GROUNDWATER RESOURCES OF THE SOUTH EAST OF SOUTH AUSTRALIA

F. Stadter, Principal Geologist, Primary Industries and Resources SA

1. INTRODUCTION

The South East of South Australia can be described as the major groundwater province in the State, with its general availability of large supplies of relatively good quality groundwater.

There is a large reliance on these groundwater resources for municipal and individual domestic supplies, and stock, irrigation and industrial uses, given the general absence of reliable surface water supplies.

Management of the groundwater resources in parts of the South East region has been progressively introduced since the early 1970s to overcome longer term problems, such as groundwater depletion and particularly groundwater quality deterioration resulting from irrigation water use.

The main groundwater resources in the South East are essentially contained within two regionally extensive aquifer systems - an upper unconfined aquifer and a deeper confined aquifer. These two aquifers are usually separated vertically by a clay aquitard.

2. THE UNCONFINED AQUIFER

2.1 Groundwater Use

The unconfined aquifer is the principal source of groundwater in the South East given:

• its relatively shallow depth
• the general availability of large supplies of groundwater from the aquifer of up to, and even exceeding in some cases, 300 litres per second
• the groundwater quality is generally suitable for a range of uses
• the relatively low economic cost in developing groundwater supplies in comparison with the deeper confined aquifer.

Groundwater from the unconfined aquifer is used for a number of different purposes - ranging from municipal supplies for towns such as Mount Gambier and Millicent, individual domestic and stock water supplies, industrial use with particular high demand by pulp and paper mills near Millicent, and widespread irrigation usage throughout the South East region.

2.2 Hydrostratigraphy

The unconfined aquifer can occur within a number of different geological formations, with the predominant unit being the regionally extensive Gambier Limestone of Tertiary age.

The Gambier Limestone is generally a richly fossiliferous marine limestone, with interbeds of marl, calcite and dolomite, and flint horizons. Extensive dissolution of the limestone has occurred in some areas through the actions of infiltrating meteoric water, groundwater flow and historic sea level fluctuations. Surface expressions of such karstic features are common in a number of areas, particularly in the Mount Gambier district with its numerous sinkholes. The thickness of the Gambier Limestone varies regionally, with the largest vertical sequence of about 300 metres occurring in the area south of Mount Gambier, whilst in other areas north of Kingston the unit is absent.

The Gambier Limestone aquifer generally has a dual porosity, with a primary inter-granular porosity which acts hydraulically as a porous medium and a secondary fracture porosity resulting from dissolution of the limestone. This secondary porosity forms conduits for preferred flow.

Other important lithological units of the unconfined aquifer are the Quaternary age Padthaway and Bridgewater Formations. These units are predominantly found in the northern part of the South East and are associated with the alternating lateral sequence of dune ranges and inter-dunal flats.

The Padthaway Formation occurs exclusively within the inter-dunal corridors and its lithology can vary from a rubbly limestone to marl and silt. This formation is characterised by its marked secondary porosity with corresponding high well yields. The formation has a maximum thickness of about 15 metres.

The Bridgewater Formation occurs predominantly within the dunal ranges and comprises a spatial mix of unconsolidated calcareous sand and sandstone, with some limestone interbeds. This formation can attain saturated thicknesses of about 40 metres. The formation can also have a well developed secondary porosity.

All the units described above, plus some other less significant horizons, are generally hydraulically connected and exhibit a wide variation in hydraulic properties.

2.3 Groundwater Salinity

There is a considerable variation in the groundwater salinity of the unconfined aquifer through the South East. The main features of the spatial distribution of groundwater salinity are:

• the general occurrence of low salinity groundwater (less than 500 mg/L) in the southern part of the region
• the general distribution of groundwater salinity ranging from 500 to 1500 mg/L through the central and eastern parts of the region
• the gradation to an area of higher groundwater salinity in the northwestern part of the region (where salinities can approach and even locally exceed seawater salinity of 35000 mg/L). The higher groundwater salinities found in this area are likely to be a result of a combination of lower vertical recharge rates, reduced aquifer flushing in an area where the aquifer thickness is relatively small, increased evaporative discharge and possibly historic surface water discharge to the area.

Distinct local variations in groundwater salinity are common throughout the region and are largely the result of spatial variability in aquifer recharge and discharge processes.

2.4 Groundwater Flow Regime

The groundwater flow regime for the unconfined aquifer in the South East is characterised by:

• the general east to west direction of groundwater flow in the central and northern parts of the region, with flow across the State Border from western Victoria into South Australia
• the general north to south direction of groundwater flow in the southern part of the region
• northwest to southeast trending zones of steeper hydraulic groundwater gradients - in the area from Millicent through Mount Gambier to the State Border, and in the area from Padthaway through Naracoorte to the State Border. These are related to geological faults respectively referred to as the Tartwaup and Kanawinka Faults.
• a groundwater divide occurring in the Nangwarry Tarpeena area north of Mount Gambier.

With the large spatial variability in the transmissivity of the unconfined aquifer, there will be a corresponding variability in the groundwater velocity. Rates of groundwater flow ranging from 50 to 100 metres per year can generally be expected.

Local variations in groundwater flow directions can occur and are largely the result of localised aquifer recharge or discharge impacts.

2.5 Aquifer Recharge

Recharge to the unconfined aquifer can occur by a number of different processes, such as:

• from groundwater inflow from up-gradient areas. This is referred to as lateral groundwater recharge and is governed by the hydraulic gradient and the transmissivity of the aquifer.
• from the infiltration of a proportion of the annual rainfall through the soil profile into the aquifer. This is termed diffuse recharge and its magnitude is principally controlled by factors such as the amount of rainfall, nature of soil type, depth to the water table and type of vegetative land cover.
• from direct discharge of surface water into the aquifer in some areas through naturally occurring sink-holes or specifically constructed drainage wells. Direct recharge can also occur by localised infiltration of surface water from the numerous swamps that occur in the area. These forms of recharge are termed point recharge.
• from upward leakage of groundwater from the deeper confined aquifer. This is governed by the hydraulic gradient between the aquifers and the vertical hydraulic conductivity of the aquitard which separates the aquifers.

The vertical recharge received by the unconfined aquifer comprises the total of the diffuse recharge and any point recharge.

Given the variability in rainfall, soils, depth of water table and vegetation cover throughout the South East, the vertical recharge will vary according to the significance of these factors.

Studies undertaken over the last few years have indicated that vertical recharge rates can range from 2 to 130mm per annum.

3. THE CONFINED AQUIFER

3.1 Groundwater Use

Groundwater from the confined aquifer is utilised mainly by the agricultural industry and for municipal water supplies. In the last few years there has been some increase in the use of the confined aquifer for the provision of water for a small number of aquaculture developments.

Artesian supplies of groundwater of up to 100 litres per second are available from the confined aquifer in parts of the South East, with the main artesian area occurring in the general Kingston - Lucindale - Beachport district. In this area there is a reliance on the confined aquifer for water supplies for irrigation, stock and domestic requirements, and some aquaculture enterprises.

Large scale extraction of groundwater for the irrigation of pasture for fodder and fat lamb production in the artesian area accounts for a significant percentage of the water used from the aquifer.

There are other districts in the South East where the confined aquifer groundwater is used for irrigation purposes, but these are relatively small developments which are usually located in areas where it is difficult to obtain adequate supplies from the unconfined aquifer (eg the Glencoe district).

A number of towns in the South East (eg Naracoorte, Kingston, Robe and Kalangadoo) rely on the confined aquifer for a reliable and contaminant free water supply. The confining clay beds which generally separate the confined and unconfined aquifer systems offer some protection to the confined aquifer from any potential groundwater pollution.

The potential for aquaculture development in the South East using the confined aquifer has been realised in recent years, given the general low salinity of the groundwater and its higher temperature relative to that of the unconfined aquifer. Temperatures of the groundwater at the well head are generally 22°C but can be as high as 30°C as individual well completion depth increases.

3.2 Hydrostratigraphy

The confined aquifer occurs within an interbedded sequence of sands, gravels and clays referred to as the Dilwyn Formation, which is of early Tertiary age. Minor sand horizons of the Mepunga Formation, which overlies the Dilwyn Formation, are also generally considered to be part of the confined aquifer.

The aquifer is essentially an unconsolidated, quartz-rich sand and gravel which was deposited in a shallow marine, deltaic environment. There are both lateral and vertical variations in grain size distribution which can determine individual well yields and aquifer properties.

There is generally a clay unit at the top of the Dilwyn Formation which forms part of the aquitard separating the confined and unconfined aquifers. Clay and marl units that occur at the base of the Gambier Limestone and within intermediate units between the Dilwyn Formation and the Gambier Limestone also form part of the aquitard.

The Dilwyn Formation aquifer sequence increases in thickness towards the south of the region, with a maximum recorded thickness of 800 metres off-shore.

Available hydraulic data for the aquifer is sparse but does not exhibit the large variability as observed for the unconfined aquifer.

3.3 Groundwater Salinity

The groundwater salinity of the confined aquifer indicates little variation over a large portion of the region and ranges from about 500 mg/L near Mount Gambier to about 800 mg/L in the Kingston area. North of Kingston, the confined aquifer groundwater salinity increases quite rapidly to about 10 000 mg/L in an area where the Dilwyn formation is discontinuous against a shallow basement high.

3.4 Groundwater Flow Regime

The potentiometric head (pressure) distribution for the confined aquifer indicates that regional groundwater flow in the confined aquifer is quite similar to that of the unconfined aquifer and largely emanates from recharge areas in western Victoria, together with some recharge in the general Nangwarry area.

The potentiometric head difference between the confined aquifer and the unconfined aquifer indicates that in the western and most southerly parts of the region, there is potential for upward leakage from the confined aquifer into the unconfined aquifer. Elsewhere in the region, there is potential for downward leakage from the unconfined aquifer into the confined aquifer.

Potentiometric head measurements in the artesian area show a significant seasonal drawdown in head, generally over the period from October to March each year, and this is due to the groundwater extractions. Two main cones of drawdown in the areas of more intense irrigation development are evident. Seasonal conditions have a major influence on the drawdown of the potentiometric head, and in years with a longer than usual irrigation season (such as in 1989/90) the cone of drawdown can exceed 14 metres.

A longer term decline in potentiometric head of about 0.3 metres per annum is being observed in the southern part of the artesian irrigation area.

3.5 Aquifer Recharge

Recharge to the confined aquifer is of two forms - from lateral groundwater inflow from up-gradient areas and from downward leakage of groundwater from the unconfined aquifer. The latter is considered to occur in the eastern part of the region and particularly in the Nangwarry area north of Mount Gambier.

It is considered that the recharge rates are relatively small and in the order of a few millimetres per annum.

4. MANAGEMENT OF THE UNCONFINED AQUIFER GROUNDWATER RESOURCES

4.1 Management Issues

The main management issues in relation to extraction of groundwater from the unconfined aquifer are the potential for a permanent lowering of the water table, the seasonal impact of mutually interfering water supply extractions and groundwater quality deterioration associated with re-cycling of groundwater used in irrigation developments. The latter is considered to be the most serious issue facing both current and future users of the groundwater resources from the unconfined aquifer.

Such management problems are presently being experienced in the South East, as illustrated by the following:

• in parts of the Padthaway irrigation area, a longer term increase in groundwater salinity has occurred over the last twenty years. The original groundwater salinity of 800 mg/L has increased to about 1800 mg/L in some areas, and threatens the longer term viability of some irrigated viticultural developments.
• a marked deterioration in groundwater quality with salinity increases of up to 300 mg/L per year and an associated decline in groundwater levels of about 1 metre every ten years have occurred in the irrigation area around Keith in the Upper South East.
• a permanent decline in groundwater levels of about 14 metres has been experienced in the Snuggery area south of Millicent due to large groundwater extractions, in the order of 14 000 ML per annum, for established pulp and paper mills.

Another important management issue throughout the South East is the protection of groundwater quality. The relatively high rainfall, the general occurrence of permeable soils and shallow depths to groundwater can result in a high risk of groundwater contamination for the unconfined aquifer. Groundwater pollution has been detected at various sites in the South East as a result of both localised point source and more widespread diffuse contamination.

Another significant issue in the Upper South East is the problem of dryland salinity and water-logging. An increase in the vertical recharge rates to the aquifer brought about by the clearance of native vegetation and the loss of previously established high water use perennial pastures has caused groundwater levels to rise, with increased deposition of salts at the land surface. An estimated 250 000 hectares of land in the Upper South East are experiencing these problems.

4.2 Groundwater Management Approach

The management approach that has been adopted for the unconfined aquifer in the South East is to maintain groundwater allocations at the level of vertical recharge within defined management areas.

This approach maintains the maximum possible lateral groundwater throughflow in order to minimise the effects of any longer term deterioration in groundwater quality, especially for irrigators and other groundwater users located down-gradient of areas with intense irrigation development.

5. MANAGEMENT OF THE CONFINED AQUIFER GROUNDWATER RESOURCES

5.1 Management Issues

The main management issues for the confined aquifer are a longer term decline in potentiometric head and seasonal drawdowns resulting from large scale groundwater extractions, and the poor condition of a number of supply wells in the artesian irrigation area.

A longer term decline of potentiometric head in the southern part of the main artesian area and extending as far south as the Millicent to Penola road is evident in some monitoring wells. A long term reduction in potentiometric head may result in a permanent reversal in the head difference between the confined and unconfined aquifers. This would allow groundwater from the often more saline unconfined aquifer to leak downwards into the confined aquifer, thereby causing a deterioration of its groundwater quality.

The earliest artesian wells were drilled in the 1960s and the majority of the wells drilled from this time to about 1975 were poorly constructed, with either an inadequate amount of well-casing or with well-casing that was not cemented to ensure isolation of the separate aquifers.

Various investigations have shown that these early constructed wells are allowing uncontrolled flow of groundwater from the confined aquifer into the unconfined aquifer. The magnitude of this leakage is difficult to assess and has not been quantified. This leakage will increase with time as the condition of the well-casing in individual wells continues to deteriorate. There are about 120 known wells of such condition.

In addition to this sub-surface leakage, there are a number of artesian wells which have poorly maintained or inadequate head-works with the groundwater flowing to waste during part or all of the year.

5.2 Groundwater Management Approach

Investigations are currently being undertaken to establish sustainable volumes of groundwater extraction within defined management areas in the region.

Individual applications for groundwater use from the confined aquifer, apart from those for domestic and stock water supplies, are currently hydrogeologically assessed by determining the likely drawdown in potentiometric head and the impacts that this may have on the groundwater resource and established users of the confined aquifer.