Geochemistry and groundwater flow beneath an abandoned coal mine reclaimed with pressurized fluidized bed combustion by-products

A seven-acre abandoned coal mine was reclaimed with 125 tons per acre of pressurized fluidized bed combustion (PFBC) byproduct in 1994. This by-product was produced by a scrubber fitted to a coal-fired boiler designed to reduce sulfur emissions. The by-product was alkaline and contained elevated concentrations of sulfate, magnesium, boron, and other trace elements associated with coal. Reclamation with this material was intended to neutralize acid mine drainage and reduce element solubility. However, documentation of the fate and transport of elements derived from the by-product is needed before large-scale use of this material can be accepted by regulatory agencies.; The site was instrumented with lysimeters and ground-water monitoring wells to obtain water-quality samples and hydrologic data. The sole source of recharge to a three-dimensional ground-water flow model of the flow system was from meteoric precipitation; whereas, the only discharge from the flow system was through downgradient springs. Backward tracking of hypothetical particles to recharge areas showed that flow rates at the site were slow, and water may take up to 24 years to travel from the recharge area to the discharge springs.; Interstitial waters were influenced by PFBC by-product leachate. The most conspicuous evidence of such leachate was elevated sulfate and boron concentrations and elevated magnesium to calcium ratios. Speciation modeling using WATEQ4F indicated interstitial waters were supersaturated with magnesium-bearing minerals such as dolomite and magnesite. Therefore, dissolution of these minerals from the spoils was not a source of elevated magnesium to calcium ratios.; A sulfur isotope mixing model of interstitial water indicated that up to 75 percent of the sulfate in interstitial water was derived from PFBC by-product. Downgradient ground-water contained elevated concentrations of elements including sulfate with sulfur isotope signatures indicative of the surface-mined coal. However, there was no chemical evidence that leachate derived from the PFBC by-product had reached ground water during the study period (through 1997). Mass-balance calculations for sulfate in water at the mine site showed that, for the mass of sulfate to balance, oxidation and dissolution of rocks must exceed precipitation of secondary minerals by at least 25 metric tons of sulfate per year.