Groundwater is rain and snow melt that percolates into the ground and accumulates to a depth that depends on the amount of rainfall and/or snowmelt. This depth is known as the water table, which can rise and fall with the changing of the seasons.
Groundwater moves downhill, and where erosion cuts below the water-table level, there will be leakage of groundwater into a stream area (or, if the flow is low, there will be springs and seeps).
Groundwater moves very slowly; if pollutants are poured into the ground and reach the water table, it might take centuries for the pollutants to be flushed from the local area.
As depicted in the diagram above, each water well that takes out groundwater lowers the water table in the immediate vicinity of the well; if too many water wells are pumping out groundwater, then the water table will drop an the wells will have to be deepened = an expensive undertaking.
The topic of my next post concerns what happens to groundwater in limestone terrains.
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