Waihopai

The Waihopai Groundwater Management Zone (GMZ) covers an area of approximately 42,000 ha, encompassing the Waihopai River and Waikiwi Stream catchments on the Southland Plains.

Topography: extensive flat to undulating alluvial terraces locally dissected by a lower order drainage network.
Main surface water catchments: Waihopai River, Waikiwi Stream, Otepuni Creek

Boundaries – follow the surface water catchment boundaries of the Waihopai River, Waikiwi Stream and Otepuni Stream.

Physical setting

The Waihopai GMZ occupies a spatially extensive, late Quaternary alluvial terrace (referred to as the Kamahi Terrace) that extends from Invercargill to Gore. The terrace surface is dissected by an extensive surface water drainage network to form the characteristic undulating to gently rolling topography of the Eastern Southland downlands.

Aquifer type: Lowland

Geological setting

The near-surface geology of Waihopai GMZ comprises a sequence of mid-Quaternary (Q8) alluvium deposited by the ancestral Mataura River. Due to their age and fluvioglacial nature, the gravel deposits consist of poorly sorted quartz sand and gravel in a highly weathered clay matrix. The alluvial deposits vary in thickness between 20 and 40 metres.

The Quaternary alluvium overlies a thick sequence (>200 metres) of Tertiary lignite measure sediments (Gore Lignite Measures) of the East Southland Group. These sediments comprise alternating layers of carbonaceous mudstone and lignite interspersed with localised sand and gravel layers. The lignite measure deposits are underlain by sand and shellbed deposits of the Chatton Formation.

Mesozoic basement rocks of the Murihiku Terranes occur at depth (see diagram below).

Hydrogeology

Soils in the Waihopai GMZ generally comprise imperfectly to well drained silt loam soils formed in deep wind-blown loess deposits accumulated across the Kamahi Terrace surface.

The alluvial deposits of the Waihopai GMZ host a spatially extensive unconfined aquifer system. Due to the highly weathered nature of the alluvial materials bore yields are typically low to moderate. The rolling topography of the Waihopai GMZ forms the characteristic setting for a lowland aquifer system where the major component of groundwater flow occurs to local rivers and streams. A limited, deeper component of groundwater circulation follows the general catchment drainage pattern.

Depth to groundwater varies across the Waihopai GMZ. In lower-lying areas along the margins of the surface drainage network the water table typically occurs 2 to 3 metres below ground level. Depth to groundwater increases to over 10 metres below ground level under intervening ridges. Seasonal groundwater levels generally vary between 2 to 3 metres following temporal variations in land surface recharge. The magnitude of seasonal groundwater level variation may decrease along the margins of the surface drainage network.

Locally significant, low yielding confined aquifers occur in sand and gravel layers of the lignite measure sediments within the Waihopai GMZ. The occurrence and extent of such aquifers varies spatially across the zone.

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

Water age

Mean residence time ranges from 2 to 5 years for most of the zone
Mean residence time ranges from 5 to 10 years for small areas across the zone

Depth to groundwater

typically 2-3 metres below ground level adjacent to the surface drainage network
increasing to >10 metres below ground level under intervening ridges

Seasonal groundwater variation

typically 2 to 3 metres, decreasing along the margins of the surface drainage network

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

Groundwater recharge in the Waihopai GMZ is derived from infiltration of local rainfall.

Average annual rainfall recharge: 304 mm per year
Average annual rainfall recharge volume: 127 million m3 per year

Discharge

A majority of groundwater discharge occurs via baseflow to the surface drainage network. In places this drainage is augmented by artificial drainage networks, particularly in lower-lying areas. Some of this deeper groundwater throughflow may discharge directly to the New River Estuary.

Groundwater flow

Shallow groundwater flow in the Waihopai GMZ generally occurs obliquely toward the surface drainage network. A small component of deeper groundwater flow may occur to the south-west following the overall surface water drainage pattern.

Abstraction and water use

Groundwater is utilised for domestic and farm water supply in the Waihopai 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 Waihopai GMZ is variable. Groundwater generally contains low concentrations of dissolved ions. Hardness is low. Iron concentrations are generally low but can be elevated in some areas of the unconfined aquifer. Iron concentrations are typically high in water-bearing layers within the lignite measure deposits. Nitrate concentrations are variable.

Soils

The potential for contaminants to drain from the land surface to the underlying water table in the Waihopai GMZ is moderated by a combination of imperfectly drained soils, surficial loess deposits and the relatively thick unsaturated zone in places.

Nutrients

Groundwater nitrate concentrations in the Waihopai GMZ are variable. In places, the combination of well drained soils, oxidising groundwater and low rate of groundwater throughflow results in elevated nitrate concentrations in the unconfined aquifer. Elsewhere, this is moderated by imperfectly drained soils and interaction with reducing sediments and groundwater of the underlying lignite measure sediments. In general nitrate concentrations are higher in the northern half of the zone where more oxidising soils dominate.

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. The potential for microbial contaminants to be attenuated in the soil zone and underlying groundwater is increased by the thick, fine grained nature of overlying soils and the low rate of groundwater throughflow.

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: oxidizing or mixed

Nitrate: moderate to high

Phosphorus: low

Microbial contamination: low, but localised risk may be elevated close to source

Major ions: hardness is low, iron concentrations are variable in the unconfined aquifer and high in lignite measure aquifers

Water quality - human health

Main issues in this zone

Nitrate:Nitrate levels can be elevated where there are well drained soils, oxidising groundwater, and a low rate of groundwater throughflow.
Microbial contamination: Groundwater quality in this zone may be compromised by elevated nitrate and microbial contamination levels in some locations.
Iron and manganese: Iron concentrations are variable in unconfined aquifer resources and higher in lignite marine aquifers.

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.