To understand how the construction of a landfill may influence the subsurface transport of water and contaminants, since 1990 we have conducted a series of field, lab, and theoretical investigations comparing a simulated hazardous waste trench with nearby undisturbed soil (Shakofsky and Nimmo, 1996; Nimmo and others, 1999). We are investigating unsaturated-flow processes in actual and simulated waste trenches, and, as a control for the effect of artificial modifications, on the nearby undisturbed soil. Results show a wide variety of structural effects, some substantial and others virtually negligible. We are collaborating in these efforts with personnel of the USGS INL Project, including Joe Rousseau, Brennon Orr, DeWayne Cecil, and Linda Davis, and previously including Larry Mann, Gary Lords, and John Pittman of the USGS INL Project, and Jon Kaminsky of Idaho State University.
The surficial soil at this site, especially where it has not been disturbed for landfill construction, has impeding layers and significant macropore structure. For this soil we developed a numerical simulation of unsaturated flow in 2 dimensions, for 9 surficial layers, based on measured hydraulic properties. We successfully applied the VS2DT code, modified for hysteresis, using wetting curves determined from minimal data using various hysteresis models. Hysteresis showed less effect than macropore flow. We represented macropore effects by developing a technique involving time-staggered initial conditions. Macropore flow modeled through time-staggered initial conditions shows that 3 hours of data from ponded infiltration embodies essential information concerning macropores.
To conduct two infiltration experiments at the USGS Test Trench Area (TTA) adjacent to the Subsurface Disposal Area (SDA) of the Radioactive Waste Management Complex (RWMC) of the INL, we instrumented the site for water content and water pressure measurements, conducted a controlled 24-hour ponded infiltration experiment, and measured subsurface moisture conditions during and after infiltration for three months. The main purposes were (a) to obtain direct field measurements, by the instantaneous-profile method, of unsaturated hydraulic conductivity and moisture retention on the INL soil at various depths; and (b) to investigate the nature of subsurface water flow under conditions comparable to natural flooding, likely a main source of aquifer recharge at the site. The results show good confirmation with our earlier lab hydraulic property measurements on soil core samples, and indicate distinct differences in flow between the artificial landfill and the adjacent natural soil profile.The layered structure of the undisturbed profile significantly retards downward flow below 2 m depth in a way that the landfill does not.
In the lab we have measured unsaturated hydraulic properties (hydraulic conductivity, moisture retention, and others) of core samples from both the simulated waste trench and undisturbed soil. We found (a) good agreement among measurement techniques for lab and field unsaturated hydraulic properties, (b) that there are important field-plot-scale phenomena not predicted by the standard Darcy/Richards approach, (c) that the undisturbed profile has layers that differ substantially in hydraulic properties, and (d) that the waste trench is more homogeneous.
The lab and field results suggest that, if preferential flow is not dominant, downward fluxes may be greater in the waste trench, where the flow-inhibiting effects of the natural layering are largely absent. Preferential flow, likely due to the presence of macropores, causes rapid infiltration of water to 2 m depth in the undisturbed profile but not the landfill. However, the layered structure of the undisturbed profile significantly retards downward flow below this depth. The results are significant for the design of waste-burial trenches, and delineate the limitations of standard modeling approaches.
The data from our study of disturbed and undisturbed soil provided the required basis for development of a soil structure model (Nimmo, 1997b). We also did a numerical simulation of infiltration and redistribution with sensitivity tests of the following refinements: hysteresis, two-dimensional flow, increased detail of layering structure, and initial conditions reflecting preferential flow. The results show that preferential flow is by far the most critical effect on the reliability of model results for these conditions. They also lead to ways of incorporating preferential flow into a Richards-equation-based model.
With DeWayne Cecil of the USGS INL Project, we obtained core samples from the actual SDA waste trench in 1996 for the purpose of measuring concentrations of bomb-derived isotopic tracers. Some of the cores were also used in our hydraulic property measurements noted above. Previous bomb-tracer investigations, near to but not at this site, showed 30-year-old water at 1-2 m depths in undisturbed soil. Our measurements on the waste-trench soil test the hypothesis that flooding of the waste-site in the 1960s carried the tracers (and possibly also dissolved contaminants) far below these depths.
| National Research Program | USGS | Water Resources | UZ Flow Homepage |