Monitoring unsaturated flow and transport using ...

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XVI International Conference on Computational Methods in Water Resources (CMWR-XVI) Ingeniørhuset

Monitoring unsaturated flow and transport using cross-borehole geophysical methods
Author:Majken C Looms <> (Geological Institute, University of Copenhagen)
Karsten H Jensen <> (Geological Institute, University of Copenhagen)
Lars Nielsen <> (Geological Institute, University of Copenhagen)
Andrew Binley <> (Department of Environmental Science, Lancaster University)
Hans Thybo <> (Geological Institute, University of Copenhagen)
Presenter:Majken C Looms <> (Geological Institute, University of Copenhagen)
Date: 2006-06-18     Track: Special Sessions     Session: Hydrogeophysical data fusion

Recent research has shown that cross-borehole georadar and electrical resistivity tomography (ERT) can provide data on soil moisture content and conductivity variations in the vadose zone at a more appropriate spatial scale than traditional techniques. A field site has been established in Denmark on a 20-30 m layer of unsaturated melt water sand and gravel deposits. Two identical field setups have been established each having four ERT and four georadar boreholes. The boreholes are drilled to a depth of 12 m, and form a cross consisting of two lines. Along each line the outer two boreholes (7 m apart) are equipped with ERT instrumented PVC-tubes (electrodes every 50 cm) while the inner two boreholes (5 m apart) have access tubes for georadar. Two different tracer infiltration experiments have been performed; (i) natural infiltration and (ii) forced infiltration experiments. In the natural infiltration tracer experiment, 200 L tracer was distributed on a 10 m by 10 m area, simulating a 2 mm rain event. In the forced infiltration tracer experiment, tracer was applied during the first 4 hours of the experiment at a constant rate of 250 l/hr (5.1 mm/hr) followed by irrigation of clean water at the same rate to accelerate flow during the successive 10 days. When using the two geophysical monitoring techniques simultaneously, it is possible to monitor variations in fluid conductivity resulting from tracer infiltration by combining the water content images from cross-borehole georadar with the bulk conductivity images achieved from the cross-borehole ERT. In both experiments, water content and conductivity data were collected prior to and after the applied tracer. The two sets of experiments provide data describing two vastly different flow conditions. In the natural infiltration experiment the upper boundary is determined by the naturally occurring precipitation and actual evapotranspiration. In this case little temporal variation in soil moisture content is observed and the tracer migration mainly takes place in autumn and winter where a surplus net-precipitation exists. In contrast, flow and transport velocities are much higher in the forced infiltration experiment and the upper boundary condition for this experiment is much more well-defined. Both experiments are interpreted by numerical hydraulic models which allow estimation of large-scale unsaturated hydraulic and transport parameters by appropriate inverse modeling.