Quantitative Evaluation of Pb-Zn Mine Wastes for Subgrade Disposal - Characterization Versus Classification in a Superfund Environment

Mississippi Valley Type Pb-Zn deposits are widespread in the midcontinent U.S. and have been mined from the 1800's to present. Some tens of thousands of acres of mined land in the Tri-State District have been designated as Superfund sites as a result of surficial mine wastes and ground waters with elevated levels of metals. EPA considers the surficial mine wastes to be hazardous and a major source of acid mine drainage fAMD) that degrades water resources and threatens human health and the environment. To mitigate potential risks at one site near Galena, Kansas, a group of mining companies who had been identified by EPA as Potentially Responsible Parties IPRPs) proposed subgrade disposal of the mining wastes in shafts and subsidence features. However, this proposal was challenged by the Agency on grounds related to AMD and the potential for degradation of ground water quality. The PRP team developed a program of materials characterization, laboratory-scale leaching, and finally large-scale, pilot testing in the field. Static and dynamic leaching tests at both laboratory scale fusing artificial rainwater under both variably and fully saturated conditions) and field scale (using pH 4.5 ground water) showed that the surficial wastes do not produce acidic leachate and are not a long-term source of metals. Laboratory chemical and mineralogical tests show that the mine wastes contain substantial carbonate, but very limited iron sulfides, consistent with the leach test results. EPA "characterization" tests fEP Toxicity and TCLP) compare very poorly with tests simulating the expected leaching conditions. While these tests would classify the surficia1 wastes as "hazardous, they do not characterize the behavior of the high-volume, low-concentration, coarse-grained materials with respect to environments and disposal options of interest. To be meaningful, characterization of mining wastes must be performed in a manner that simulates the environmental conditions that are actually of interest with respect to a specific decision, such as subgrade disposal in the saturated zone.