EFFECT OF VARIABLE-PH LANDFILL LEACHATE ON A CARBONATE ROCK AGGREGATE

Carbonate rock aggregate is commonly used as drainage stone in leachat e collection systems of RCRA Subtitle D landfills. U.S. EPA technical guidance for waste containment facilities states that excessive carbon ate 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 experi mental evidence with typical landfill leachate or documented instances of clogging at typical Subtitle D landfills. This paper describes a p ractical laboratory experiment that measured the effects of landfill l eachate-maintained at three distinct pH levels-on a mixed limestone an d dolomitic limestone aggregate. Equilibrium concentrations of dissolv ed calcium carbonate in dilute aqueous solutions can be calculated rea dily for various pH levels. However, such calculations do not predict the kinetics of such a reaction. Moreover, formation of complexes, con sisting of calcium and magnesium ions derived from the aggregate and a nions from organic acids contained in the leachate, is a complicated p rocess not easily accounted for by simple equilibrium chemistry of dil ute aqueous solutions. The experimental approach employed for this pro ject circumvents these theoretical obstacles to predicting the rate an d extent of dissolution. Four column-type laboratory models were const ructed 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 cylinde rs were continuously recirculated and maintained at different target p H 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 cond itions; 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 a naerobic atmosphere of carbon dioxide mixed with nitrogen. Trends in t he chemistry of the cylinder fluids were measured over a 20-week perio d, during which time the cylinder fluids were sampled 12 times for alk alinity, total dissolved solids, specific conductance, total and disso lved calcium, and total and dissolved magnesium. The cylinders functio ned essentially as batch reactors, with fluids removed only for sampli ng and fluids added only to replace the sampled volumes. After five an d 17 weeks, the cylinders were opened, the fluids emptied, fresh fluid s added, and the cylinders resealed with anaerobic atmosphere for subs equent test intervals. Results of the experiment demonstrated negligib le weight loss in the aggregate sample with leachate maintained at a p H of 6.0-6.5, conditions typical of a RCRA Subtitle D landfill. Leacha te 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 sa mples collected from the same cylinder showed a sharp decrease in alka linity, and sharp increases in TDS and dissolved calcium, indicating s ignificant dissolution of the aggregate material. Fluid chemistry chan ges in the three cylinders maintained at higher pH were judged to be i nsignificant. Based on the results of the completed experiment, 100-pe rcent carbonate rock aggregate is suitably dissolution-resistant for u se in leachate collection systems containing typical landfill leachate (pH = 6.0-6.5). The experimental results further suggest that the exa ct 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.