What is Groundwater?
Groundwater is a significant part of our environment and needs to be carefully managed.
Eighty percent of Australian people depend partially on groundwater. About 60% are totally dependent and for 20% groundwater is their main source of water. Groundwater is the principal supply of drinking water for many communities in Australia.
When people think of water they think of rain, dams, rivers, lakes, but rarely of groundwater.
The distribution of the world's water is as follows:
- Oceans 97.3 %
- Fresh water 2.7 %
The location of fresh water is:
- Frozen ice caps and glaciers 77.2 %
- Groundwater and soil moisture 22.4 %
- Other .4 %
In other words, 95% of the world's available, fresh water is groundwater.
Enormous
volumes of water occur underground. Unfortunately the groundwater is not
all good
quality nor uniformly distributed. Records of bores and wells reveal extreme
variations: depths from a few metres to some 1350 metres; yields from zero
up to 330 litres per second; and salinity levels from not much more than
that of rainwater to about three times that of seawater.
The largest groundwater deposit in Australia is the Great Artesian Basin.
This 1.7 million square kilometre basin was created more than 195 million
years ago. It underlies 22% of the Australian continent and is situated
under parts of ueensland,
South Australia, New South Wales and the Northern Territory.
How Groundwater Occurs
Because groundwater is out of sight, it is the least obvious part of the hydrologic cycle. When rain falls on the earth, some will run off directly to creeks and rivers, some will evaporate from the wetted surfaces of vegetation and soil, and some will soak into the ground. Most of the water in the ground will be retained in the upper soil horizons as soil moisture and will gradually either evaporate or be transpired by vegetation. But if infiltration conditions are favourable, and there is more than enough water to satisfy soil moisture requirements, some will gravitate into voids in the underlying rocks. Or, if there is no soil cover, some surface runoff will infiltrate the outcropping rocks. In either case, the voids in the rocks will fill to form a saturated zone, and the water in this zone is called groundwater. The upper surface of the zone, where the water is at atmospheric pressure, is called the water table. Above it is the unsaturated zone where water is being held against the force of gravity by molecular attraction to the soil particles.
Having entered the groundwater system, water does not stay in one place but moves downwards in response to gravity. The rate of movement is usually very slow. Once water enters a groundwater system, it may be there a very long time, sometimes tens of thousands of years, before it enters another part of the hydrologic cycle.
The water table may intercept surface water bodies such as swamps and
lagoons. Here, groundwater contributes to the surface water bodies and
can then return to the atmosphere by evaporation. If the water table gets
close to the soil surface,
groundwater can be transpired by vegetation, or capillary action may bring
it to the
surface from where it will evaporate.
Groundwater also discharges to rivers and streams. It is the contribution of groundwater that maintains the permanence of flow in the non-tidal reaches of streams, even in drought periods. Groundwater again becomes part of the surface water system from where it can evaporate from the river, be transpired by bank vegetation, or can be transported out to sea.
Underground water-bearing formations that can yield useful supplies of
water are called aquifers. The volume of water that can be held in an aquifer,
the rate at which water can get into it and be extracted from it, and the
rate at which water can move through it, are all controlled by the geology
of the aquifer. Variations in groundwater occurrence, salinity and yield
are due to interrelationships of geology, climate, topography, soils and
vegetation, but geology is the dominant factor. This means that even in
areas of high rainfall, and with other factors favourable, if the geological
conditions are unsuitable, groundwater will be correspondingly limited.
Conversely, with favourable geological conditions, substantial supplies
of good
quality water can be obtained in areas of low rainfall.