Lower Aparima

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.

Physical setting

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

Geological setting

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).

Hydrogeology

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

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

Discharge

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 flow

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.

Groundwater quantity

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: