Tiwai

The Tiwai Groundwater Management Zone (GMZ) covers an area of approximately 2,400 ha encompassing the Tiwai Peninsula, which extends along the south coast, east of Bluff Harbour.

Topography: elongated barrier beach separating Awarua Bay from Foveaux Strait.
Main surface water catchments: n/a

Boundaries – generally follow the shoreline around the margin of the Tiwai Peninsula.

Physical setting

The Tiwai GMZ encompasses the Tiwai Peninsula, which comprises a barrier beach that separates Awarua Bay to the north from Foveaux Strait to the south, and forms the eastern entrance to Bluff Harbour.

Aquifer type: Terrace

Geological setting

The Tiwai GMZ comprises a surficial layer of well sorted quartz gravels (pea gravels) between 5 and 10 metres thick. These deposits grade into layers of sand and sand gravel that extend to a total depth of 20 to 25 metres (see diagram below).

The sand and gravel deposits of the Tiwai Peninsula represent an extensive accumulation of barrier beach sediments deposited as a result of longshore drift and wave action along the south coast. Similar deposits are recognised further inland across the Awarua Plain and elsewhere, representing various positions of the coastline resulting from Holocene sea level variations.

The Quaternary sediments overlie a thick (>100 metre thick) sequence of Tertiary lignite measure sediments of the East Southland Group. These lignite measure sediments comprise thick layers of carbonaceous mudstone interspersed with irregular layers of lignite, sand and gravel. These sediments in turn overlie sand, gravel and sandy limestone sediments of the Chatton Formation.

Hydrogeology

Soils in the Tiwai GMZ are typically coarse and well drained.

The sand and gravel deposits of the Tiwai GMZ host a shallow moderately to highly permeable unconfined aquifer system. This aquifer effectively forms a shallow freshwater lens underlying the Tiwai Peninsula, which is surrounded to the north and south by brackish to salty water.

Depth to groundwater ranges from 2 to 5 metres, increasing under higher elevation areas along the central axis of the Tiwai Peninsula. Seasonal variation in groundwater levels is generally <1 metre reflecting hydraulic connection to Awarua Bay and the south coast around the peninsula margin.

Tertiary sediments underlying the Tiwai GMZ contain a limited confined groundwater resource hosted in layers of sand and gravel.

The diagram below depicts a generalised conceptual hydrogeological understanding of the Tiwai GMZ.

Depth to groundwater

<1 metre below ground level around the coastal margin
increasing to 5 metres below ground level along the central axis of the Tiwai Peninsula

Seasonal groundwater variation

<1 metre

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 in this zone is derived from the infiltration of local rainfall.

Average annual rainfall recharge: 300 mm per year
Average annual rainfall recharge volume: 7.3 million m3 per year

Discharge

Discharge occurs to Awarua Bay and the south coast.

Groundwater flow

Groundwater flow in the Tiwai GMZ occurs perpendicular to the central axis of the Tiwai Peninsula.

Abstraction and water use

Groundwater is extensively utilised for industrial water supply in the Tiwai GMZ.

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 highlighted below:

More information about these effects is available in our guide to groundwater ecosystem health monitoring.

Water quality pressures

Groundwater quality in the Tiwai GMZ is generally good. Groundwater generally contains slightly elevated concentrations of dissolved ions (particularly sodium and chloride) reflecting marine aerosol deposition. Hardness is moderate and iron concentrations typically below detection. Nitrate concentrations are low due to natural state landcover across a majority of the zone.

The primary risk to groundwater quality in this zone is from saline intrusion, and industrial land use along the western margin.

Soils

Soils across the Tiwai GMZ are well drained, limiting the potential for attenuation of nutrient concentrations via denitrification.

Nutrients

Nutrient concentrations across a majority of the Tiwai Peninsula are low reflecting the natural state landcover.

Phosphorus is typically strongly bound to soils.

Microbial contamination

Microbial contamination of groundwater is typically limited by natural attenuation in the soil zone and underlying aquifers. Due to well drained soils and the coarse nature of the underlying gravel deposits the potential exists for microbial contamination to infiltrate from the land surface into underlying groundwater.

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 pathway for contamination to reach groundwater is via deep drainage.

Water quality state summary

Redox state: oxidising

Nitrate: low

Phosphorus: low

Microbial contamination: low, but localised risk may be elevated due to the well-drained soils, shallow water table and coarse alluvial deposits

Major ions: hardness is generally moderate, iron concentrations are low

Water quality - human health

Main issues in this zone

Salt water
Industrial waste
Microbial contamination

The main threats to water quality in this zone are salt water intrusion and contamination from industrial land use. There may be a risk of microbial contamination in some parts of the zone, due to the presence of well drained soils and coarse gravel deposits.

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.