The Aerated Intake Zone

A typical well passes through two zones an upper zone, usually of soil-covered rock, that contains air and water in pores and fissures and a lower zone, often of rock, that contains only water in pores and fissures. This separation into two parts is observed everywhere, and the two zones have been named the aerated (air-containing) zone and the saturated (water-saturated) zone. The top of the saturated zone (where water is hit in a well) is called the water table (Fig. 2.1). The water table is described either in terms of its depth from the surface or as the altitude above sea level (section 2.2). 

The aerated zone is also called the vadose zone (from Latin vadosus, shallow). Its thickness varies from zero (swamps) to several hundred meters (in regions of elevated and rugged topography and in arid climates), but it is commonly 5-25 m. In most cases the aerated zone has two parts soil at the top and a rocky section beneath. 

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Intake of water in the aerated zone is either by infiltration into the soil cover or, on bare rock surfaces, by infiltration into intergranular pores (as in sand-stone), fissures and joints (as in igneous rocks or quartzite), or dissolution conduits and cavities (limestone, dolomite, gypsum, rock salt). Only pores and fissures that are interconnected, or communicate, are effective to infiltration. 

Water infiltration in a sandy soil may be described as water movement in a homogeneous granular medium (or as in a sponge). Water descends in this setting in downward-moving fronts, or in a piston-like flow (Fig. 2.2). In such a mode of flow a layering exists, the deeper water layers being older than the shallower ones. Measurements of tritium and other contaminants 

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