Knapdale

The Knapdale Groundwater Management Zone (GMZ) covers an area of approximately 4,900 ha on the east side the Mataura River. The zone extends along the northern margin of the Mataura Valley, between Pyramid and East Gore.

Topography: series of undulating alluvial terraces
Main surface water catchments: Otama Creek, Okapuka Creek, Gold Creek

Boundaries – the northern boundary follows the approximate contact between Quaternary alluvium and Gore Lignite Measure sediments, and older Gore Piedmont Gravels. The southern boundary follows the northern boundary of the Croydon GMZ.

Physical setting

The Knapdale GMZ occupies a series of partially dissected elevated alluvial terraces along the northern margin of the Mataura River Valley, between Otama and Gore. These alluvial terraces are elevated above the recent Mataura River floodplain and grade into rolling hill country to the north.

Aquifer type: Terrace

Geological setting

The subsurface geology of the Knapdale GMZ consists of sequence of alluvial terraces that decrease in age and elevation (Q2, Q4 and Q6+) as they descend toward the recent floodplain of the Mataura River. These alluvial deposits are typically moderately to highly weathered and contain a relatively high percentage of fine-grained material (silt and fine sand) in the gravel matrix, particularly on the older, elevated terraces. Depth of the alluvial materials ranges from less than 5 metres to 40 metres, generally increasing under older, elevated terraces toward the valley margins.

Quaternary gravels overlie a thick sequence of Tertiary Gore Lignite Measure sediments, comprising of mudstone with layers of siltstone and lignite. These sediments are up to 200 metres thick in places.

Underlying basement rock comprises sandstone and volcaniclastic sediments to the south of the Dun Mountain-Maitai Terrane (see diagram below).

Hydrogeology

Soils overlying the Knapdale GMZ are generally imperfectly to moderately well drained, formed in a thin layer of loess overlying gravelly alluvium.

The Knapdale GMZ contains an unconfined aquifer system hosted in the alluvial gravel materials. Bore yields are typically low reflecting the weathered nature of the alluvial materials.

Depth to groundwater typically ranges from 2 to 10 metres below the ground surface, increasing in depth under higher alluvial terraces. Seasonal groundwater level variation is typically of the order of 1 to 1.5 metres. Groundwater levels are generally highest in winter and lowest in early autumn.

Sand and gravel layers within the Tertiary lignite measure sediments contain a limited groundwater resource.

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

Water age

Mean residence time is approximately 3 years near the Mataura River and longer in more distal parts of the zone.

Depth to groundwater

2 to >10 metres below ground, increasing under higher alluvial terraces

Seasonal groundwater variation

1 to 1.5 metres

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 Knapdale GMZ is primarily derived from infiltration of local rainfall.

Rainfall recharge: 159 mm per year
Average annual rainfall recharge volume: 7.8 million m3 per year

Discharge

Most surface waterways in the Knapdale GMZ are perched above the unconfined aquifer due to the depth to groundwater, which is over 5 metres below ground level.

A majority of groundwater discharge occurs via throughflow to the Croydon GMZ along the riparian margin of the Mataura River. Some groundwater discharge occurs as baseflow to streams such as the Okapua Creek and Gold Creek, as they flow across the lower elevation Q2 terrace.

Groundwater flow

Groundwater in this zone generally flows in a south-westerly direction following the topographic gradient towards the Mataura River.

Abstraction and water use

Groundwater is utilised for domestic and farm water supplies across the Knapdale GMZ. However, bore yields from the shallow unconfined aquifer are generally modest.

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:

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

Water quality pressures

Natural groundwater quality in the Knapdale GMZ is generally good, however, many areas now show moderate to very high levels of contamination from land use activities. Groundwater generally contains low concentrations of dissolved ions. Hardness is typically low and concentrations of iron and manganese are generally low. Very high nitrate concentrations are observed in some parts of the Knapdale GMZ.

Soils

Soils in the Knapdale GMZ are typically imperfectly to moderately well drained, with limited potential to reduce nutrient concentrations through denitrification.

Nutrients

Groundwater in the unconfined aquifer is oxidising. Combined with little 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.

Many areas of the Knapdale GMZ exhibit moderate to very high nitrate concentrations.

Microbial contamination

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 Knapdale 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: moderate to very high

Phosphorus: low

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

Major ions: low to moderate hardness; 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 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.