area of approximately 7,300 ha around the coastal margin of the southern end of the Longwood Range.
Topography: undulating to rolling alluvial and marine terraces around the southern margin of the Longwood Range.
Main surface water catchments: Waimeamea River, Rurikaka Creek, Ourawere Stream, Lake George
Boundaries – follow the mapped extent of Quaternary alluvium and marine terrace deposits along the narrow coastal plain south of the Longwood Range.
The Orepuki GMZ encompasses a series of alluvial and marine terraces that form a narrow coastal plain extending between Colac Bay and Waihoaka.
Aquifer type: Terrace
The Orepuki GMZ has a diverse geology despite its small size.
Alluvial fan deposits along the base of the Longwood Range interfinger with marine terraces formed during a late Quaternary sea level highstand toward the present-day coastline. An extensive marine terrace comprising sand and fine gravel extends west of Orepuki township. Extensive areas of peatland also occur across the zone (see diagram below).
Quaternary alluvial deposits and marine terrace vary in thickness from 5 to 30 metres west of Orepuki to greater than 30 metres toward the base of the Longwood Range. These sediments represent late Quaternary deposition in the lee of several outcrops of Median Batholith volcanic intrusives, which form a prominent headland extending from Monkey Island to Wakaputa Point.
The Quaternary alluvium and marine terrace deposits are underlain by sand, gravel and conglomerate deposits of the Orepuki Formation.
Basement rocks comprise a mix of Median Batholith intrusives and volcanic rocks of the Brook Street Terrane that form the Longwood Range to the north.
The majority of soils in the Orepuki GMZ are deep, fine-grained and imperfectly to poorly drained. This increases the potential for runoff of sloping areas and may require the use of artificial drainage on flatter-lying areas to maintain agricultural productivity.
The Orepuki GMZ hosts a limited groundwater resource in the Quaternary alluvium and marine terrace deposits. Bore yields are generally low to very low, reflecting the high proportion of fine material (silt) in the gravel and sand matrix and the limited saturated thickness of the water-bearing materials.
Around the Orepuki township, the unconfined aquifer is hosted in coastal sand deposits. Establishing a groundwater supply may be difficult in this area with many existing bores restricted to infiltration galleries adjacent to streams and in wetland areas.
Groundwater levels typically range from 3 to 5 metres below ground level but may be shallower in low-lying wetland areas and deeper closer to the base of the Longwood Range.
Sand and gravel layers in the Orepuki Formation host localised, low yielding confined aquifers.
Basement rocks contain a very limited groundwater resource.
The diagram below depicts a generalised conceptual hydrogeological understanding of the Orepuki GMZ.
Depth to groundwater
- <1 to 10 metres below ground level, generally increasing inland
Seasonal groundwater variation
- <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 to the Orepuki GMZ is primarily derived from infiltration of local rainfall. Some recharge may occur via infiltration from small streams that drain from the Longwood Range.
- Rainfall recharge: 415 mm per year
- Average annual rainfall recharge volume: 30.1 million m3 per year
Discharge predominantly occurs along the coastal margin to the Foveaux Strait. Some discharge may also occur to small streams along the coastal margin and via evapotranspiration from wetland areas.
Some groundwater discharge may occur to Lake George.
Groundwater in this zone generally flows perpendicular to the coastline.
Abstraction and water use
Limited utilization of groundwater for domestic and farm water supplies occurs in the Orepuki GMZ.
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:
Water quality pressures
Groundwater quality in the Orepuki GMZ is not well characterised. Groundwater generally contains low concentrations of most dissolved ions but exhibits elevated concentrations of sodium and chloride, reflecting marine aerosol deposition. Hardness is typically low. Elevated iron concentrations may occur in Tertiary and alluvial aquifers, particularly in wetland areas. Nitrate concentrations are generally low, reflecting the reducing nature of soils and groundwater.
Soils across the Orepuki GMZ are generally poorly drained, increasing the potential for reduction of nutrient concentrations via denitrification.
The redox state of groundwater in alluvial aquifers is generally reducing. This increases the potential for nitrate concentrations to be attenuated by denitrification.
Microbial contamination of groundwater is typically limited by natural attenuation in the soil zone and underlying aquifers. The potential for microbial contamination of groundwater in the Orepuki GMZ is typically low due to the poorly drained nature of soils and the relatively slow rate of groundwater flow.
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.
The main pathway for contamination to reach groundwater is via artificial drainage in peat areas.
Water quality state summary
Redox state: reducing
Nitrate: generally low but can be elevated due to intensive land use where groundwater is more oxic
Phosphorus: generally low but can be come elevated in areas with strongly reducing groundwater
Microbial contamination: low, but risk can be elevated close to source
Major ions: hardness generally, elevated iron and manganese concentrations may be common
Water quality - human health
Main issues in this zone
- Microbial contamination: Groundwater quality in this zone may be compromised by elevated microbial contamination levels in some locations.
- Iron and manganese: Groundwater quality in this zone may be compromised by elevated iron and manganese concentrations.
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.