CMSMDC (KDI) Seminar
Wednesday, October 13, 1999, 4:00pm, 120 Physics
Doug Sutton (Civil and Environmental Engineering, Duke)
Dipole Flow Tests with Tracers
Abstract:- We propose a new aquifer characterization test,
{\sl the dipole-flow test with a tracer} (DFTT), and develop its
interpretation methodology. Combining the dipole-flow test (DFT) and a tracer
test, the DFTT is a single-borehole, forced-gradient tracer test.
The DFTT device isolates an injection and an extraction chamber
in a well with inflatable packers, and utilizes a small pump to create a
dipole-flow pattern. After a steady-state flow field is reached and the
pumping rate and chamber drawdowns are measured, a tracer is released into
the injection chamber, and the concentration breakthrough curve is
recorded in the extraction chamber. We use a streamtube modeling approach
to semi-analytically simulate the tracer transport in a DFTT, conducted in a
homogeneous aquifer with no skin zone around the well, and determine the
necessary relationships for estimating the longitudinal dispersivity as
well as the radial and vertical hydraulic conductivities. The arrival time
of the peak concentration is linearly related to the anisotropy ratio, and
the arrival time of the tracer front is related to the longitudinal
dispersivity. We present data from preliminary DFTTs conducted with Rhodamine
WT as a tracer at the Lizzie Field Site near Greenville, North Carolina.
Our results demonstrate that this single-borehole tracer test is feasible
and that its estimates of dispersivity are consistent with those reported in
literature whereas its estimates of hydraulic conductivities are approximately
an order of magnitude higher than those obtained from a flowmeter test.
The sorption of RWT and its composition of two differently sorbing isomers
complicates the nature of the DFTT breakthrough curve and its
interpretation. The use of a conservative tracer, such as bromide, will
eliminate this complication. The skin effects readily manifest themselves
in the DFTT breakthrough curve as as spurious and/or recirculated peaks.
The presented interpretation methodology applies to cases with
insignificant skin effects.
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