Reactive transport of metal contaminants in alluvium - model comparison and column simulation

A comparative assessment of two reactive-transport models, PHREEQC and HYDR OGEOCHEM (HGC), was done to determine the suitability of each for simulatin g the movement of acidic contamination in alluvium. For simulations that ac counted for aqueous complexation, precipitation and dissolution, the breakt hrough and rinseout curves generated by each model were similar. The differ ences in simulated equilibrium concentrations between models were minor and were related to (1) different units in model output, (2) different activit y coefficients, and (3) ionic-strength calculations. When adsorption proces ses were added to the models, the rinseout pH simulated by PHREEQC using th e diffuse double-layer adsorption model rose to a pH of 6 after pore volume 15, about 1 pore volume later than the pH simulated by HGC using the const ant-capacitance model. In PHREEQC simulation of a laboratory column experiment, the inability of t he model to match measured outflow concentrations of selected constituents was related to the evident lack of local geochemical equilibrium in the col umn. The difference in timing and size of measured and simulated breakthrou gh of selected constituents indicated that the redox and adsorption reactio ns in the column occurred slowly when compared with the modeled reactions. MINTEQA2 and PHREEQC simulations of the column experiment indicated that th e number of surface sites that took part in adsorption reactions was less t han that estimated from the measured concentration of Fe hydroxide in the a lluvium. (C) 1999 Elsevier Science Ltd. All rights reserved.