The Longridge Groundwater Management Zone (GMZ) covers an elongated area of approximately 8,200 ha, located on the north-eastern margin of the Waimea Plain.
Topography: Flat-lying alluvial terrace along the southern margin of the Mataura River Valley.
Main surface water catchments: Longridge Stream
Boundaries – the northern boundary follows the approximate contact large terrace riser marking the southern extent of the Mataura River Valley. The southern boundary follows the approximate alignment of a north-west to south-east trending ridge in the basement rock underlying the alluvial deposits.
The Longridge GMZ occupies a flat-lying Q8 alluvial terrace that extends along the north-eastern margin of the Waimea Plain. The terrace is bordered to the south by a partially exposed bedrock ridge, which follows a north-west to south alignment across the Waimea Plain. West of Mandeville this structure forms a catchment divide between the Mataura River to the north and the Waimea Stream to the south.
Aquifer type: Terrace
The subsurface geology of the Longridge GMZ comprises a relatively thin layer of late Quaternary alluvium (Q8 – mQa) overlying Tertiary and basement rocks. The alluvial deposits typically consist of moderately to highly weathered, compact, silty, sandy gravel. The thickness of the alluvial materials ranges from 10 to 40 metres, generally increasing to the west.
Across the western portion of the Longridge GMZ the alluvial deposits are underlain by Tertiary sediments of the East Southland Group (Chatton Formation, Forest Hill Formation), which are exposed at the surface near sandstone.
Basement rocks of the Dun Mountain-Maitai Terrane are exposed at the surface along the southern boundary, east of Dunn and Cody Road and occur at shallow depths (<10 metres below ground level) across much of the eastern section of the Longridge GMZ (see diagram below).
Soils types in the Longridge GMZ are generally moderately well drained. Soils formed in a thin layer of loess overlying gravelly alluvium.
The Longridge 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. Across the eastern section of the Longridge GMZ (i.e. the Sandstone Stream catchment) groundwater availability may be restricted due to the limited thickness and low permeability of the alluvial materials.
Depth to groundwater typically ranges from 5 to 7 metres below the ground surface, becoming shallower toward the western extent of the zone. Seasonal groundwater level variation generally ranges between 2.5 and 3.5 metres. Groundwater levels are generally highest in winter and lowest in early autumn.
Tertiary sediments and Dun Mountain basement rocks underlying the Longridge GMZ host a limited groundwater resource.
The diagram below depicts a generalised conceptual hydrogeological understanding of the Longridge GMZ.
Depth to groundwater
- 5 to 7 metres below ground level, decreasing to the north of Balfour township
Seasonal groundwater variation
- 2.5 to 3.5 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 to the Longridge GMZ is primarily derived from infiltration of local rainfall.
- Rainfall recharge: 162 mm per year
- Average annual rainfall recharge volume: 13.4 million m3 per year
A majority of discharge is inferred to occur via throughflow to the Riversdale GMZ.
Some groundwater discharge occurs via small spring-fed streams (including the McKellar Stream), which originate along the northern margin of the Longridge GMZ. Groundwater discharge also maintains limited baseflow in the Sandstone Stream. The Longridge Stream appears to be perched above the unconfined aquifer for much of its course across the Longridge GMZ.
Groundwater in this zone generally flows in a north-easterly direction towards the Riversdale GMZ.
Abstraction and water use
Groundwater is utilised for domestic and farm water supplies across the Longridge GMZ. However, bore yields from the shallow unconfined aquifer are generally modest.
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 Longridge GMZ is generally good, however many areas now show moderate to 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. Moderate to high nitrate concentrations are observed in many parts of the Longridge GMZ.
Soils in the Longridge GMZ are typically moderately well drained, with limited potential to reduce nutrient concentrations through denitrification.
Groundwater in the unconfined aquifer is oxidising. Combined with limited recharge from low nutrient surface waters and the low rate of groundwater throughflow, these factors increase the potential for elevated nitrate concentrations in the unconfined aquifer in response to intensive land use.
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 Longridge 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 high
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 and microbial contamination levels in some locations.
- 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.