Groundwater occurs where the soil and underlying parent material are saturated (see Figure 19.3) and the water table marks the top of this saturated zone, which fluctuates over the seasons, normally being much higher in winter. In wetlands the water table is very near the soil surface and the land is not suitable for horticulture until the water table of the whole area is lowered (see drainage). Where water flows down the soil profile and is impeded by an impermeable layer, such as saturated clay or silty clay, a perched (or crown) water table is formed. Water from above cannot drain through the impermeable barrier and so a saturated zone builds up above it. Springs appear at a point on the landscape where an overlying porous material meets an impermeable layer at the soil surface, e.g. where chalk hills or gravel mounds overlie clay.
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