Comparative analysis of laboratory and field tracer tests for investigating preferential flow and transport in mining waste rock

A comparison of different tracer tests, conducted in mining waste rock from the Aitik site in northern Sweden, is made by use of temporal moment analysis and advection-dispersion modelling of experimental breakthrough curves. The tracer tests were carried out in both laboratory columns and held lysimeters, and the experimental conditions included both saturated and unsaturated flow, ranging from being close to steady-state to being highly transient. The objectives were to investigate the possible occurrence of preferential flow and the transport behaviour of two tracers, bromide and uranine, under geochemical conditions prevailing in mining waste rock. No indications of significant preferential flow were found in the laboratory columns. In the field, there were indications of water flowing preferentially in about 55-70% of the total water content. In relatively old waste rock, the tracers bromide and uranine behave differently, with uranine being both retained and delayed relative to bromide. These effects on uranine are most probably due to low pH conditions in parts of, or along the entire length of the tracer pathway in the mining waste rock, suggesting that uranine may be useful as an indicator of limited acidic zones within a larger transport domain; in mining waste rock, acidic zones are associated with higher pollutant generation. (C) 1997 Elsevier Science B.V. All rights reserved.