Effect of Variable-pH Landfill Leachate on a Carbonate Rock Aggregate

Carbonate rock aggregate is commonly used as drainage stone in leachate collection systems of RCRA Subtitle D landfills. U.S. EPA technical guidance for waste containment facilities states that excessive carbonate in drainage stone may result in dissolution and clogging at sites of reprecipitation (Daniel and Koerner, 1993, p. 197). Unfortunately, this theoretical concern does not appear to be confirmed by any experimental evidence with typical landfill leachate or documented instances of clogging at typical Subtitle D landfills. This paper describes a practical laboratory experiment that measured the effects of landfill leachate—maintained at three distinct pH levels—on a mixed limestone and dolomitic limestone aggregate. Equilibrium concentrations of dissolved calcium carbonate in dilute aqueous solutions can be calculated readily for various pH levels. However, such calculations do not predict the kinetics of such a reaction. Moreover, formation of complexes, consisting of calcium and magnesium ions derived from the aggregate and anions from organic acids contained in the leachate, is a complicated process not easily accounted for by simple equilibrium chemistry of dilute aqueous solutions. The experimental approach employed for this project circumvents these theoretical obstacles to predicting the rate and extent of dissolution. Four column-type laboratory models were constructed of plexiglass and filled with carbonate aggregate and leachate from an operating RCRA Subtitle D sanitary landfill to simulate actual conditions within a leachate collection system. Fluids in the cylinders were continuously recirculated and maintained at different target pH values of: a) as close to 3.0 as possible, to simulate a worst-case dissolution scenario; b) 6.0 to 6.5, to simulate typical landfill conditions; c) equilibrium pH; and d) distilled water at 6.0 to 6.5, as an experimental control. The cylinders were sealed and subjected to an anaerobic atmosphere of carbon dioxide mixed with nitrogen. Trends in the chemistry of the cylinder fluids were measured over a 20-week period, during which time the cylinder fluids were sampled 12 times for alkalinity, total dissolved solids, specific conductance, total and dissolved calcium, and total and dissolved magnesium. The cylinders functioned essentially as batch reactors, with fluids removed only for sampling and fluids added only to replace the sampled volumes. After five and 17 weeks, the cylinders were opened, the fluids emptied, fresh fluids added, and the cylinders resealed with anaerobic atmosphere for subsequent test intervals. Results of the experiment demonstrated negligible weight loss in the aggregate sample with leachate maintained at a pH of 6.0–6.5, conditions typical of a RCRA Subtitle D landfill. Leachate maintained at equilibrium pH, and distilled water maintained at pH 6.0–6.5, also experienced negligible weight loss. The aggregate sample maintained with leachate at approximately pH 3.0 experienced a weight reduction of 12 percent. Consistent with the weight loss, leachate samples collected from the same cylinder showed a sharp decrease in alkalinity, and sharp increases in TDS and dissolved calcium, indicating significant dissolution of the aggregate material. Fluid chemistry changes in the three cylinders maintained at higher pH were judged to be insignificant. Based on the results of the completed experiment, 100-percent carbonate rock aggregate is suitably dissolution-resistant for use in leachate collection systems containing typical landfill leachate (pH = 6.0–6.5). The experimental results further suggest that the exact chemical\mineralogical composition of carbonate drainage stone and minor changes in leachate pH may be much less important in controlling dissolution than factors such as particle size.