Edendale

The Edendale Groundwater Management Zone (GMZ) covers an area of approximately 12,400 ha on the Edendale Terrace in the Lower Mataura Valley

Topography: broad, flat alluvial terrace
Main surface water catchments: Ota Creek, Oteramika Stream, Ives Creek, Clear Creek

Boundaries – the western boundary follows the boundary between Q4 alluvium and the exposure of lignite measure sediments in the terrace riser marking the western margin of the Mataura River valley. The eastern boundary follows the terrace rise that marks the eastern margin of the Edendale Terrace.

Physical setting

The Edendale GMZ encompasses an extensive, flat-lying alluvial terrace that extends along the western margin of the Mataura River Valley between Glendhu Road (west of Mataura township) and Seaward Downs.

Aquifer type: Terrace

Geological setting

During successive interglacial and post-glacial periods during the late Quaternary, the ancestral Mataura River eroded and reworked existing sediments to form a broad valley extending up to 11 km wide in the Edendale area. The Edendale Terrace comprises an extensive remnant of a late Quaternary (Q4) alluvial terrace, which comprises poorly sorted quartz and greywacke gravel with accessory lenses of silt and clay (see diagram below). The alluvial deposits range in thickness from 10 to 30 metres, increasing in thickness along the central axis of the Edendale Terrace.

The alluvial deposits overlie a thick sequence of lignite measure sediments of the East Southland Group that comprise alternating layers of mudstone and lignite interspersed with sand and gravel. Lignite measure sediments exposed along the margin of the Mataura River Valley form the western boundary of the Edendale GMZ.

Mesozoic basement rocks of the Murihiku and Brook Street Terranes occur at depth (not shown in diagram below).

Hydrogeology

Soils overlying the Edendale GMZ range from well drained soils along the eastern side of the Edendale Terrace to poorly drained toward the western margin. Infiltration of water from areas of well drained soils is moderated by the presence of fine-grained loess deposits. These deposits range from 2 to 5 metres thick and overlie alluvial materials.

The alluvial materials of the Edendale Terrace host a spatially extensive unconfined aquifer system. Bore yields are moderate to high, particularly along the central axis where the alluvial deposits are thickest. Higher yielding intervals typically occur near the base of the gravel sequence.

Depth to groundwater ranges from over 10 metres below ground north of Edendale township to around 5 metres below ground at Seaward Downs. Groundwater levels exhibit relatively regular seasonal fluctuations reflecting the slow infiltration of land surface recharge through the overlying soils and loess deposits. Inter-annual groundwater level variations reflect long-term rainfall departure from normal.

Surface waterways, including Ota Creek and Oteramika Stream, are perched above the unconfined aquifer across the Edendale Terrace.

Tertiary lignite measure sediments comprising an alternating sequence of marginal marine (mudstone and lignite) and terrestrial (sand and gravel) sediments occur at depths ranging between 10 to 30 metres below ground level (bgl) across the Edendale GMZ. Isolated sand and gravel layers within the lignite measures host low yielding confined aquifers that are typically restricted in spatial extent.

The diagram below depicts a generalised conceptual hydrogeological understanding for the Edendale GMZ.

Water age

Mean residence time is generally less than 10 years

Depth to groundwater

>10 metres below ground level north of Edendale township, reducing to 5 metres below ground in the Seaward Downs area

Seasonal groundwater variation

Approximately 3 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

The majority of recharge to the Edendale GMZ is derived from infiltration of local rainfall and runoff from the surrounding hills. Throughflow from neighbouring aquifers is likely to be negligible.

Rainfall recharge: 270 mm per year
Average annual rainfall recharge volume: 33.5 million m3 per year

Discharge

Discharge from the Edendale GMZ predominantly occurs via discharge to spring-fed streams (Clear Creek and Ives Creek) that originate along the base of the Q4 terrace in the Seward Downs area. Surface waterways including Ota Creek and Oteramika Stream also gain baseflow along the base of the Edendale Terrace. Some throughflow may also occur to the Lower Mataura GMZ along the eastern boundary.

Groundwater flow

Groundwater in this zone generally flows from north to south, moving to a more south-easterly direction in the Seaward Downs area, reflecting discharge to spring-fed streams in this area.

Abstraction and water use

Groundwater is utilised for domestic and farm water supplies across the Edendale GMZ. Significant quantities of groundwater are also utilised for municipal, industrial and irrigation water supply.

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 Edendale 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 to moderate and concentrations of iron and manganese are generally low. Very high nitrate concentrations are observed in many parts of the Edendale GMZ.

Nutrients

Many areas of the Edendale GMZ exhibit moderate to very high nitrate concentrations. The zone has an increased potential for elevated groundwater nitrate concentrations, resulting from intensive land use. This is due to limited dilution from low nutrient surface waters, the relatively slow rate of groundwater thoughflow, and the oxidising conditions present in the unconfined aquifer.

Microbial contamination

Microbial contamination of groundwater is typically limited by natural attenuation in the soil zone and underlying aquifers. In the Edendale GMZ the potential for attenuation of microbial contaminants is increased due to the presence of slowly permeable loess deposits underlying the soil profile and the relatively deep water table. However, the use of soak holes to augment surface drainage has the potential to allow contaminants to bypass the soil zone and rapidly infiltrate to groundwater.

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 sources such as soak holes and septic tanks

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 is susceptible to elevated nitrate concentrations in areas of 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.