Calculation results are first presented as color plots of calculated soil gas concentration profiles, in through . The dramatic differences in soil gas concentration profiles shown in these figures show the difficulty in using soil gas data by themselves to characterize vapor intrusion risks, if there is not at the same time a proper understanding of the site’s geology to help in interpreting such data.
Figure 5 Soil gas concentration profile and pressure field plots for A. a soil containing a continuous clay layer, B. discontinuous clay layer, and C. soil with miscellaneous obstructions. In pressure field plots (right hand side figures) , qualitative soil gas (more ...)
was calculated for the case of no building depressurization, and thus no induced advective flow, in order to show the dominant influence of pure diffusional transport on the soil gas concentration profiles. The topmost panel of shows soil gas concentration profiles for different permeability/diffusivity soils, deliberately selected to show different bounds of what might be normally encountered in the field. The left hand side of shows the model results for a homogeneous soil with relatively high diffusivity (and permeability, though advection does not play a role here), and the right hand side the same model run for rather impermeable, low diffusivity soil. The concentration profiles on the two sides of are strikingly similar, as they must be, since in a steady state diffusional process in homogeneous media with no biodegradation or other reaction processes taking place, the concentration profiles must be the same, even if the diffusion rates themselves are vastly different (the rate of diffusion in the left hand panel is much higher than that in the right hand panel). Thus, it is immediately apparent that attempting to characterize a vapor intrusion situation using only soil gas measurements, or soil gas concentration profiles, will not be reliable for establishing the rate of contaminant migration, and hence, vapor intrusion potential.
It is also worth noting here how the soil gas concentration profiles always neck upwards in the shadow of the structure. These subsurface concentration profiles are similar to those shown previously for other cases by Abreu and Johnson (2005)
. This is also to be expected, since the structure’s foundation is a solid barrier to upward diffusion of the vapor. Thus subslab contaminant gas concentrations will always be higher than those measured at the same depth outside of the building footprint. This is true irrespective of the existence or absence of advective transport, which plays no role here (and which would affect the results only at high flowrates).
present layered soil computations also under pure diffusion conditions. In , the left hand panel shows results for when the surface soil layer is characterized by high diffusivity and the right hand panel for when the surface soil layer is of low diffusivity. Both these cases involve an intervening soil layer of intermediate diffusivity (diffusivities are those given in ). As in any multilayer diffusion problem, the lowest diffusivity layer controls the overall rate of transport. The right hand panel of shows the “capping” effect of the topmost low diffusivity soil layer. The resistance to diffusion of contaminant to the atmosphere is greatest across this low diffusivity layer, which requires the steepest concentration gradient to be located in this layer. Concentration profiles in the higher diffusivity layers below this are much more uniform and the actual concentrations nearer the source concentration. These differences in the soil gas concentration profiles are entirely a result of the assumed diffusion process and are a consequence of the differences in the effective diffusivities for each layer, since there is no advection involved. However, these profiles do not yet by themselves speak to the indoor air concentrations, discussed below. These results by themselves do not permit reliable assessment of vapor intrusion potentials, as will be seen.
presents a different arrangement of the different diffusivity layers, with the low diffusivity layer being in the middle of the high and low diffusivity layers. The similarity of the profiles on both sides of this panel is notable, even though subtle differences are evident. In both cases, the presence of the middle low diffusivity layer has the effect of establishing a high contaminant concentration zone below it. Both the left and right panels of show relatively low contaminant concentration in the top soil layers, because the main diffusional resistance exists below those top layers. Hence, knowledge of the diffusivity of a particular soil layer will not by itself necessarily allow prediction of a contaminant concentration level in that layer, even if the distance to a source is know; it is the interplay of the properties of different diffusivity layers that can determine the observed in-ground concentration profiles.
Although the soil gas concentration profiles are different in the various scenarios of , the contaminant flux from source to ambient would be almost equal in all those scenarios. This is not quite true only because the bottom layer is one meter thinner than the top and middle layers (we chose the total 8 m thickness so that the results would be comparable to results in Pennell et al. 2008
, while maintaining whole meter values for layer thicknesses). If the thicknesses of the soil layers were all equal the fluxes in all cases would have to be equal, because that is the requirement in a steady state series resistance diffusion (or conduction) problem such as this. An electrical analogy would consist of a series circuit of three resistors operating across a constant voltage difference - the current would be the same, regardless of the ordering of resistors, and only the voltage at the junctions between the resistors would be different (the voltage drops across the resistors are analogous to the soil gas concentration gradients in the present problem). Numerous publications highlight calculation of effective diffusivity values for layered soils (e.g. Johnson et al. 1998
shows soil gas concentration profiles and pressure fields for layered soil geologies, compared with a simple base case of a homogeneous soil in . For the cases shown here, the building was always depressurized relative to atmospheric pressure (−5 Pa), so as distinct from , advection does play a role. It is, however, immediately apparent in comparing and that the basic soil gas concentration profiles are qualitatively similar. This confirms the basic nature of the phenomenon- diffusion of contaminant vapor in the soil determines the general profile around a building, but it is details of the flow in the vicinity of the building that determine the indoor air concentrations.
Arrows on the pressure field plots of show qualitative vectors for soil gas flow (the size of the vectors do not indicate magnitude – the arrows are only intended to convey direction). These plots show the common feature that air is drawn into the soil from the surface by the pressure gradient within the soil, and the depressurized structure is the sink for this airflow. Flow rates into the foundation crack are in the usual range, of order liters per minute. To the extent that airflow crosses a region of non-zero contaminant vapor concentration, it carries the contaminant with it, including into the structure. This is a well-established feature of the vapor intrusion problem.
shows the impact of different soil layer arrangements on the flow rate into the building, and resulting indoor air concentrations. It can be seen that if the soil consists of multiple layers with different soil properties, the indoor air concentration is highest when the permeability/diffusivity of the topmost layer is high, despite the fact that the soil gas concentrations beneath the building actually are then lowest. To understand why, reference is made to the air flow rates entering the building, values for which are shown inset into the basement areas in the right hand side of . Higher permeability surface soil permits relatively higher air entry rates into the building, and this flow carries with it more mass of contaminant into the structure from the soil near the foundation. It is the product of flow rate and concentration that determines the all-important mass entry rate.
Soil gas flow field is strongly affected by the arrangement of the different soil layers. The right hand panels of qualitatively show the soil gas flow field for the homogeneous soil case, and show the flows in the layered soil cases. When a low permeability soil is beneath a high permeability soil, such as in , the low permeability layer creates resistance to soil gas flow through that layer, and a pressure gradient of some magnitude can be sustained. If the order of the layers is changed, as in , then the pressure gradient is dissipated in the high permeability layer located beneath a low permeability layer. Thus significant soil gas flow takes place in that higher permeability layer, but there is no pressure gradient to drive flow from the lower, low permeability layers.
The indoor air concentration for the homogenous soil case with k=10−11
() is similar to the indoor air concentrations calculated for the layered soil cases with the high permeability/diffusivity soil at the top (). The value of soil permeability around the foundation has an apparent effect on indoor air concentrations, but the different soil layers that are adjacent to this high permeability layer can also influence the indoor air concentration. The presence of the lower permeability layers beneath the high permeability soil layer results in nearly an order of magnitude lower indoor air concentration, as compared to a homogenous soil with similar properties (i.e. k=10−10
) (comparison here is with results reported in Pennell et al. 2008
). It should be noted that Pennell et al. (2008)
considered the effect of porous material (10 inches thick) beneath the foundation for otherwise homogenous geology. The presence of a porous subbase resulted in slightly higher indoor air concentrations, as compared to a homogenous geology without a porous subbase. This trend is consistent with the results presented herein.
illustrate that due to high air flow rate in the soil around the building foundation, scenarios with high soil permeability values often show a low concentration area near the foundation breaches. Again, this does not mean that the indoor air concentration will be low, as the results from this calculation demonstrate. As already noted above, it is always the product of air inflow and concentration that determines indoor air concentration.
When the top layer of the multi-layered soil is assigned a low permeability/ diffusivity value (), this layer again acts as a cap, just as in the pure diffusion cases of . Here, the low permeability limits the flow of atmospheric air into the soil, which limits dilution of the contaminant in the soil near the foundation. Concentration of the contaminant at the foundation is high in this scenario. Nonetheless, the indoor air concentration is much lower than the scenarios with high soil permeability values, due to lack of airflow carrying contaminant into the building.
For many sites, it may be possible to approximate soils at sites of concern as relatively homogeneous, save for discrete features with very different permeability and diffusivity characteristics. Some examples are shown in . Insertion of a continuous clay layer between the source and the building is illustrated in . The presence of such a clay layer drastically reduces the species transport into the building. The clay layer is seen to “bottle up” the contaminant below itself. The insert on the left hand side of shows that there is actually a concentration gradient across the clay layer itself, but the resistance to diffusion of the contaminant across that layer is so high as to leave the soil immediately beneath the structure almost free of contaminant. The wet clay layer effectively acts as a shallow source that absorbs the contaminants from beneath and only slowly releases them to the upper soil layers. Because the contaminant concentration near the foundation is very low, then the air flowing into the building does not carry much contaminant, and indoor air concentration is low.
On the other hand, a discontinuous clay layer beneath the building footprint has far less effect in reducing indoor air concentrations (). The clay layer splits the domain horizontally into two regions. Beneath the clay layer contaminant concentration is almost equal to the source concentration just as for the continuous clay layer. Where there is no clay layer, the concentration profiles are similar to those for homogeneous soil (see ).
There is often a question as to how discrete impervious zones might influence the vapor intrusion process. There are of course a great many configurations that could be examined, but here we considered a case of a number of boulders or large rocks spread out at a given depth below the structure. As seen in , such small obstructions in the domain do not significantly influence concentration profiles, nor do they influence the soil gas flow rate into the building. The indoor air concentration also remains as same as in the homogenous soil scenario.
While the above color plots are useful in providing a quick, qualitative understanding of the soil gas concentrations, for many purposes more traditional graphs of concentration as a function of position better illustrate the phenomena. shows the soil gas concentration values for a base case of k=1×10−11 m2, and Deff = 8.68×10−7 m2/s, at different depths and distances away from the building of interest. The line in connects soil gas concentration values at a distance of 40 meters away from the building. This is far from where there is any induced advective flow and where simple diffusion theory demands a linear concentration gradient for steady state diffusion. also shows that even at a distance of only 2 m from the foundation wall, the linear concentration profile of diffusion-dominated transport is already observable. Thus a soil gas concentration measured at two meters depth only two meters away from the foundation would be much different than a subslab concentration at essentially the same depth. is also a very close approximation to the calculated profiles in the case of scattered obstructions in homogeneous soil. The obstructions act locally, and when widely dispersed as assumed, their impact is not large.
Figure 6 Soil gas concentration values at different depths and distances from building foundation for a homogeneous soil scenario. k=1×10−11 m2, and D eff = 8.68×10−7 m2/s. Building depressurization of 5 Pa was assumed in this case. (more ...)
illustrates the concentration profiles in the different layered soil scenarios. The top two panels show the high permeability soil at the top, the middle two panels show the medium permeability soil as the surface layer, and the lowest panels show the low permeability soil at the surface. Again, it is instructive to consider the concentration profiles far from the building foundation. While the slopes of the lines connecting the points at 40 m distance remain linear within each layer, they change from one layer to another.
Soil gas concentrations in layered soil scenarios.
The low diffusivity layer always has the flattest slope whereas the high diffusivity layer always has the steepest slope in . Because of how these results are plotted, with depth as the ordinate and concentration as the abscissa, low slope actually indicates the steepest concentration gradient, i.e., low diffusivity gives the highest gradient with depth, dC/dz, and high diffusivity the lowest.
allows predicting what would happen in characterization of soil gas concentrations, when dealing with different soil types. One can compare soil gas concentration results for what are characterized as the “low” diffusivity soil to medium, or high, diffusivity soils (the latter two are similar to one another). It can be observed that in the higher diffusivity (and permeability) soils, subslab concentrations are closer to concentrations taken far from the building at that same depth. For example, compare the horizontal distance from the fitting line representing the concentration gradient far from the building to the subslab points (triangles) for the case of the top soil layer being low vs. high diffusivity/permeability. In low permeability/diffusivity layers, larger differences can exist between these values.
shows the soil gas contaminant concentration values for the discontinuous clay layer (4 m bgs) scenario. , shows the profile under the side of the domain that has a clay layer. At distances beyond the building wall, there is a very abrupt decline in concentration across the clay layer, identical to what is seen in the continuous clay layer case (results not shown). Immediately below the center of the building foundation, the influence of the side without a clay layer is apparent. shows the profiles observed on the side of the building with no clay layer. In this case, the continuous gradient associated with diffusion-dominated transport in a homogeneous medium is obtained already within 2 m of the foundation wall. This illustrates that the effect of a barrier to vertical vapor migration (such as a clay layer) dissipates at distances close to the barrier’s point of termination.
Figure 8 Soil gas concentration values in the discontinuous clay layer scenario. Permeability of top and bottom homogeneous soil for this scenario are k=1×10−11 m2, and D eff = 8.68×10−7 m2/s. A. Profiles beneath the side with clay (more ...)
The influence of soil permeability/diffusivity was examined with respect to its effect on indoor air contaminant concentration and on soil gas flowrate into the house. shows the results for different soils in a homogeneous soil cases. Again, the indicated value of permeability was associated with a corresponding value of diffusivity, as shown in . It is clear that the indoor air concentration rises as the flow rate of soil gas into the building increases. The diffusional process in the soil is important for establishing the soil gas concentration profile around the building, but it is the actual entry of soil gas into the structure that determines the magnitude of the indoor air problem. Low permeability soils do not allow much airflow, and thus protect the house from high indoor air concentrations. As mentioned previously, the results of Pennell et al. 2008
illustrated that a porous subbase can result in (slightly) higher soil gas flowrates, and therefore result higher indoor air concentrations. For sites where a porous subbase is known or thought to exist, the effect that this highly permeable zone may have on vapor intrusion rates should be considered.
Indoor air concentration and volumetric flowrate into building, for different soil permeabilities/diffusivities in homogeneous soil case. The results are identical for the case of homogeneous soil with scattered obstructions.