Page 278 - Soil and water contamination, 2nd edition

Gas exchange 265

Note that because of our incomplete understanding of the air–water gas exchange
processes and the fact that so many factors influence the exchange rate, the empirical
relationships presented above for estimating the gas exchange coefficient are subject to
considerable uncertainty. So, the relationships should not be applied blindly. The most
accurate determination of the gas exchange coefficient requires careful field experiments in
the water body of interest, using a tracer gas (e.g. propane (C H )).
3 8
Example 14.4 Gas exchange controlled on the air side

-1
The wind speed at 10 m above the surface of a river is 3.5 m s . Estimate the gas
exchange coefficient at 20 °C for nitrobenzene (C H NO ), given that its molecular
6
2
5
2 -1
-2
diffusion coefficient in air is 7.6 10 cm s .
Solution
2 -1
-5
2 -1
The molecular diffusion coefficient of water vapour at 20 °C is 2.4·10 m s = 0.24 cm s .
The gas exchange coefficient for nitrobenzene is calculated using Equation (14.24):
5 . 0
. 0 076 -1
k . 1 607 3 7 . 2 m d
L
. 0 24
14.6 GAS EXCHANGE IN THE SUBSURFACE ENVIRONMENT
Obviously, the abovementioned relationships between the gas exchange coefficient and
wind speed and rainfall intensity apply to surface water bodies in direct contact with the
atmosphere. Nevertheless, Henry’s law (Equation 14.1) and the thin film model (Equation
14.5) are also applicable to soil water and groundwater. Because the movement of soil water
and groundwater is slow and laminar , the surface renewal model is inappropriate to describe
gas exchange in the subsurface. Moreover, because the water movement is slow, equilibrium
may be assumed between the soil air and the aqueous phase (soil water and groundwater near
the water table) and in most cases Henry’s law is sufficient for calculating the distribution
of chemicals between the aqueous and gas phases. However, the soil air and free atmosphere
differ in their chemical composition. The most notable difference is the smaller oxygen
concentration and larger carbon dioxide concentration in the soil air due to decomposition

of soil organic matter . The average oxygen content of the atmosphere is about 21 volume
percent and the oxygen content in soil generally varies between 9 and 21 volume percent.
The average CO content of the free atmosphere is 0.03 volume percent and generally
2
ranges between 0.021 and 0.044 volume percent above land, although the concentrations
may be larger around industries, cities, thermal springs, and volcanoes (Mathess, 1994). In
general, the CO content of soil air is 10 to 100 times larger, with frequent values between
2
0.2 and 5 volume percent. In areas with natural CO releases or intensive anthropogenic soil
2
contamination by organic pollutants (e.g. waste disposal sites), values of up to 25 volume
percent or larger are found. In general, natural gases occurring in soil air with elevated
concentrations compared to the free atmosphere are – like CO – products of the oxic or
2
anoxic decomposition of organic matter, or radioactive decay products. Besides CO , these
2
gases include nitrous oxide (N O)) , hydrogen sulphide (H S), methane (CH ), and radon
2 2 4
(Rn).
Gas exchange between the upper soil layers (typically up to about 2 m depth) and
the free atmosphere is primarily due to Fickian diffusion as a result of differences in gas
concentration, although advective gas transport may also occur due to fluctuations in
groundwater level, infiltrating rainwater , or biogas generation. If the gas flow velocity is

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