6246lect04_S10

6246lect04_S10 - Advanced Environmental Geochemistry, GLY...

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Advanced Environmental Geochemistry, GLY 6246, ©David Warburton, 2010 1 LECTURE 4 - Details of the Hydrologic Cycle and the Concept of Residence Time Water is distributed in many reservoirs in the hydrosphere, atmosphere, and lithosphere. Estimates of the contents of each reservoir are available. Geochemists are interested not only in the distribution of water but also in the circulation of that water between reservoirs. Flux rates (the rate of transfer of water from one reservoir to another) may also be estimated. These rates are of great interest because of their implications for the transfer of energy and substances dissolved in the water. Lect04, slide 2 here Table 4-1 gives a list of many reservoirs with stored mass and percent of the total water. This compilation neglects all water below four thousand meters as inaccessible. Another way of stating this is to say that water below four kilometers is part of the very long cycle and that we are interested only in the short and long cycles. The table is given in geograms (Gg) to facilitate comparison with Table 1-1. Most figures compare well but the figure for lakes in Table 4-1 is about five times higher than that in Table 1-1.
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Advanced Environmental Geochemistry, GLY 6246, ©David Warburton, 2010 2 Reservoir Mass (Gg) Percent of Total Oceans 13,700 97.25 Ice caps and glaciers 290 2.05 Shallow groundwater 42 0.30 (< 750 meters) Deep groundwater 53 0.38 (750-4000 meters) Soil Moisture 0.65 0.005 Atmosphere 0.13 0.001 Lakes 1.25 0.01 Rivers 0.017 0.0001 Biosphere 0.006 0.00004 Total 14,087 100 Table 4-1 (after Berner and Berner, 1987, p.13) 4-1 Lect04, slide 3 here The concept of residence time, which plays a large role in geochemistry, may now be defined. The residence time is the volume of a substance in a reservoir divided by the flux (addition of material to a reservoir or removal of the substance from the reservoir). Mass may be substituted for volume if the reservoir capacity and flux are both expressed as mass. In dimensional analysis, The residence time may be thought of as the average time a molecule spends in a reservoir.
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Advanced Environmental Geochemistry, GLY 6246, ©David Warburton, 2010 3 4-2 Lect04, slide 4 here Example Calculation: Calculate the residence time of water in the ocean given that the mass of water in the oceans is 13.7 x 10 3 Gg and the flux rate is 0.37 Gg/yr. A similar calculation for the atmosphere gives a residence time of about ten days. The concept of residence time is general. It can be applied to compounds such as water, carbon dioxide, or DDT, or elements such as neon, argon, or to particular isotopes such as deuterium, ( 1 2 H, often represented as D). It should be noted that isotopic fractionation effects can lead to residence times, for specific isotopes in a compound, that are different from for the average molecule of the compound. For example, water composed of H 2 18 O will spend longer in the ocean then H 2 16 O, because lighter molecular weight compounds evaporate faster than heavier compounds. H
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This note was uploaded on 05/04/2011 for the course GLY 6246 taught by Professor Warburton during the Spring '11 term at FAU.

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6246lect04_S10 - Advanced Environmental Geochemistry, GLY...

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