The dipole-flow test with a tracer: a new single-borehole tracer test for aquifer characterization

We propose a new aquifer characterization test. the dipole-flow test with a tracer (DFTT), and develop its interpretation methodology. Combining the d ipole-flow test (DFT) and a tracer test, the DFTT is a single-borehole, for ced-gradient tracer test, The DFTT device isolates an injection and an extr action 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 relea sed into the injection chamber, and the concentration breakthrough curve is recorded in the extraction chamber. In developing the DFTT model, we assum e that the aquifer is homogeneous on the scale of the test and that the wel l has no skin zone. We use a streamtube approach to semi-analytically simul ate the tracer transport in a DFTT and determine the necessary relationship s for estimating the longitudinal dispersivity as well as the radial and ve rtical hydraulic conductivities. The arrival time of the peak concentration is linearly related to the anisotropy ratio, and the arrival time of the t racer front is related to the longitudinal dispersivity. We present data fr om preliminary DFTTs conducted with Rhodamine WT (RWT) as a tracer at the L izzie Field Site located between Farmville and Maury, NC. Our results demon strate that this single-borehole tracer test is feasible and that its estim ates of dispersivity are consistent with those reported in literature, wher eas its estimates of hydraulic conductivity differ from the flowmeter-test estimates by less than an order of magnitude. This difference is most likel y caused by the natural aquifer heterogeneity and different characterizatio n scales of the two tests. The sorption of RWT and its composition of two d ifferently sorbing isomers complicate the nature of the DFTT breakthrough c urve and its interpretation. The use of a conservative tracer, such as brom ide, should eliminate this complication. The skin effects readily manifest themselves in the DFTT breakthrough curve as extra and/or recirculated peak s. The presented interpretation methodology applies to cases with insignifi cant skin effects. (C) 2000 Elsevier Science B.V. All rights reserved.