Ewan Ko

Chapter 1

Ground water

If a hole is dug, only the water that flows freely into the hole is groundwater. Since

the air in the hole is at atmospheric pressure, the pressure in the groundwater must

be higher if it is to flow freely into the hole. This is what distinguishes groundwater

from the rest of the underground water.

Definition 1.1. Water, below the surface, at a pressure greater than atmospheric

pressure, which thus flows freely into a hole through interconnected void spaces,

is groundwater.

Groundwater and soil water together make up approximately 0.5% of all water

in the hydrosphere. Figure 1.1 illustrates the various zones of water found beneath

the surface. Water beneath the surface can essentially be divided into three zones:

1) the soil water zone, or vadose zone, 2) an intermediate zone, or capillary fringe,

and 3) the ground water, or saturated zone. The top two zones, the vadose zone and

capillary fringe, can be grouped into the zone of aeration, where during the year

air occupies the pore spaces between earth materials. Sometimes, especially during

times of high rainfall, those pore spaces are filled with water.

Vadose zone

Pw < P a

Capillary fringe

Water table Pw = Pa

Fig. 1.1 Soil moisture and

distribution of fluid pressure

Pw relative to atmospheric Pa

in the ground.

Saturated zone

Pw > P a

Beneath the zone of aeration lies the zone of saturation, or the groundwater zone.

Here water constantly occupies all pore spaces. The water table divides the zone of

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1 Ground water

aeration from the zone of saturation. The elevation of the water table is determined

to be where the pore water pressure, Pw , is equal to atmospheric pressure, Pa . The

height of the water table will fluctuate with precipitation, increasing in elevation

during wet periods and decreasing during dry. In general, the water table has an

undulating surface which generally follows the surface topography, but with smaller

relief.

1.1 Porosity

Ground water and soil moisture occur...