The Riversdale Groundwater Management Zone (GMZ) covers an area of approximately 11,000 ha, encompassing an extensive Q2 alluvial terrace in the middle reaches of the Mataura catchment.
Topography: flat alluvial terrace.
Main surface water catchments: Mataura River, Meadow Burn
Boundaries – extend from the channel of the Mataura River to the prominent alluvial terrace marking the northern edge of the Waimea Plains west of Mandeville.
The Riversdale GMZ encompasses the extensive flat-lying Q2 alluvial terrace on the true right (southern) bank of the Mataura River, between Ardlussa and the Otamita Bridge.
Aquifer type: Riparian
The subsurface geology of the Riversdale GMZ consists of a typical sequence of heterogeneous alluvial gravels associated with a moderate to large braided river. The gravel deposits comprise of moderately to poorly sorted gravel, claybound gravel, and sand and silt. Deposits extend to a depth of up to 30 metres below ground level to the west of the Riversdale township, and reduce to less than 10 metres towards Mandeville (see figure below).
Tertiary sediments of the East Southland Group (Gore Lignite Measures) underly the Quaternary alluvium across the Riversdale GMZ, cropping out at the land surface at locations along the southern boundary. These sediments comprise thick layers of carbonaceous mudstone interspersed with localised layers of sand and gravel.
Basement rock consists of sandstone and volcaniclastic sediments of the Dun Mountain-Maitai Terrane, and semi-schist of the Caples Terrane.
A majority of soil types in the Riversdale GMZ are alluvial, shallow, and well drained. Water is able to readily infiltrate from the land surface into underlying groundwater.
The Q2 alluvial deposits in the Riversdale GMZ host a spatially extensive unconfined aquifer system that is hydraulically connected to the Mataura River and local spring-fed streams. Bore yields are generally high, declining toward the southern margin of the Q2 terrace.
Depth to groundwater ranges from 1 to 3 metres below ground level across a majority of the Riversdale GMZ. Seasonal groundwater level variations reflect temporal rainfall variability and are generally less than 1 metre, reflecting the hydraulic connection to surface water.
Low yielding confined aquifers in occur in localised sand and gravel layers within the Gore Lignite Measure sediments underlying the Quaternary alluvium.
The diagram below depicts a generalised conceptual hydrogeological understanding of the Riversdale GMZ.
- Mean residence time is approximately between 7 and 11 years
Depth to groundwater
- 1 to 3 metres below ground, decreasing toward the Mataura River
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 to the Riversdale GMZ is derived from infiltration of local rainfall and riverine recharge from the Mataura River. Flow measurements indicate loss of between 1.2 to 2.0 m3 per second between Ardlussa and the Riversdale Bridge. A majority of this recharge is interpreted to flow through the Riversdale GMZ.
- Rainfall recharge: 170 mm per year
- Average annual rainfall recharge volume: 18.7 million m3 per year
The Riversdale GMZ also receives throughflow from the Longridge GMZ to the west.
Groundwater discharge from the Riversdale GMZ predominantly occurs via baseflow to the Mataura River and a number of spring-fed streams that originate across the Q2 terrace. A flow gain approximately 25 percent higher than that occurring over the upstream section of the Mataura River is observed between Pyramid and the Otamita Bridge.
Groundwater flow in the Riversdale GMZ occurs in a south-easterly direction toward the downstream reach of the Mataura River.
Abstraction and water use
Groundwater is extensively utilised for domestic, farm and irrigation water supplies across the Riversdale GMZ.
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:
Water quality pressures
Natural groundwater quality in the Riversdale GMZ is generally good, however many areas now show moderate to very high levels of contamination from land use activities. Groundwater contains low concentrations of dissolved ions reflecting the significant recharge contribution from the Mataura River. Hardness is low, and iron and manganese concentrations are typically below detection. Nitrate concentrations are generally lower in areas close to the Mataura River due to high throughflow rates, but are commonly high in areas away from the river and associated with intensive land use.
Soils across the Riversdale GMZ are well drained, limiting the potential for attenuation of nutrient concentrations via denitrification.
Groundwater in the Riversdale GMZ is oxidising. This means there is limited potential for denitrification in groundwater once it has passed/infiltrated through the soil zone. However, the potential for accumulation of elevated nitrate concentrations can be mitigated by the high rate of groundwater throughflow.
Phosphorus is typically strongly bound to soils.
Microbial contamination of groundwater is typically limited by natural attenuation in the soil zone and underlying aquifers. Due to the well-drained nature of soils, there is potential for microbial contaminants to be transported through the soil zone into groundwater where 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.
The main pathway for contamination to reach groundwater is via deep drainage.
Water quality state summary
Redox state: oxidising
Nitrate: low in areas close to the Mataura River, generally moderate to high away from the river
Microbial contamination: low, but risk can be elevated close to source, particularly on well drained soils
Major ions: hardness, iron and manganese are low
Water quality - human health
Main issues in this zone
- Nitrate: There is potential for elevated nitrate concentrations in areas of intensive land use where groundwater is oxic.
- Microbial contamination: Groundwater quality in this zone may be compromised by elevated nitrate and microbial contamination levels in some locations.
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.