Abstract: Better understanding of field-scale unsaturated zone
transport mechanisms is required if the fate of contaminants released at
the surface is to be predicted accurately. Interpretation of results from
direct tracer sampling in terms of operative hydraulic processes is often
limited by the poor spatial coverage and the invasive nature of such techniques.
Cross-borehole electrical imaging during progress of saline tracer migration
is proposed to assist investigation of field-scale solute transport in
the unsaturated zone. Electrical imaging provides non-destructive, high
density and spatially continuous sampling of saline tracer transport injected
over an area of the ground surface between two boreholes. The value of
electrical imaging was tested at a field site on an interfluve of the UK
Chalk aquifer. Improved understanding of active transport mechanisms in
the unsaturated zone of the UK Chalk is required to predict its vulnerability
to surface pollutants.
In a tracer experiment in May 1996, a conductive
saline tracer was infiltrated over 18 m2 at an average rate of 47 mm day-1
for 56 hours. Cross-borehole images obtained during and after infiltration
show a large, homogenous, resistivity reduction in the top 3 m, no change
between 3 m and 6 m depth, and smaller, inhomogeneous, resistivity reductions
below 6 m depth. The resistivity has reduced at down to 15 m depth less
than 2 days after tracer infiltration began. Hydrological interpretation
of a sequence of electrical images obtained prior to, during, and up to
three months after tracer injection suggests: (1) rapid tracer entry into
the soil zone and upper 2 m of weathered Chalk, (2) intergranular transport
of the bulk of the tracer, (3) a significant fissure flow component transporting
tracer to at least 15 m depth in 31 hours, and (4) vertical changes in
transport mechanisms possibly caused by interception of fissures by marl
layers. The results of this experiment suggest that electrical imaging
can assist the description of unsaturated zone hydraulic mechanisms through
visual identification of spatial and temporal variations in transport processes. |