MEASUREMENT OF EVAPORATIVE FLUXES FROM CANDIDATE COVER SOILS

In geotechnical engineering applications, such as the use of moisture- retaining soil covers for mitigating acid rock drainage in mine waste, a knowledge of the rate of evaporation at the soil-atmosphere interfa ce is required to assess the water content of the cover during dry per iods. In this study, the actual rates of evaporation from a coarse san d, a fine sand, a clay, and a top soil were measured in the laboratory under controlled conditions. The data for all the soils showed three phases of evaporation, typical of soils. In phase I (constant-rate pha se) evaporation, the rates observed in the coarse and fine sands were similar to the potential or ''maximum'' rate of evaporation measured f rom a free water surface. The compacted clay and top soil gave slightl y lower initial rates of evaporation? probably because of their lower unsaturated hydraulic conductivities compared with those of the sands. Water content measured in the soil columns showed drying fronts that penetrated into the soil profile as evaporation continued to the end o f phase II (the first falling-rate phase), where the rate of evaporati on was low. During phase III, the second falling-rate phase, the water content profiles were nearly uniform in the sands and top soil. The c lay, however, continued to show a drying front. The utility of the mea sured evaporation fluxes is demonstrated with a numerical analysis of a soil cover over mine tailings.