The Centre Hill Groundwater Management Zone (GMZ) covers an area of approximately 6,000 ha, and encompasses the alluvial deposits along the upper Oreti River, upstream of Rocky Point. These deposits include alluvial fans on the surrounding hills, which grade onto a series of alluvial terraces that infill a relatively narrow, elongate valley.
Topography: a sequence of alluvial terraces infilling
an elongate river valley.
Main surface water catchments: Oreti River, Weydon Burn, Windley River
The Centre Hill GMZ encompasses the alluvial sediments infilling the Oreti River Valley between Mossburn and Three Kings. This area includes alluvial fans from the surrounding hills as well as a sequence of late Quaternary alluvial terraces, including an extensive flat-lying glacial outwash (Q8 to Q10) terrace in the Centre Hill area.
Aquifer type: Riparian
The geology of the Centre Hill GMZ consists small areas of Q1 alluvium located along the margins of the Oreti River and tributary streams. Areas of Q4 and Q6 alluvium on intermediate terraces are underlain by a layer of older Q8 to Q10 alluvium, which forms a broad, flat-lying terrace north of Centre Hill. The alluvial deposits have a thickness of 15 to 20 metres or more.
The Quaternary gravels are underlain by basement rock of the Murihiku and Dun Mountain-Maitai terranes.
Soils on the lower alluvial terraces in the Centre Hill GMZ are generally thin, stony, and well drained. Water is able to readily infiltrate from the land surface into underlying groundwater. The proportion of surface run-off increases on imperfectly to poorly-drained soils, which occur on older, higher elevation terraces and on alluvial fans along the base of the surrounding hills.
Limited information is available to characterise the hydrogeology of the Centre Hills GMZ. Recent alluvial deposits along the margins of the Oreti River and tributaries host a moderate to high yielding unconfined aquifer. This aquifer system typically has a saturated thickness of less than 30 metres. Older, more weathered alluvial underlying higher terraces is typically moderate to low yielding.
Basement rocks underlying the alluvial deposits host a very limited groundwater resource.
Depth to groundwater ranges from 1 to 2 metres below ground on the recent Oreti river floodplain (Q1) terrace, increasing to greater than 5 metres below ground under higher terraces toward the valley margins.
The diagram below depicts a generalised conceptual hydrogeological understanding for the Centre Hill GMZ.
Mean residence time is about 1 year at a depth of 12 metres Mean residence time increases to:
- about 60 years at depths of 12 - 15 metres
- 110 years at depths of 15 - 18 metres
- 190 years at depths of approximately 31 metres
Depth to groundwater
- 1 to 5 metres below ground level, increasing under higher alluvial terraces
Seasonal groundwater variation
- Less than 2 metres, reducing close to the Oreti River
Recharge and discharge
The movement of water into (recharge) and out of (discharge) the shallow unconfined aquifer resource for this zone is depicted below.
Groundwater recharge is derived from the infiltration of local rainfall and runoff from surrounding hills. There is a high level of hydraulic connectivity between the Oreti River and major tributaries and groundwater in the adjacent Q1 alluvium. Streams crossing higher alluvial terraces are perched above the water table.
- Rainfall recharge: 250 mm per year
- Average annual rainfall recharge volume: 15 million m3 per year
Discharge of groundwater in the Centre Hill GMZ primarily occurs via baseflow to the Oreti River. Small spring-fed streams also originate along the base of major alluvial terraces.
Groundwater in this zone generally flows from north to south, following the topographic gradient.
Abstraction and water use
A relatively small volume of groundwater is utilised for domestic and farm water supplies across the Centre Hill 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:
Water quality pressures
Limited data exists to characterise groundwater quality in the Centre Hills GMZ. It is assumed that groundwater quality will be generally high in this zone due to the oxidizing nature of soils and groundwater, the hydraulic connection with the Oreti River, and generally low intensity land use.
Soils in the Centre Hills GMZ are typically well drained, with limited denitrification potential and 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 overlying older, elevated terraces, although these soils have a greater potential for contaminant loss via overland flow.
Oxidising conditions are prevalent in the thin, permeable alluvial aquifer underlying the Q1 alluvium of the Oreti River floodplain. However, given the high degree of hydraulic connection with the Oreti River, there is likely to be significant attenuation of contaminant losses due to the high rate of groundwater throughflow.
Elevated alluvial terraces have a lower rate of groundwater throughflow but the potential for nutrient losses to groundwater is mitigated to some extent by elevated denitrification potential in imperfectly to poorly drained 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: low to moderate
Microbial contamination: low, but risk can be elevated close to source
Major ions: low hardness; low iron and manganese concentrations
Water quality - human health
Main issues in this zone
- Contaminants infiltrating to the unconfined aquifer along the riparian margin of the Oreti River may be rapidly exported to surface water.
- The susceptibility of groundwater to nitrate contamination may increase under higher alluvial terraces.
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.