The Upper Aparima Groundwater Management Zone (GMZ) covers an area of approximately 49,000 ha in the Aparima River catchment upstream of Otautau.
Topography: a series of extensive flat-lying to undulating alluvial terraces across the middle reaches of the Aparima catchment
Main surface water catchments: Aparima River, Hamilton Burn, Moss Burn, Sheepwash Creek, Otautau Stream
Boundaries – lateral boundaries generally follow the boundary between Quaternary alluvium and bedrock of the surrounding hills. The northern boundary follows the prominent terrace margin the boundary between older Q8 alluvium in the Aparima catchment and reworked alluvium (Q1/Q2) of the Oreti catchment.
The Upper Aparima GMZ encompasses the Aparima River catchment upstream of Otautau. This area includes extensive outwash terrace deposits extending out from the surrounding hills, which have been reworked along the margins of the Aparima River and tributaries during the late Quaternary.
Aquifer type: Terrace
The geology of Upper Aparima GMZ consists of a series of broad Quaternary alluvial terraces that flank the recent floodplain deposits of the Aparima River. In many areas older Quaternary (Q4 to Q10+) alluvial terrace deposits have been incised by lower order streams to form an undulating topography that descends onto the flat-lying (Q1) floodplain of the Aparima River (see diagram below).
The gravel deposits underlying the higher terrace surfaces tend to be relatively claybound and weathered, comprising early-mid Quaternary alluvium deposited by the Aparima River along with locally derived alluvial fan deposits sourced from the surrounding foothills. These materials have been reworked along the central axis of the valley by progressive entrenchment of the Aparima River during the late-Quaternary. This has formed a series of alluvial terraces that increase in age and toward the margins of the valley. The thickness of the alluvial deposits vary but typically exceed 40 to 50 metres under older elevated terraces, particularly along the alignment of the Aparima River across northern parts of the zone.
An extensive area of peat deposits occurs within the Castle Downs Wetland.
Tertiary sediments of the East Southland Group (Forest Hill Formation, Winton Hill Formation) underlie Quaternary alluvium across southern areas of the Upper Aparima GMZ. Mudstone dominated sediments of the Waicoe Group underlie northern parts of the zone and crop out along the foot of the Takitumu Mountains west of Mossburn.
Basement rock is greywacke of the Murihiku Terrane, which is extensively faulted along the base of the Takitumu Mountains.
Soil types in the Upper Aparima GMZ range from shallow, stony, well drained soils along the margins of the Aparima River through to imperfectly to poorly drained soils developed on thick loess deposits overlying older, elevated terraces along the valley margins.
The Upper Aparima GMZ hosts a spatially extensive unconfined aquifer system hosted in the Quaternary alluvial terrace deposits. This aquifer system is typically comprised of highly weathered gravel deposits in a tight silt and clay matrix. Bore yields vary with position in the landscape. Areas of Q1 alluvium along the margins of the Aparima River hosts may be moderately permeable but bore yields generally decline in older, weathered alluvium under higher terraces. For example, in some areas west of Wreys Bush, yields can be so low that it may be difficult to establish viable groundwater supplies.
Some localised, higher yielding layers comprising outwash gravel deposits occur at depth in the northern part of the zone.
Depth to groundwater in the Aparima GMZ typically ranges from less than 2 metres below ground level along the margins of the Aparima River, to >10 m below ground level under elevated terraces toward the valley margins. Seasonal variation in groundwater levels is generally <2 to 3 metres, reducing on lower terraces adjacent to the Aparima River.
Tertiary sediments underlying the Upper Aparima GMZ contain a limited groundwater resource.
The diagram below depicts a generalised conceptual hydrogeological understanding of the Upper Aparima GMZ.
- Average saturated thickness = 40 metres
- Approximate volume of groundwater storage = 3,900 million m3
- Mean residence time is approximately 4 years
Depth to groundwater
- 1-2 metres below ground along the margins of the Aparima River
- increasing to >10 m below ground level under higher terraces toward the valley margins
Seasonal groundwater variation
- <2 to 3 metres, reducing on lower terraces adjacent to the Aparima 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 in this zone comes from the infiltration of local rainfall and runoff from the surrounding hills.
- Average annual rainfall recharge: 290 mm per year
- Average annual rainfall recharge volume: 163 million m3 per year
Some interaction may occur between the Aparima River and riparian groundwater, with flow gains and losses occurring across intervening reaches.
Discharge predominantly occurs to the Aparima River and its tributaries. Some discharge to spring-fed streams occurs along the base of terrace risers.
Groundwater flow in the Upper Aparima GMZ generally occurs sub-parallel to the overall surface water drainage pattern.
Abstraction and water use
Groundwater is utilised for domestic and farm water supplies across the Upper Aparima 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
Natural groundwater quality in the Upper Aparima GMZ is generally good, however many areas now show moderate to high levels of contamination from land use activities. Groundwater generally contains low concentrations of dissolved ions. Hardness is low. Iron concentrations are generally low but elevated concentrations occur in places, particularly in older alluvial terrace deposits. Nitrate concentrations are variable. They are generally high in the east of the zone adjacent to the Aparima River, and lower in the west where reducing conditions are more prevalent.
Well drained soils along the margins of the Aparima River enable rapid drainage from the land surface to the underlying water table reducing the potential for contaminant attenuation in the soil zone. Imperfectly to poorly drained soils on higher terraces increase the potential for attenuation of nutrient concentrations via denitrification.
Elevated nitrate concentrations tend to occur on lower alluvial terraces where soils are well drained, and groundwater is generally oxidising.
Phosphorus is typically strongly bound to soils.
Microbial contamination of groundwater is typically limited by natural attenuation in the soil zone and underlying aquifers. Localised microbial contamination may occur where soils are well drained 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.
The main pathways for contamination to reach groundwater in areas of this zone that are not peat are via deep drainage (top left) and artificial drainage (top right).
The main pathways for contamination to reach groundwater in areas of peat are via deep drainage (bottom left) and artificial drainage (bottom right).
Water quality state summary
Redox state: variable, generally reducing under higher terraces, oxidizing under lower terraces
Nitrate: variable, with some areas having high concentrations
Microbial contamination: low, but localised risk may be elevated where soils are well-drained soils and the water table is shallow.
Major ions: hardness is generally low, elevated iron concentrations may occur under elevated terraces.
Water quality - human health
Main issues in this zone
- Nitrate: Groundwater quality in this zone may be compromised by elevated nitrate and microbial contamination levels in some locations.
- Microbial contamination: Microbial contamination may be elevated in areas where soils are well-drained and the water table is shallow.
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.