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# GG381-groundwaternotes1.pdf

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School
Wilfrid Laurier University
Department
Geography
Course
GG381
Professor
Andrew Medeiros
Semester
Fall

Description
GG381 HYDROLOGY Notes CH3 GROUNDWATER Groundwater is important for a transport mechanism, as well as storage of freshwater. The total 6 3 storage of groundwater on earth is ~ 8(10) km , which exceeds the capacity of all the world's rivers and lakes. This ground water is largely confined between grains of sediments, soils, and in small fissures in rock formations. Near the surface these spaces are partly filled with air, which is known as the vadose zone (zone of aeration). Below the vadose zone, where the pore spaces are saturated, lies the water table. The water table can establish at the level at which the pore water pressure in the ground equals the air pressure of overlying air (standard air pressure 101.325 kPa). The volume of soils that can be occupied by water is known as Porosity. If a column of soil 1 metre high has a porosity of P=0.4, the water itself would be a column 0.4 m high. Generally, the porosity of soils is about 20-40% and decreases with depth. Porosity can be related to a leaky sponge. A dripping sponge will not release all of its water by gravity alone. The percentage of water that can drip is known as the specific yield, Sy(Generally about 10-20%). When the soil has dripped all it can the amount of water remaining is known as the field capacity. This water is held by capillary action, which increases as grain size decreases. At field capacity plants can still extract water until they reach the wilting point. For most soils water volume at wilting point is < 10%. In a confined aquifer water can’t escape, available water is due to compressibility of the soil and water layer (and is less than the specific yield). This quantity is the Storativity, S; volume of water -3 -5 extracted from an aquifer per unit area and unit lowering of the head. S is typically 10 and 10 for a confined aquifer but equals specific yield, Sy for an unconfined aquifer. Why? Most soils (except fine clays) and some rocks, such as sandstone, are permeable. Even Crystalline rocks such as granite are permeable when riddled with joints and cracks (most are no
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