The Lower Aparima Groundwater Management Zone (GMZ) covers an area of approximately 29,000 ha in the lower reaches of the Aparima River catchment.
Topography: Flat to rolling alluvial terraces in the lower reaches of the Aparima River catchment.
Main surface water catchments: Aparima River, Pourakino River, Opouriki Stream
Boundaries – the western boundary follows the approximate contact between Quaternary alluvial deposits and basement rocks of the Longwood Hills. The eastern boundary follows the distinct terrace riser marking the lateral extent of the Waimatuku Stream catchment. The southern boundary follows the margin of Jacobs River Estuary west of Riverton, and a late Quaternary marine terrace to the east.
The Lower Aparima GMZ encompasses remnants of a late Quaternary (Q8) alluvial terrace that is been partially dissected by the Aparima River and its tributaries. To the west the terrace grades onto alluvial fans along the eastern margin of the Longwood Hills. The zone also includes the Jacobs River Estuary at the mouth of the Aparima River.
Aquifer type: Terrace
The Lower Aparima GMZ encompasses remnant terraces of a mid-Quaternary (Q8) alluvial terrace that has been locally reworked due to post-glacial entrenchment of the Aparima River and tributaries. The alluvial deposits comprise moderately to highly weathered silty, sand gravel that range in thickness from 10 to 40 metres. Gravels increase in thickness toward the current alignment of the Aparima River.
The Quaternary alluvium overlies Tertiary mudstone and limestone sediments of the East Southland Group (Forest Hill Formation, Winton Hill Formation, Gore Lignite Measures). Limestone deposits of the Forest Hill Formation crop out in the terrace marking the eastern margin of the Lower Aparima GMZ between Otautau and Isla Bank. These limestone deposits occur at shallow depth (<10 m below ground level) under the alluvial terrace between Isla Bank and the Aparima River. Limestone deposits also underlie Quaternary alluvium on the western side of the Aparima River between Gropers Bush and Gummies Bush.
Basement rock consists of greywacke of the Murihiku Terrane and volcaniclastic sediments of the Brook Street Terrane (see diagram below).
Soil types on higher alluvial terraces in the Lower Aparima GMZ are generally moderately-well to well drained soils formed in loess deposits. Imperfectly to poorly drained soils often occur along the margins of the surface water drainage network.
Quaternary alluvial deposits in the Lower Aparima GMZ contain a variable groundwater resource. Bore yields are typically low, reflecting the weathered nature of the alluvial deposits but increase where they have been reworked along the margins of the Aparima River. In some areas (such as Isla Bank) the low permeability and limited saturated thickness of the alluvial materials limit the viability of this resource.
Areas of limestone of the Forest Hill Formation host a spatially extensive groundwater resource across large parts of the Lower Aparmia GMZ. This aquifer system is hosted in secondary permeability (joints and fracturing) within the limestone rock mass, and typically forms a low to moderate yielding unconfined to semi-confined aquifer (depending on depth and hydraulic properties of the overlying alluvial materials). In the Isla Bank area, this limestone aquifer hosts the primary groundwater resource. To the west of the Aparima River, water-bearing limestone of the Forest Hill Formation occurs at depth and forms a moderately permeable, spatially extensive, semi-confined aquifer system.
Depth to groundwater ranges from less than 2 metres below ground on the Aparima River floodplain to 20 metres below ground in limestone aquifers underlying higher alluvial terraces. Seasonal groundwater level variations are typically less than 2 metres in alluvial deposits but may be up to 10 metres in limestone aquifers, reflecting the low effective porosity of these materials.
The diagram below depicts a generalised conceptual hydrogeological understanding of the Lower Aparima GMZ.
Depth to groundwater
- <2 to 20 metres below ground, increasing in limestone aquifers under higher alluvial terraces.
Seasonal groundwater variation
- <2 metres in alluvial aquifers; up to 10 metres in limestone aquifers
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 Lower Aparima GMZ is primarily derived from infiltration of local rainfall, augmented by limited infiltration of runoff from the Longwood Hills.
- Rainfall recharge: 330 mm per year
- Average annual rainfall recharge volume: 92.6 million m3 per year
Surface waterways draining higher alluvial terraces are generally perched above the unconfined aquifer due to the depth to groundwater.
Groundwater discharge maintains flow in a number of spring-fed streams that originate at the base of the terrace at the lateral margins of the Aparima River flooplain. Spring discharges also occur from limestone sediments on the flanks of the Longwood Ranges. Baseflow to the lower reaches of the Aparima River also occurs from the surrounding alluvial aquifer.
Groundwater discharge is also inferred to occur to the Jacobs River Estuary and the south coast.
Groundwater in this zone generally flows obliquely towards the Aparima River, following the general topographic gradient.
Abstraction and water use
Groundwater is utilised for domestic and farm water supplies across the Lower Aparima GMZ. The limestone aquifer in the Gummies Bush area is also utilised for municipal supply.
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
Groundwater quality in the Lower Aparima GMZ is variable. Groundwater generally contains low concentrations of most dissolved ions. However, elevated Calcium and Bicarbonate concentrations occur in limestone aquifers and contribute to elevated hardness. Concentrations of iron and manganese are generally low. Nitrate concentrations are generally low to moderate, although concentrations may be elevated in some shallow limestone aquifers.
Soils in the Lower Aparima GMZ are typically moderately well drained, with limited potential to reduce nutrient concentrations through denitrification.
Groundwater in the unconfined aquifer is oxidising. Combined with limited recharge from low nutrient surface waters and the low rate of groundwater throughflow, these factors increase the potential for elevated nitrate concentrations to accumulate in the unconfined aquifer in response to intensive land use.
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 Lower Aparima GMZ is typically low due to the relatively deep water table and 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 pathways for contamination to reach groundwater in this zone are via deep drainage (left) and artificial drainage (right).
Water quality state summary
Redox state: oxidising
Nitrate: generally low but may be elevated, especially in shallow limestone aquifers
Microbial contamination: low, but risk can be elevated close to source
Major ions: variable hardness (elevated in limestone aquifers); 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 and microbial contamination levels in some locations.
- Groundwater quality in this zone may be compromised by elevated nitrate concentrations associated with intensive land use.
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.