The Dipton Groundwater Management Zone (GMZ) covers a roughly triangular area of 11,700 ha between the Hokonui Hills to the east and the Taringatura Hills to the west. This area contains a sedimentary basin that is distinct from the Southland Plains to the south.
Topography: broad, flat to undulating alluvial terrace
Main surface water catchments: Oreti River, Stag Stream, Dipton Stream
Boundaries - follow the approximate boundary between alluvial sediments and basement rock of the surrounding Hokonui and Taringatura Hills. Caroline Valley is excluded from the Dipton GMZ due to the absence of significant water-bearing sediments.
The Dipton GMZ comprises alluvial deposits and underlying Tertiary sediments infilling a sedimentary basin that extends along the Oreti River valley between Ram Hill and Benmore. Landform in the Dipton basin comprises a series of broad, flat-lying alluvial terraces that grade from the recent Oreti River floodplain onto slightly higher elevation remnant alluvial terraces toward the western basin margin.
Aquifer type: Terrace
The surficial geology of the Dipton GMZ consists of a narrow band of Q1 alluvium that runs along the riparian margin of the Oreti River. Further from the river channel, broad flat-lying terrace Q2 and Q4 alluvial terrace grade onto alluvial fans on the lower slopes of the surrounding hills. These terrace deposits overlie layers of older Q6 to Q8+ alluvium. The alluvial deposits range in thickness from 30 to >60 metres.
Quaternary gravels overlie Tertiary Gore Lignite Measure sediments, containing mudstone and layers of lignite. Underlying the Gore Lignite Measure sediments are layers of shelly sand and sandy limestone of the Forest Hill Formation.
Basement rocks in this zone primarily comprise Mesozoic sandstones, mudstones, and conglomerates of the Murihiku Terrane.
The geological setting of the Dipton GMZ is depicted in the diagram below.
Alluvial soils along the margins of the Oreti River in the Dipton GMZ are generally free draining, enabling water to readily infiltrate from the land surface into underlying groundwater. Imperfectly to poorly drained soils on the older Q2 and Q4 terrace surfaces increase the potential for surface runoff and require the use of artificial drainage to maintain agricultural productivity.
The Dipton GMZ hosts a spatially extensive unconfined aquifer system in the shallow alluvial deposits. Bore yields are typically low to moderate and generally decline in the older Q4 gravel deposits. This aquifer system is hydraulically connected to the Oreti River and tributaries.
Groundwater generally occurs within 2 metres of the ground surface across much of the Dipton GMZ. Seasonal groundwater level variation is typically of the order of 2 metres, reducing along the margins of the Oreti River and tributaries.
In some areas the alluvial gravels may be interspersed with layers of silty clay or tight silty gravels, which form an aquitard restricting hydraulic connection with the shallow aquifer. As a result, low yielding semi-confined aquifers can be found in deeper alluvial deposits in parts of the zone.
A limited groundwater resource is found at depth within the shelly sand and sandy limestone of the Forest Hill Formation toward the northern end of the Dipton basin.
The diagram below depicts a generalised conceptual hydrogeological understanding for the Dipton groundwater zone.
- Mean residence time = 2 years
Depth to groundwater
- Generally <2 metres below ground level
Seasonal groundwater variation
- <2metres, less adjacent 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.
The majority of recharge comes from infiltration of local rainfall and runoff from the surrounding hills. Some recharge may also occur from surface water along the margins of the Oreti River. Throughflow from neighbouring aquifers is likely to be negligible.
- Rainfall recharge: 233 mm per year
- Average annual rainfall recharge volume: 27.2 million m3 per year
Discharge predominantly occurs via baseflow to the Oreti River and major tributaries (Dipton Stream, Stag Stream).
Groundwater in this zone generally flows from north to south, obliquely towards the Oreti River.
Abstraction and water use
Groundwater is utilised for domestic and farm water supplies across the Dipton GMZ. Rates of abstraction are generally restricted by the low-yielding nature of the aquifers. Some higher yielding irrigation takes have been established from deeper alluvial or limestone layers.
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 Dipton 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 the Dipton GMZ range from well drained to poorly drained. Well drained soils along the margins of the Oreti River have an 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 is likely to occur in imperfectly to poorly drained soils on older terraces, although these soils also have a greater potential for contaminant loss via overland flow or artificial drainage.
Oxidising conditions are prevalent in the thin, unconfined aquifer in the Dipton GMZ, increasing the potential for elevated groundwater nitrate concentrations to occur as a result of intensive land use.
However, the potential for nutrient losses to groundwater maybe somewhat mitigated by the elevated denitrification potential in imperfectly to poorly drained soils on older terrace surfaces.
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 well drained 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 to moderate hardness; iron and manganese generally low
Water quality - human health
Main issues in this zone
- Groundwater quality in this zone may be compromised by elevated nitrate concentrations associated with intensive land use.
- Contaminants infiltrating to the unconfined aquifer along the riparian margin of the Oreti River may be rapidly exported to surface water.
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.