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Groundwater systems (hydrogeology)

An understanding of groundwater flow systems is essential to understanding salinity.

Water that is not used by plants seeps through the soil until it reaches a layer that will not let it pass any further. This is usually a layer of impervious rock or clay. The porous rocks and soil pores begin to fill with water from the bottom up, forming a saturated zone. There may be more than one saturated zone separated by layers of impervious rock. These saturated zones are called aquifers.

Water contained in the saturated zone is called groundwater and the top layer of the highest saturated zone is the watertable. The process of water entering the groundwater system is called recharge and water leaving the system is called discharge.

Usually the watertable remains many metres below the earth's surface. However, human activities such as land clearing, urban development, over-irrigation and disruption to natural drainage lines can increase groundwater recharge and contribute to rising watertables.

As the watertable rises, it dissolves salt stored in the soil and carries it to different parts of the landscape. Eventually, saline water reaches the land surface, usually at lower positions in the landscape. Salts are then concentrated at the surface as a result of evaporation.

Rising groundwater by itself does not automatically cause salinity, but the wide distribution of saline soils in Australia means it is highly likely that rising groundwater will mobilise salt stored in the soil.

Local groundwater flow systems

Salinity is often a local problem, affecting a few properties over a relatively small area. A local groundwater flow system is usually less than 5 km long and the flow direction has a similar pattern to surface drainage. Rainfall recharges the groundwater system through open rock fractures at the surface in highland areas, as well as through better drained soils in the mid and lower slopes. This water enters the soil and rock fractures and flows under the influence of gravity towards an adjacent area low in the landscape where the groundwater discharges.

Local groundwater flow systems usually respond quickly to both recharge events (ie rainfall) and remedial actions such as tree planting at a recharge site. Response times are generally less than 10 years, but could be as short as 2 years.

Intermediate groundwater flow systems

The flow path of an intermediate groundwater flow system is usually between 5 and 50 km. Intermediate groundwater flow systems have a greater storage capacity than local systems and take longer to 'fill' following increased recharge. The response times for intermediate groundwater flow systems are generally between 10 - 50 years, but may be as long as 100 years.

Regional groundwater flow systems

Regional groundwater systems extend over larger areas and go to much greater depths than local groundwater systems. Groundwater movement is usually independent of local landforms, being determined by the regional geology. The following characteristics are typical of a regional system:

• groundwater flow paths may be long (to hundreds of kilometres) and deep;
• volumes of stored groundwater may be very large;
• recharge and discharge areas may be far apart (possibly hundreds of kilometres);
• they may be overlain by a number of local groundwater systems;
• the waters may be mineralised or hot, eg mineral springs or thermal pools;
• response times may be measured in hundreds of years.

In NSW, there are many local, intermediate and regional groundwater flow systems. The local and intermediate systems predominate where dryland salinity is a problem, particularly in the tablelands and slopes. This is not the case in other parts of Australia such as Western Australia, so one must be cautious when applying solutions based on experience from elsewhere.