FINAL TECHNICAL REPORT

September 1, 1996, through August 31, 1998

Project Title: MONITORING MINES RECLAIMED WITH MIXTURES OF SPOIL AND COAL COMBUSTION RESIDUES

ICCI Project Number: 96-1/7.1A-3

Principal Investigator: Dr. Steven P. Esling, Department of Geology, Southern Illinois University

Other Investigators: Dr. Bradley C. Paul, Department of Mining Engineering, Southern Illinois University

Project Manager: Dr. Dan D. Banerjee, ICCI

ABSTRACT

Three separate large-scale field demonstration projects begun within the last six years at abandoned mines, have involved in one way or another, the application of both flue gas desulferization (FGD) and fluidized bed combustion (FBC) residues in mine land reclamation. The first, at Forsythe-Energy, used FGD residues as well as fly ash as substitute fill to a surface mine pit; the second, at Thunderbird, created seven caps composed of varying proportions of FBC fly and bottom ashes; and the third, at Harco, involved amending coal processing waste with FGD residues to control infiltration and to induce alkaline recharge. Previous studies monitored the groundwater at the Forsythe-Energy site and the surface water at the Thunderbird site for short term performance, and no significant environmental impacts were detected. The long-term capacity of the residues to control acid mine drainage and their long-term impact on the environment, however, are unknown. The widespread application of the residues in reclamation as well as industry acceptance of these methods hinges on the long-term behavior.

This two year project continued monitoring the impact of reclamation with coal combustion residues on groundwater at the Forsythe-Energy site and surface water at the Thunderbird site. In addition, the study began an investigation of the impact of reclamation on groundwater at the Harco and Thunderbird sites. To date, leachate generated by the residues has not degraded groundwater at the Forsythe-Energy site. In addition, evidence suggests that the fill can ameliorate groundwater degraded by mining. Data collected thus far at the Harco site on the impact of the residues on groundwater quality are largely inconclusive. Evidence from the Thunderbird site suggests that although the caps do not degrade groundwater or surface water, they do not improve existing groundwater quality. Qualitative data from both the Thunderbird and Harco suggest that the FGD soil amendments and FBC fly ash and bottom ash caps may leak at these sites. Of the three reclamation strategies; application of residues as a bulk fill, cap, or soil amendment; the bulk fill approach shows the most promise.

EXECUTIVE SUMMARY

Three separate large-scale field demonstration projects begun within the last five years at abandoned mines, have involved in one way or another, the application of both flue gas desulferization (FGD) and fluidized bed combustion (FBC) residues in mine land reclamation. The first, at Forsythe-Energy, used FGD residues as well as fly ash as substitute fill to a surface mine pit; the second, at Thunderbird, created seven caps composed of varying proportions of FBC fly and bottom ashes; and the third, at Harco, involved amending coal processing waste with FGD residues to control infiltration and to induce alkaline recharge. Previous studies monitored all three demonstration sites for short term performance, and in general, no significant environmental impacts were detected. Long-term impacts and long-term performance of FBC and FGD residues in preventing acid mine drainage, on the other hand, are unknown. Industry and regulatory agency acceptance of new reclamation strategies employing FGD or FBC residues hinges on an analysis of long-term environmental impacts. This report summarizes continued environmental monitoring for the past two years at all three demonstration sites.

The monitoring network at the Forsythe-Energy site has only monitoring wells (including a drain beneath the residues). The network at Thunderbird includes a rain gauge, monitoring wells, survey points, lysimeters and surface water sampling points along Brier Creek. The network at Harco, once reclamation work was completed, included a rain gauge, monitoring wells, suction and free-drainage lysimeters, infiltrometers, tensiometers, and TDR probes. Work on this project included preparing a site for collecting water quality data in addition to sample collection, analysis, and interpretation.

Site Preparation Work

Reclamation has altered groundwater flow directions away from wells originally installed to monitor downgradient groundwater quality at the Forsythe-Energy site. Other wells now downgradient are too far from the residue fill to detect leachate. A new well was installed immediately downgradient of the fill in December, 1996 following accepted standard practices. Groundwater sampling at the Forsythe-Energy began once this well was installed.

The dedicated sampling system originally intended for the wells at the Thunderbird site required access to the well head with a vehicle that can carry the heavy and bulky gas cylinders used to drive the pumps. Access roads to the Thunderbird site were removed just before the start of this project. A wet autumn then prevented access to the monitoring wells with a vehicle. Fortunately, an alternative sampling system became an option when another project, funded by the US Department of Energy, allowed the Department of Geology to acquire an actuator for driving inertial pumps. The relatively light weight actuator can be carried to remote sites. Dedicated inertial pumps were installed at Thunderbird after acquisition of the actuator in the spring, 1997. Groundwater sampling began at Thunderbird in June, 1997.

At the time this project began, the contractor retained by the Illinois Abandoned Mine Land Reclamation Division of the Illinois Department of Natural Resources to reclaim the Harco site still had not completed the work. Because of this, groundwater monitoring wells still needed to be installed, a task not originally planned for this project. Three groundwater monitoring wells were installed at the site in May and July, 1997; one upgradient and two downgradient of soil amendments with FGD residues. Additional wells were installed as the contractor completed reclamation adjacent to the test site in the coming year. The first set of samples were collected the first quarter of the second year of the project (November, 1997). In addition, vadose zone monitoring also was attempted during the fall, 1997 and again in the summer, 1998. This work failed to provide useful information on vadose zone processes. Lysimeters installed for trapping infiltration leaked, providing unreliable readings of infiltration. Large networks of rodent burrows diverted flow from the infiltrometers. Other instruments, including soil suction lysimeters, tensiometers, and time domain reflectometry were installed to monitor the impact of induced infiltration from the infiltrometers. The failure of the infiltrometer tests made data collected with the other instruments of little value.

Geochemistry

Water quality sampling for this project followed accepted standard practices to effectively guarantee reliable field geochemical and hydrologic data. In the course of the past two years, over 140 samples were collected at the three sites from surface and groundwater monitoring points. Three samples were collected from each field instrument during a sampling event; one for cations, one for anions, and one for alkalinity. The alkalinity tests were performed using a Mettler Auto Titrator in the Mining Engineering Environmental Laboratory. The remaining tests were performed in the Mining Engineering Carterville Laboratories, or the joint water chemistry lab operated by Mining Engineering and Geology. Anions were determined potentiometrically or by Liquid Ion Chromatography using a Dionex 10. Cations were determined by ICP with more precise measurements for lead, arsenic and selenium made by graphite furnace or hydride formation AA.

An analysis of the samples collected at each site seeks an answer to two questions; Does leachate generated by the FBC and FGD residues impact groundwater or surface water quality? and, Do these residues mitigate acid mine drainage? The critical indicators of acid mine drainage are elevated concentrations of iron, magnesium, manganese, and sulfate. The fly ash which composed part of the fill at the Forsythe-Energy site generates high concentrations of boron in its leachate. The FBC residues at Thunderbird and the FGD residues at Forsythe-Energy and Harco produce high concentrations of sodium and potassium in their leachate.

This research found little value in monitoring vadose zone hydrology with free drainage lysimeters, infiltrometers, tensiometers, and time domain reflectometry. The application of soil cover by heavy equipment tends to damage lysimeters, making them unreliable instruments for quantitatively measuring infiltration. High contrast in soil permeability between animal burrows and the compacted soil matrix also biased the infiltrometer, tensiometer, and time domain reflectometry data.

Samples at the Forsythe-Energy site collected from a sand filter installed horizontally at the base of the residue fill consistently contain high concentrations of boron, and reduced levels of iron, manganese, magnesium, and sulfate relative to the native groundwater. Boron is the best indicator of residue leachate at this site. This constituent, however, is not detected in wells located downgradient at concentrations significantly different than those in wells upgradient of the residue fill. In addition, iron, magnesium, manganese, and sulfate, traditional indicators of acid mine drainage, generally have reduced levels in downgradient wells when compared to the upgradient well. This study, therefore, suggests that the FGD/fly ash fill at the Forsythe-Energy site has not impacted groundwater quality. In fact, the fill can actually improve groundwater quality downgradient of the residues. The absence of boron in levels significantly different than those found upgradient of the fill, in light of contaminant transport models, suggests that boron transport is retarded relative to groundwater flow at this site.

The Thunderbird site showed positive effects of reclamation on stream water quality. Many factors can influence the geochemistry of stream water other than drainage across the reclaimed mine site, including the duration and intensity of recent storm events. The concentration of some constituents tends to fluctuate from one sample event to another because of these factors. Certain trends in the Brier Creek data, however, are evident. Sodium and potassium, indicators of leachate generated by the FBC residues at this site, are not detected in concentrations downstream from the test plots in levels significantly different than those upstream. In addition, iron concentrations, the main indicator of acid mine drainage, dropped dramatically after reclamation. The effect of the residue caps on the improvement of the water quality in Brier Creek is unknown. The area of the caps represents only a small portion of the stream's drainage basin. The overall reclamation plan likely benefited water quality in the stream.

Surface water and groundwater quality data collected at the Thunderbird site also indicate no significant impact of the FBC residues cap. This research, however, does suggest that the cap at Thunderbird leaks and that the residues do not significantly improve groundwater quality.

Data from the Harco site are inconclusive because changes in the original plan for the distribution of FGD residues may have affected water quality in the wells upgradient of the test site. Qualitative data from Harco suggest that the layer incorporating FGD residues leaks.

This study suggests that coal combustion residues can serve as a beneficial fill material in the reclamation of abandoned mines. Groundwater degraded by mining may actually improve in quality as it percolates through residues over an extended time period and in a field setting. Large bulk fills may offer distinct advantage over thin caps or soil amendments.