ABSTRACT
Water that infiltrates into the soil, if it is in sufficient amounts, moves downwards to percolate into the rocks beneath at a rate that is proportional to the amount of water in the soil and the ease with which water can move through the underlying rocks. Truly impermeable rocks are those through which water cannot move, even though they may hold a lot of water in the pore spaces. Clays are generally impermeable, but porous. When rocks take in or release water, they expand or contract in a way that can cause surface subsidence. The main effects of pumping out too much water are therefore:
• The water table may fall. This is a very dramatic signal that the withdrawal is beyond the ‘safe yield’. For large aquifers, the quantity of water extracted that will produce a measurable fall in the water table is enormous. Conversely, small aquifers are very sensitive. Care
has therefore to be taken with figures on rates of fall (usually expressed in metres per year), otherwise it is easy to exaggerate the effects of exploitation.
