Groundwater is underneath most land and is the most available freshwater on earth.
Overall 22% of the world's freshwater is groundwater. In the Northern Territory (NT) 90% of the water supply comes from groundwater.
There are around 35,000 water bores in the NT.
The quality of groundwater changes in each place so not all groundwater is drinking water.
Aquifers are the formations made of fractured rock, cavernous limestone, gravel, sand and sandstone.
The groundwater is stored in and moves through many small interconnected spaces or fractures.
Groundwater comes from rainfall and is an important part of the water cycle.
Water not used by vegetation filters through the soil until it reaches the saturated zone. This is called groundwater recharge.
A lot of recharge can be expected during heavy rain.
The water table or level in an aquifer will rise and fall depending on variations in recharge.
Aquifers can also get water from rivers and streams draining into the ground.
Overuse of groundwater poses a high risk to springs, soaks and rivers.
Groundwater levels are low for the Darwin Rural area.
To find out more go to the Darwin Rural Groundwater Watch on the Department of Environment and Natural Resources website.
The Wet and Dry seasons in the Top End mean that rain is very seasonal, with the majority of rain falling in the Wet Season.
It is this regular recharge from rainfall that allows sustainable groundwater use.
In Central Australia rainfall is low and can happen at any time of the year.
Aquifers can also be recharged by seepage through river beds and flood out areas.
In some cases, recharge rates are so slow they are measured in geological time.
It is in these regions that groundwater resources can be limited due to the lack of rain.
There are three main types of aquifer in the NT.
Fractured rock aquifers
These are made of hard rocks where the groundwater is stored in the fractures, joints and bedding planes.
These rock types have minor porosity.
Bores in these aquifers give low to moderate yields.
Fractured and cavernous aquifers
These are made of limestone or dolostone that is often cavernous.
They can be highly productive. Springs like those at Mataranka are the outlets for this type of aquifer and they often keep stream flows for part or all of the Dry Season.
Sedimentary basin aquifers (porous)
These are made of porous sediment such as sand, gravel and sandstone.
Groundwater is stored in and moves through the spaces between the sand grains. These are not as common in the NT as in other parts of Australia.
Two key examples in the NT are the Great Artesian Basin (a small part of this is beneath the Simpson Desert) and the Ti-Tree Basin. Such aquifers can be productive.
Groundwater levels are measured by the standing water level in monitoring bores.
Groundwater levels within an aquifer vary as groundwater moves from high to low zones under the impact of gravity and with recharge from rainfall.
Groundwater levels that change slowly over time are measured by measuring/plopping the distance from the top of the bore to the water level in the bore.
Where groundwater levels are dynamic, ground water levels are measured on an hourly basis by instrumented bores.
The current monitoring program has around 150 instrumented bores and 250 plopped bores.
You can get data from past and current sites at the water data portal.
Last updated: 20 February 2017