Blackmount

The Blackmount Groundwater Management Zone (GMZ) covers approximately 14,000 ha and encompasses a series of alluvial terraces adjacent to the middle reaches of the Waiau River, the Wairaki River and lower Lill Burn.

Topography: broad, flat-lying alluvial terraces separated by steep terrace risers
Main surface water catchments: Waiau River, Wairaki River, lower Lill Burn

Physical setting

The Blackmount GMZ encompasses a series of elevated alluvial terraces that occur along the margins of the Waiau River. Terrace age and elevation above the Waiau River decrease step-wise towards the active river channel. For example, the relatively broad river flats in the areas around Otahu Flat, Dean Burn and Clifden contain a sequence of up to six distinct alluvial terraces that step down towards the current river channel.

Aquifer type: Terrace

Geological setting

The Waiau River valley follows a depression formed by faulting in the underlying bedrock (see diagram below).

The alluvial terraces of the Blackmount GMZ increase in age from recent (Q1) alluvium adjacent to the major rivers to older, weathered alluvium (Q8+) of remnant terraces along the valley margins.

The water-bearing alluvial deposits can be up to 25 metres thick in places and are underlain by Tertiary sediments of the Waiau Group that comprise mudstone, siltstone, conglomerate and limestone in places.

Hydrogeology

Soils overlying intermediate to lower terraces are generally well-drained, enabling water to readily infiltrate from the land surface into underlying aquifers. Imperfectly to poorly-drained soils occur on higher terraces where surface run-off may increase.

Alluvial deposits along the margins of rivers and streams typically comprise coarse sand and gravel that is moderate to high yielding, while older alluvial deposits underlying higher terraces along the margins of valley generally contain a higher proportion of fine material (fine sand, silt and clay) and exhibit lower yields.

Tertiary siltstone and mudstone sediments of the Waiau Group are generally not water-yielding. However, a limited aquifer resource may be found in some limestone areas. The diagram below depicts a generalised conceptual hydrogeological understanding for the Blackmount GMZ. Water yield is highest adjacent to the main rivers, and lowest on the higher terraces.

Water age

Approximate residence time= 7 years (variable)

Depth to groundwater

Higher terraces: 5 to 10 metres below ground level
Lower terraces: <2 metres below ground level

Seasonal groundwater variation

2 to 3 metres, less close to main rivers

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

The majority of recharge comes from the infiltration of local rainfall and runoff from surrounding hills. Some recharge may occur from large tributaries of the Waiau River.

Rainfall recharge: 440 mm per year

Average annual rainfall recharge volume: 21 million m3 per year

Discharge

Discharge predominantly occurs as baseflow to the Waiau River. Some discharge also occurs to local springs, which often occur along the base of terrace risers where the water table intercepts the land surface.

The Blackmount GMZ is unlikely to receive from, or discharge groundwater throughflow to, neighbouring zones.

Groundwater flow

Groundwater in this zone generally flows from north to south, following the topographic gradient. At a finer scale, groundwater is likely to flow from the higher terraces down towards the centre of the valley. Some localised variation in flow direction may occur along the margins of major tributaries.

Abstraction and water use

Groundwater is utilised for domestic and farm water supplies across the Blackmount GMZ. Larger takes may be constrained in the vicinity of surface waterways due to potential effects on surface water flows.

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

Natural groundwater quality in the Blackmount GMZ is typically good. Groundwater generally contains low concentrations of dissolved ions. Hardness is low to moderate, and concentrations of iron and manganese are generally low. Elevated nitrate concentrations may be associated with intensive land use.

Soils

Soils in the Blackmount GMZ are typically well-drained, with elevated potential for bypass flow. Bypass flow occurs when water flows through open channels in the soil (such as cracks or fissures), rather than through the soil itself. This reduces the potential for the soil to attenuate or 'filter' contaminants carried in the water. These contaminants can then enter the groundwater below.

Nitrate

Nitrate concentrations can be high in groundwater where there is intensive land use, due to:

oxidising conditions in the soil zone and underlying aquifers,
limited dilution from external river sources, and
moderate rates of groundwater throughflow.

Groundwater under lower alluvial terraces may be less susceptible to elevated nitrate concentrations due to greater dilution by river recharge.

Microbial contamination

Microbial contamination of groundwater is typically limited by natural attenuation in the soil zone and underlying aquifers. Microbial contamination may occur in shallow groundwater where soils have an elevated potential for bypass flow 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.

Water quality state summary

Redox state: oxidising
Nitrate: elevated in areas of intensive land use
Phosphorus: low
Microbial contamination: low, but risk can be elevated close to source in coarse-grained aquifers
Major ions: moderate hardness; iron and manganese are typically 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.
The susceptibility of groundwater to nitrate contamination likely reflects the predominance of oxidising conditions in the thin unconfined gravel aquifers.
Elevated terraces in this zone are at a higher risk of nitrate contamination as they receive minimal recharge (and 'flushing') from streams or rivers.

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.