Environmental Engineering Reference
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deep soil gas data to provide quantitative evidence of the degree of off-gassing
from the water table where groundwater concentrations appear to be high enough
to cause a potential vapor intrusion concern. Contaminant transport from ground-
water to the water-unsaturated zone can occur by diffusion through the capillary
fringe (i.e., tension-saturated zone), but the diffusion coefficient through the water-
saturated capillary fringe will be low. The capillary fringe thickness depends on the
texture of the soil, ranging from a few centimeters in sands and gravels (which may
impose negligible resistance to off-gassing) to many feet/meters for clays. Mass
transfer to the water-unsaturated zone will increase if the water table fluctuates,
because a falling water table leaves impacted water above the water table in ten-
sion that was previously below the water table and thereby facilitates off-gassing
(McCarthy and Johnson 1991 ). This is shown in Fig. 10.7 . Mass transfer to the
water unsaturated zone will be reduced if a high rate of rainfall recharge creates a
layer of clean water at the water table (called a fresh-water lens), providing that the
infiltration does not occur through a NAPL-impacted vadose zone or one containing
VOC vapors. This layer may act as an effective barrier to off-gassing from the water
table as shown in Fig. 10.5 . Many contaminated sites have groundwater monitoring
wells with many years of water level monitoring data that can quickly be reviewed
to assess the potential for each of these conditions.
Transport from deep soil gas to buildings typically is dominated by diffusion.
Molecular diffusion results in movement of contaminants away from areas of higher
concentrations toward areas of lower concentration. As soil is porous, with some
of the pore space air-filled and some water-filled, the overall effective vapor-phase
diffusion coefficient for a contaminant through soil can be estimated as a com-
bination of diffusion through the soil vapor and the water in the pores. Free air
diffusion coefficients are typically about 10,000 times higher than aqueous diffusion
Fig. 10.7 Effect of falling water table on off-gassing from groundwater (schematic) vapor
diffusion in soils
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