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

Abstract

We propose a new aquifer characterization test, 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. In developing the DFTT model, we assume that the aquifer is homogeneous on the scale of the test and that the well has no skin zone. We use a streamtube approach to semi-analytically simulate the tracer transport in a DFTT 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 (RWT) as a tracer at the Lizzie Field Site located between Farmville and Maury, NC. 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 conductivity differ from the flowmeter-test estimates by less than an order of magnitude. This difference is most likely caused by the natural aquifer heterogeneity and different characterization scales of the two tests. The sorption of RWT and its composition of two differently sorbing isomers complicate the nature of the DFTT breakthrough curve and its interpretation. The use of a conservative tracer, such as bromide, should eliminate this complication. The skin effects readily manifest themselves in the DFTT breakthrough curve as extra and/or recirculated peaks. The presented interpretation methodology applies to cases with insignificant skin effects.