Page 343 - gas transport in porous media

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resulting soil temperatures at several depths, as well as the chemical concentrations
of TNT, DNT and DNB expressed as total concentrations and as separate solid, liquid
and gas phase concentrations. Of note is the dramatic increase in surface gas-phase
concentrations of all three chemicals following a rainfall event.
The demonstration calculations showed that the complex interdependent inter-
actions that occur in the soil could be simulated to produce estimates of the
vapor concentrations and surface soil residues for comparison to mine dog perfor-
mance capabilities. This tool was then used to evaluate various scenarios – certain
combinations of particular mine types (leakage), soil properties, and weather patterns.
The impact of weather conditions on the movement of landmine signature chemi-
cals through the subsurface and into the atmosphere was evaluated using the T2TNT
code with specific improvements since initial development (Webb and Phelan, 2003).
These included temperature dependent mine flux and temperature/soil moisture con-
tent dependent biodegradation rates. The T2TNT code currently does not include the
effects of plants, so a bare soil version was used. Weather data from three diverse
sites were used: Kabul, Afghanistan, an arid to semi-arid climate, Ft. Leonard Wood,
Missouri, USA, a moderately wet climate with significant seasonal temperature vari-
ations, and Napacala, Mozambique, a wet climate with minimal seasonal temperature
variations.
The results for the Kabul and Ft. Leonard Wood TNT gas-phase and solid-phase
surface concentrations for the final year of the simulations are shown in Figure 21.4
and Figure 21.5. For Kabul, the year is from January to December. For Ft. Leonard
Wood, the year is from mid-July to mid-June because of the weather data. Both the
surface gas-phase concentrations shown in Figure 21.4 and the surface concentrations
in Figure 21.5 vary widely due to seasonal weather patterns, most importantly, by
precipitation. For Kabul, the large increases in gas-phase concentration occur after a
rainfall event. The gas-phase concentration is predicted to increase up to 6–7 orders
of magnitude. This increase is due to reduced vapor-solid sorption. When the soil
saturation increases, the amount of vapor that can be sorbed to the solid decreases,
resulting in a large increase in the gas-phase concentration. The Ft. Leonard Wood
variation is similar to Kabul. However, because of the increased annual precipitation
(62 cm vs. 18 cm), the dry-period values are generally lower for Ft. Leonard Wood
because the chemical signature has been transported deeper by infiltrating rainfall
and by accelerated biodegradation.
Thesurfacesolid-phaseconcentrationsshowninFigure21.5forKabulalsodecrease
after rainfall, again primarily due to “washing down” of chemical signature. The
values decrease during rain, and increase during dry periods. The range of solid-phase
concentration is much smaller than the range in gas-phase values. For Ft. Leonard
Wood, the variation is similar. With more frequent rain, there are much more frequent
decreases in solid-phase concentration, and the values are generally lower.
Differences in soil vapor emanations for scenarios in regional areas (Afghanistan,
Cambodia, Mozambique, Angola) demonstrates the critical nature of maintaining
optimal mine dog performance and the futility of certain situations where vapor levels
are well below typical mine dog vapor sensing capabilities.