THE “PUSH-PULL” TRACER METHOD: HOW DOES THE TEST DESIGN INFLUENCE THE SHAPE OF THE BREAKTHROUGH CURVES?
In a PP test, a test solution that contains a known amount of solutes and a conservative tracer is injected into the aquifer (“push”) and extracted afterwards (“pull”). Optionally, the test solution is flushed out of the well and the casing with untreated test solution with a so called “chaser” before being extracted. Also a drifting time may be included between injection and extraction. The breakthrough of the tracer is measured during the extraction phase and is used for analyses and interpretation.
In our research project, several PP Tests were performed in a sedimentary coastal basin in northern Hokkaido (Japan). The objective was to study how the test design is influencing the breakthrough curves of the PP Tests with the aim to enhance the method by a systematic test approach.
During the campaign, six different PP Tests were performed, while only single aspects of the setup were varied from test to test. The tests differed in injection and extraction rate (5 L/min and 10 L/min), in the salinity of the injected test solution (brackish water and deionized water) and in the use of a chaser solution. The general shapes of the breakthrough curves are similar and a good applicability of this method is assumed for the test side. However, the Uranine mass balances of the different tests show a wide range of recoveries between 65 % and 126 %. The maximal normalized concentrations are in a range between c/c0 = 0.58 and c/c0 = 1.22. Also, an anomaly was observed repeatedly in the breakthrough curves of one of the test designs. We assume that sorption processes and density driven flow are the main reasons for this behavior. The observations from the different PP tests are leading to the conclusion that the setup does have an influence on the measured data. Thus, depending on the objective of the test, the setup should be well planned and taken into consideration when evaluating data of single-well tests.