Leaching of heavy metals from fly ash stabilized soils.

摘要

A combined experimental and computer modeling approach was used to evaluate leaching of metals from fly ash stabilized soils used in highways. Four different tasks were undertaken: (1) water leach testing, (2) laboratory column testing, (3) field testing, and (4) numerical modeling.; The pH and aqueous concentration of metals in leachate from water leach tests, and the pH and initial effluent concentration from column leaching tests on soil-fly ash mixtures increase non-linearly with increasing fly ash content. The metal concentration in leachate cannot be estimated from simple dilution calculations. The initial effluent concentration depends primarily on the type of fly ash, and the partition coefficient depends primarily on the type of soil used in the soil-fly ash mixture. The pH of the effluent is persistent for at least 30 pore volumes of flow (30-yrs of flow in the field).; Lysimeters were installed at two field sites in a conventional pavement and two fly ash stabilized pavements. The liquid flux through the pavement was approximately 4–6% of the average annual precipitation for both types of pavements. The concentration of metals in leachate was higher for the pavements constructed with stabilized soil than the conventional pavement.; A numerical model was developed to simulate leaching that occurs in pavements incorporating stabilized soils. Result of simulations with the model showed that the maximum metal concentration decreases rapidly within the first meter below a fly ash stabilized layer, and then decreases more gradually. The maximum concentration at a given depth is independent of the retardation factor, and decreases with increasing dispersion coefficient and decreasing thickness of the stabilized layer. For advection-dominated transport, the maximum concentration is independent of the Darcy flux. The time to reach the maximum concentration at a particular depth is inversely proportional to Darcy flux, and increases as the dispersion coefficient decreases and as the retardation factor increases.; Design charts were prepared from parametric simulations conducted with the model. These charts can be used for a quick and realistic prediction of the maximum concentration and the time required to achieve the maximum concentration for a wide range of field conditions.