About the Catchment Area

Groundwater
While the majority of South Australians source their water from surface water resources, groundwater is a vital resource in many parts of the State. Groundwater supplies approximately 65% of all irrigation demand in SA.

In this section we will look at:
About groundwater
Groundwater flows
Aquifers and aquifer types
Groundwater as a resource
Threats to groundwater
Managing the use of groundwater

About groundwater
Groundwater is by far the world's largest source of fresh water. It is estimated that the amount of groundwater is 400 times greater than all the surface water in lakes, reservoirs, streams, and rivers. Groundwater is part of the water cycle. When precipitation falls on land, some water evaporates, some flows to streams and rivers, and some seeps into the soil and is absorbed by plant roots.

Excess water in the soil may percolate further down until it reaches a level known as the water table where all the pores or openings in the soil or rock are saturated with water. Water in the saturated zone below the water table is called groundwater.

Most of the spring and mineral water supplies are obtained from groundwater resources of the Mount Lofty Ranges. Soft-drink and beer manufacturers pump high quality water from the deep Tertiary aquifer beneath Adelaide. Groundwater is also pumped for industrial applications and for irrigation of recreational and sports fields throughout the metropolitan area. Irrigation for horticultural production was undertaken early in Adelaide’s development and still operates in regions around Adelaide.

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Groundwater flows
Groundwater moves very slowly through aquifers - usually less than one metre per day - until it seeps into low lying areas, streams, lakes, wetlands or the ocean. A spring is groundwater that has come to the surface. About 40% of the flow of Australian streams comes from groundwater. In turn, rivers and lakes can contribute large amounts of water to an aquifer. The process of water entering an aquifer is termed groundwater recharge. Groundwater is a renewable resource, but the ability to replenish or recharge it is limited.

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Aquifers and aquifer types
A porous rock layer which is capable of yielding useful supplies of water is called an aquifer. Aquifers are essentially layers of sand or fractured rock in which the pore spaces or fractures are filled with water. Less permeable rocks that restrict the flow of groundwater are termed confining beds (or aquitards).

An unconfined aquifer is an aquifer that extends from the land surface down to a confining base. It is generally only partially filled with water. When a bore penetrates below the water table, water will not rise up the hole. This is because the water pressure at the water table is at atmospheric pressure.

A confined aquifer exists where an aquifer is overlain by a confining bed. The confining bed prevents the upward movement of groundwater. These aquifers are usually fully saturated with water. Groundwater in confined aquifers is commonly under pressure, therefore when a bore intersects the aquifer, water can rise up the bore.

The level to which the water will rise is termed the potentiometric surface. Sometimes water will rise above ground resulting in a flowing (artesian) bore. Multiple aquifers refer to groundwater systems where two or more aquifers are present at a given site. In these systems permeable aquifers are separated by lower permeable confining beds. The groundwater in each of the aquifers may exhibit significantly different pressure regimes and water chemistry.

The pressure in the confined aquifers is normally above the top of the aquifer horizon, and the difference in pressure regimes can result in intermingling of waters from different zones via bores. The direction of inter-aquifer flow depends on the pressure differential between the aquifers.

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Groundwater as a resource
Groundwater is increasingly being seen as an alternative to surface water supplies. Unlike surface water supplies losses through evaporation are low and is groundwater is buffered against climatic variability, especially drought. It is a vast resource with relatively constant chemistry and temperature and is usually biologically pure. The most common method to tap groundwater is from a bore. Bores can be located at the location where the water will be used therefore saving on piping costs. The costs of tapping groundwater are low and associated with drilling, construction and pumping. The main disadvantages of using groundwater are that over-extraction can occur, water chemistry can be high (saline water) and once groundwater is polluted it is difficult to treat or remedy.

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Threats to groundwater
Urban and rural communities literally live above their groundwater resources, therefore it is important to recognise that activities, in terms of waste disposal and land and water use, may adversely affect groundwater resources.

Apart from salinity, pollution also affects groundwater quality and may have a negative effect on the water. Potential sources of pollution include microbiological contaminants from sewage and effluent, heavy metals, petroleum fuels and solvents, nutrients (phosphates and nitrogen) salt and detergents. These are potentially significant pollutants and their occurrence is widespread.

The high risk of contamination by these compounds is because they are very mobile (as leachate) in groundwater environments. Herbicides can pollute shallow groundwater systems if not managed correctly and high nitrate levels may result from concentrated livestock excretion, the use of nitrogenous fertilisers and nitrogen-fixing plants. Inappropriate land use management and activity is often a significant cause of this diffuse-source pollution.

Groundwater is a significant part of the environment and needs to be managed and cared for in the same way that it is important to plant trees, not to light fires on total fire ban day and to dispose of rubbish properly.

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Managing the use of groundwater
Groundwater systems have generally established, over times past, a balance between the rates of inflow (or recharge), outflow (or discharge) and the volume of water stored within the aquifer. This is called the water balance (or budget) of a groundwater system. Any new use (or extraction) of groundwater from a system will cause a change in this balance. There is a need to utilise these groundwater resources to foster economic development throughout the state. Management of these resources is crucial to ensure that these systems will continue to provide quality water into the future. Groundwater resource management aims to secure longevity of the State’s groundwater systems by using the understanding of how these systems operate and establishing the sustainable groundwater yield for each system.

Where the use of groundwater is concentrated or strategic, the resources have been protected by Prescription under the Water Resources Act 1997.

The Adelaide and Mt Lofty Ranges NRM Board is responsible to Water Allocation Plans within its region. These plans are developed with the local community and irrigators to ensure that valuable groundwater resources are protected and used sustainably.

Unfortunately the level of development of some of the State’s groundwater resources already exceeds their estimated sustainable yield and these planning tools should eventually redress this. Other examples of improper use of groundwater systems have occurred where industrial wastes have been discharged into the aquifers through bores as a means of waste disposal. Examples of this occurred in Adelaide where septic effluent was discharged into the shallow aquifers and in the South East where wastes from diary product factories were disposed of into drainage bores. These practices are now prohibited.

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Information sourced from-
SA Department of Water, Land and Biodiversity Conservation - Resource Assessment, Fact Sheet 5 - Groundwater

Vic Department of Natural Resources and Environment - Randal Nott, Catchment & Water Resources - Groundwater Notes - May 1998.