6246lect09_S10 - Advanced Environmental Geochemistry GLY...

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Advanced Environmental Geochemistry, GLY 6246, ©David Warburton, 2010 1 LECTURE 9 - Soil Water and Groundwater Introduction When precipitation reaches the earth's surface it initiates a series of reactions that change the surface and change the chemistry of the water itself. The two factors that most profoundly affect the chemistry of the precipitate and the earth's surface are biogeochemical cycling and weathering reactions. These processes are responsible for the formation of soil . The presence of soil distinguishes the earth from other planets and moons that have only regolith . Regolith is merely broken bedrock. The chemistry of the regolith is essentially identical to the rock from which it formed. Soil has been altered and is often different from the parent rock material. It is also living in the sense that it is home to countless microorganisms that affect the soil. One goal of geochemistry is to make accurate estimates of the inputs and outputs from various cycles. Rock weathering has been extensively studied in this regard. It is very important not only in soil formation but also in estimating the flow of several substances into surface, soil and ground waters. Before discussing biogeochemical cycling and weathering reactions, a few terms need to be introduced to make discussion easier. When rainfall hits the earth's surface, it may enter the soil in a process called infiltration . Such water is then called soil water . Soil water gradually works its way downward toward the water table , which separates the vadose zone , or zone of aeration, from the phreactic zone , or zone of saturation. Lect09, slide 2 here In the vadose zone the soil water occupies some empty, or pore, spaces. The rest of the pore space is occupied by gas, a mixture of air and gases released by the soil organisms. Below the water table in the phreactic
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Advanced Environmental Geochemistry, GLY 6246, ©David Warburton, 2010 2 zone the pore spaces are entirely filled with what is called ground water. The term ground water also may be applied to any water below the surface of the earth but we will not use that definition in this course. While in the vadose zone, soil water may be taken in by plant roots. The plant then pumps the water upward until it is above the surface in the tissues of the plant. Water is often then lost through the process of transpiration , or evaporation from the plant's surface. Water lost through transpiration is returned to the atmosphere. Rain falling to the earth's surface may strike plants before reaching the surface, in a process called interception , which is defined as the capture of precipitation by the plant canopy. Lect09, slide 3 here It may subsequent return to the atmosphere through evaporation or sublimation or, if it does not return to the atmosphere, will be involved in one of several subsequent processes. Lect09, slide 4 here
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6246lect09_S10 - Advanced Environmental Geochemistry GLY...

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