A groundwater basin is an underground reserve of water which may take the form of a single aquifer or a group of linked aquifers. Because groundwater reserves are important, many governments allow government agencies to take over the management of groundwater basins, when they are identified, in the interests of keeping groundwater supplies clean and safe. It is usually possible to obtain a map of known groundwater basins in a particular region from a government agency concerned with geology, hydrology, or general safety of water supplies.
It can sometimes be difficult to determine where the boundaries of a groundwater basin lie. In some cases, there are clear geologic boundaries such as deposits of rock strata which make it clear where the deposit of water begins and ends, and can be used to find out whether or not aquifers are contiguous. In other instances, the distinction may be less subtle, making it challenging to identify which areas are actually within the groundwater basin.
Another issue which can complicate the classification of a groundwater basin is the existence of aquifers at various depths. In these cases, subbasins may be designated to indicate where differing supplies of groundwater can be found. This can be critical when making decisions about groundwater allocation, as one area of a groundwater basin may be able to support heavier usage than another due to the existence of deep aquifers which extend below the level of the aquifer in the rest of the basin.
One of the big concerns with a groundwater basin is the risk of groundwater contamination. Contaminants spilled in the basin can work their way into the aquifer, spreading to contaminate all of the water. In the best case, this adds some filtering to the steps needed to process the water. In the worst case, it renders the water unusable and means that people need to seek out alternative water supplies. It can also result in long-term damage as the pollutants work into surface waterways and other aquifers in the area.
Another concern is overutilization. While groundwater does recharge, it takes time, and if people take out water more quickly that the aquifer can recharge, a water shortage can develop. As water drops below critical levels, it may sink below the current depths of wells, making it inaccessible. Thus, hydrologists study groundwater basins to find out how much water is available, and to determine how much can be safely used.