Drift pump back test for residence time distribution in engineered treatment system

Tracer tests use conservative dyes to estimate hydraulic parameters of aquifers such as permeability and velocity. However, deionized water is rarely used in tracer tests. A novel permeable reactive barrier (PRB) designed and installed to collect and treat acidic water seeps from spoil heaps was investigated. In the present work, deionized water as a tracer was used to estimate the residence time and velocity distribution in reactive media placed in the PRB. Drift pump back tracer tests were carried out in multilevel boreholes in the reactive media using deionized water. Breakthrough curves of relative electrical conductivity were analyzed. The velocity and residence time differed horizontally and vertically at less than the optimum values predetermined in the design phase for effective treatment. The treatment efficiency is higher in the lower level than that at upper level inside the PRB. The quantity of deionized water used as a tracer should be big enough to force the tracer to move out of the well bore zone and travel significant distances into the reactive media. Consequently, representative breakthrough curves are obtained and better estimate the residence time and velocity. Using deionized water as a tracer has advantages over other tracer dyes such as fluorescein as it is inexpensive and requires no lab analysis for water samples and has less ionic strength and sorption. The disadvantage of using deionized water in reactive media is that it is recharged with high saline acidic water drainage from spoil. In drift tests the velocity is in direct proportion to permeability, drift time and time to recover half tracer mass. To overcome porosity reduction due to mineral fouling in the reactive media, the contaminated water should be forced to flow into several avenues in the reactive media in a direction parallel to PRB length instead of crossing the reactive media to increase the residence time inside the reactive media.