Groundwater is the water located beneath the Earth's surface and is in direct contact with the soil. Much of our water supply is under the surface. These waters are used as sources of drinkingwater or for irrigation in agriculture.
The impermeable layer is a layer of rock with low permeationcapacity, so it does not allow water to pass through. The water found between the surface and the impermeable layer is called groundwater, while water under the impermeable layer is called an aquifer.
Groundwater is found above the topmost impermeable layer, where water accumulates via run-off and seepage. It is located at a depth of 2–5 m (6.6–16.4 ft) on average. The groundwater level depends on the weather and it directly affects agriculture. After high rainfall, the groundwater may rise so much that it fills the deeper areas of the surface. This is called groundwater flooding. In the event of low rainfall, the groundwater level drops. Toxic substances may infiltrate from the surface into the groundwater, so it is not suitable for human consumption. The underground flow of groundwater may transport these substances a long distance, so it is extremely important to protect groundwater.
Confined aquifers are located below and betweenimpermeable layers. They make up most of the groundwaters. Confined aquifers are a main source of drinking water, as they are not polluted easily. However, they replenish themselves more slowly.
An artesian aquifer is an aquifer betweentwo impermeable layers containing water under positive pressure. This typically occurs in basins. When an artesian well is made, the upper impermeable layer is drilled through and the water rises through the hole due to the natural pressure.
In karst areas, the source of karst water is the water seeping through fractures in the rock.
Bank-filtered waters occur next to surfacewaters. Here the water of the river is filtered by the layersofpebbles and sand on the river bank.