A CHROMATOGRAM OF GEOCHEMICAL REACTIONS: INTERPRETING HUMIDITY CELL TESTS

Humidity cell tests (HCTs) are longer-term leach tests that are considered by some to be the gold standard for predicting the environmental behavior of mined materials. The HCTs represent an untapped resource of geochemical data, with a year-long test generating ~2,500 individual chemical data points. The tests were designed to mimic weathering reactions in the laboratory for remediation planning, yet few really questioned whether HCTs were meaningful when applied to a large-scale mine waste site. The HCT results can be thought of as a “chromatogram” of reactions that occur in the field, running from sulfate salt dissolution through sulfide oxidation to dissolution of rock-forming and neutralizing minerals. Some of these reactions occur simultaneously, but through geochemical evaluation they can be teased apart. Our detailed examination of HCTs from three projects with different geology and mineralization shows that sulfide oxidation dominates over a limited period of time that is rarely at the end of the test. No simple rules of thumb reliably link laboratory and field results, although the first flush concentrations, which are uniformly ignored in HCT interpretation, are often similar to those from scaled-up field tests. Predictions based on HCTs often fail, and some of the factors that complicate the use of HCTs for predicting field behavior of mine wastes include: sample representation, time for microbial oxidizers to grow, consideration of hydrogeologic conditions, the availability of oxygen in the waste pile, sample storage before testing, geochemical reactions that add or remove constituents, and the HCT results chosen for modeling. Improved guidance is needed for more consistent interpretation of the results of HCTs that relies on identifying the geochemical processes, the mineralogy – including secondary mineralogy, the available surface area for reactions, and the influence of hydrologic processes on leachate concentrations in runoff, streams, and groundwater before mining begins.