The Cattle Flat Groundwater Management Zone (GMZ) covers an area of approximately 3,000 ha. It is located within the Mataura River valley, situated between Ardlussa and the Nokomai Gorge.
Topography: flat alluvial terrace along the riparian margin of the Mataura River, including some higher terrace remnants along the valley margins
Main surface water catchments: Mataura River, Tomogalak Stream, Sheepwash Creek
The Cattle Flat GMZ encompasses the alluvial deposits that infill the Mataura River valley between Parawa and Ardlussa. These deposits include the recent floodplain of the Mataura River as well as older remnant alluvial terraces along the valley margins.
Tomogalak Stream and Sheepwash Creek are the major surface water tributaries in the Cattle Flat GMZ. Both waterways drain the foothills of the Mataura Range, which lie to the west.
Aquifer type: Riparian
The Cattle Flat GMZ encompasses the alluvial deposits of various ages that lie within the Mataura River valley upstream of Ardlussa. These gravel deposits exhibit variable thickness and generally comprise a relatively thin (<10 m) sequence of sandy gravel overlying claybound gravels.
The alluvial gravels overlie semischist basement of the Caples Terrane. The alignment of the southern portion of the Mataura River valley appears to be significantly influenced by two prominent outcrops of basement rock along the western margin of the Wendonside Terrace.
The geological setting of the Cattle Flat GMZ is depicted in the diagram below.
Soils overlying the Cattle Flat GMZ generally comprise thin, alluvial soils that are well to excessively drained, enabling water to readily infiltrate from the land surface into underlying groundwater.
Alluvial deposits along the margins of the Mataura River and main tributaries typically comprise coarse sandy gravel that is moderate to high yielding. 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.
Depth to groundwater ranges from less than 2 metres below ground along the margins of the Mataura River increasing to >5 metres below ground under higher alluvial terraces. Seasonal groundwater level variation is typically less than 2 metres.
The diagram below depicts a generalised conceptual hydrogeological understanding for the Cattle Flat GMZ. It is noted the hydrogeology of this zone is not particularly well defined due to the limited amount of data available.
Depth to groundwater
- 2 to 5 metres below ground level, increasing under higher terraces
Seasonal groundwater variation
- Less than 2 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 Cattle Flat GMZ is sourced from rainfall and infiltration of runoff from the surrounding hills. Some recharge may also occur via flow loss from tributary streams where they exit the foothills onto the Mataura River floodplain.
- Rainfall recharge: 223 mm per year
- Average annual rainfall recharge volume: 6.8 million m3 per year
- Discharge from the Cattle Flat GMZ occurs as baseflow to the Mataura River.
- Some small spring-fed streams originate from groundwater discharge along terrace risers toward the valley margins.
- There is also likely to be a significant degree of interaction between the Mataura River and adjacent riparian aquifer. Groundwater recharge/discharge may occur locally adjacent to the Mataura River according to relative river stage.
Groundwater flow in the Cattle Flat GMZ is inferred to occur largely sub-parallel to the Mataura River.
Abstraction and water use
Groundwater is utilised for domestic and farm water supplies in parts of the Cattle Flat GMZ. Bores typically exhibit moderate to high yields reflecting the permeable nature of the shallow alluvial deposits.
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:
Water quality pressures
Natural groundwater quality in the Cattle Flat GMZ is generally good, reflecting the recharge contribution from the Mataura River and tributary streams. Groundwater contains low concentrations of dissolved ions similar to those occurring in river water. Hardness is typically low and concentrations of iron and manganese are below detection. Nitrate concentrations are typically low to moderate.
Soils in the Cattle Flat GMZ are typically well drained, with elevated potential for bypass flow. This reduces the potential for natural attenuation of contaminants contained in recharge from the land surface infiltrating to groundwater.
Oxidising conditions are prevalent in the thin, unconfined aquifers. However, the potential for elevated nitrate concentrations to occur as a result of intensive land use is mitigated by the high rate of groundwater throughflow in the unconfined aquifer.
- Phosphorus is typically strongly bound to soils and occurs in low concentration in groundwater.
Microbial contamination of groundwater is typically limited by natural attenuation in the soil zone and underlying aquifers. Local microbial contamination may occur in areas of free draining soils 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.
Water quality state summary
- Redox state: oxidising
- Nitrate: low to moderate
- Phosphorus: low
- Microbial contamination: low, but risk can be elevated close to source
- Major ions: low hardness, low iron and manganese
Water quality - human health
Main issues in this zone
- Groundwater quality in this zone may be compromised by microbial contamination, particularly close to source.
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.