Final Report: Evaluation of Natural Amelioration of Acidic Deep Mine Discharges for Watershed Restoration

EPA Grant Number: R825794
Title: Evaluation of Natural Amelioration of Acidic Deep Mine Discharges for Watershed Restoration
Investigators: Dzombak, David A.
Institution: Carnegie Mellon University
EPA Project Officer: Packard, Benjamin H
Project Period: June 1, 1998 through May 31, 2001 (Extended to September 30, 2001)
Project Amount: $610,484
RFA: Ecosystem Restoration (1997) RFA Text |  Recipients Lists
Research Category: Ecological Indicators/Assessment/Restoration , Hazardous Waste/Remediation , Land and Waste Management , Ecosystems

Objective:

The overall goal for this research project was to identify the hydrologic and geochemical factors responsible for improvements over time in the quality of water discharges from abandoned underground coal mines. A related goal was to evaluate the effects of mining methods and abandoned mine management practices on long-term changes in mine water quality.

Summary/Accomplishments (Outputs/Outcomes):

The project involved the study of a set of mine water discharges associated with abandoned, interconnected mines in the Uniontown-Connellsville area of western Pennsylvania. Almost all the deep-minable coal was removed from this area before 1970, and an extensive baseline study of mine discharges in the area was conducted in 1974-1975 in conjunction with Pennsylvania's Operation Scarlift. New water quality monitoring was performed from July 1998, to August 2000, at 21 of the abandoned mine drainage (AMD) sites studied in 1974-1975. The degree of mine water quality improvement since 1974-1975 was documented and assessed. In addition, the local environment for each discharge was studied to relate observed changes in mine water quality to factors such as the extent of mine flooding and the chemistry of the overburden rock overlying the mined area. Overburden rock analysis and modeling were performed to estimate the chemistry of the water infiltrating into the mine voids. Hydrologic and chemical modeling for the flooded and unflooded mine voids gave further insight into factors controlling the changes in water quality over time.

AMD Discharge Data Analysis. Changes in water quality over 25 years have been documented for discharges from an extensive network of abandoned underground coal mines in the Uniontown Syncline in Fayette County, Pennsylvania. A baseline study of 136 mine discharges in the syncline was conducted in 1974-1975. In 1998-2000, followup water flow and quality monitoring was conducted at 21 selected discharges for 2 years to assess the degree of mine water quality improvement since 1974-1975. The degree and rate of water quality improvement was found to be highly dependent on the amount of flooding in the mine voids. Water quality in the completely flooded mine voids improved significantly, going from acidic water with high sulfate and iron concentrations in 1974-1975 to alkaline water with lower sulfate and iron concentrations in 1998-2000. The water quality in the unflooded mines showed very little improvement over the 25 years between studies. The water discharging from the unflooded mines in 1974-1975 was acidic with high sulfate concentrations, and in 1998-2000, was still acidic but showed some improvement in the sulfate and iron concentrations.

Overburden Rock Analysis and Modeling. Geochemical modeling was employed to estimate the chemistry of water infiltrating into the flooded and unflooded coal mines of the Uniontown Syncline to evaluate factors governing the long-term improvement of coal mine discharge water quality. The subsurface structure and lithology was delineated using borehole, shaft, and stratigraphic information. Most of the recharge to mine voids was determined to occur in the zones of shallow overburden less than 60 ft (18.28 m) in thickness. The water-rock contact period in the recharge areas of the overburden was estimated to be 5 days or more for the flooded mines and a day or less for the unflooded mines. Flow-through reaction path models were applied to the topsoil and shale-sandstone lithological units identified in the recharge areas. The model predictions for the flooded and unflooded mines are consistent with average groundwater data from the study site and indicate the usefulness of this approach in evaluating the contribution of overburden chemistry to evolution of mine discharge quality.

Hydrologic and Chemical Analysis and Modeling for Flooded Mine Voids. Discharges from underground flooded coal mines have exhibited increases in pH and reductions in contaminant loadings with time. Data from a field study of mine water quality evolution in interconnected flooded mine voids of the Uniontown Syncline were evaluated with the aid of modeling to elucidate the hydrologic and geochemical factors responsible for these changes. Coal barriers left in place from mining operations define three hydraulically distinct zones (the southern, central, and northern pools) in these mines. The steady-state state flow rates into the mine and between mine pools were calculated based on measured discharge flow rates and estimated recharge areas. Assuming that each of the interconnected mine pools behaves as a completely mixed tank reactor, a steady-state tanks-in-series model was developed to describe system hydraulics. Chemical modeling components were then coupled with the tank reactor hydraulic model to simulate inputs to the mine voids, acid generation from pyrite dissolution, and discharge water quality. Constant empirical production terms were estimated for sulfate, iron, alkalinity, and total carbonate based on discharge data from 1974-1975 and 1998-2000. The production terms were then used to simulate discharge water quality for each of the mine pools over a 50-year period. Differences in the production terms in the northern and central mine pools can be attributed to differences in the time since the initial flooding for each of the mine pools. The simulated water quality in the northern and central mine pools reached steady-state conditions approximately 25 years after the mine pools flooded, evolving over time to reflect the recharge water quality. The simulation results indicate that the evolution of mine water quality in the flooded mine voids has been governed by alkaline recharge water slowly displacing acidic "first flush" water. Over time, the mine water quality evolves to be that of the recharge water because of the reduction in acid production after flooding.

Hydrologic and Chemical Analysis and Modeling for Unflooded Mine Voids. In the Uniontown Syncline, unflooded coal mine discharges are acidic with high sulfate concentrations and have shown little improvement in water quality over 25 years. Flow and water quality data obtained in 1974-1975 and 1998-2000 for a number of unflooded mine discharges in the Uniontown Syncline were compared and evaluated using a tank reactor model to elucidate the hydrologic and geochemical factors responsible for the quality of the discharge water. A simple tank reactor fill-and-draw concept was used as the basis for a dynamic hydraulic model that could describe the seasonal variations in outflows over time observed for unflooded mines. The hydraulic model was coupled with chemical mass balance considerations, including estimates of recharge quality and in-mine chemical production/loss, resulting in a hydrogeochemical tank reactor model for description and prediction of discharge water quality from unflooded mines. From the 1974-1975 and 1998-2000 field data it was observed that the concentrations of sulfate, iron, and acidity were fairly constant, even when flow varied greatly. Flow-related mass production functions for these constituents were obtained by fitting of the 1998-2000 field data. The hydrogeochemical model was used to describe the dynamic sulfate and acidity production from pyrite dissolution and total carbonate loss in the mine. The model simulations indicated that in-mine acid production in 1998-2000 correlated with recharge rate and was little changed from 1974-1975, due to the sustained presence of oxygen that drives pyrite dissolution; and that recharge water chemistry has a significant influence on discharge characteristics. For the Uniontown Syncline, alkaline recharge mitigates somewhat the acidity of the discharges.

Conclusions:

Comparison of the 1998-2000 and 1974-1975 data for the Uniontown Syncline abandoned mine drainage discharges provides clear evidence for natural improvement of the quality of drainage from some kinds of discharges. The water quality changes that occurred over time seemed mainly dependent on the degree of flooding within the mine voids contributing to the discharges, and the time elapsed since the completion of flooding. In flooded mines of the Uniontown Syncline, acidic discharges have become alkaline in less than 25 years. In the discharges from unflooded mines, improvements in water quality also have occurred over 25 years, but to a much smaller extent than observed for the flooded mines.

Factors responsible for the long-term improvement in the quality of discharges from abandoned, flooded underground coal mines in the Uniontown Syncline were evaluated with the aid of a multi-tank reactor model. Simulation of mine water quality evolution, based on fitting of mine discharge data from 1974-1975 and 1998-2000, indicated that the largest flooded mine pools of the Uniontown Syncline reached steady-state conditions about 25 years after flooding of the mine voids. The reduced acid production after flooding coupled with the alkaline nature of the recharge water leads to the result that over time the mine water becomes more alkaline, evolving to reflect the recharge water quality. Basically, the evolution of mine water quality in the flooded mine voids appears to be governed by alkaline recharge water slowly displacing acidic “first flush” water.

Unflooded mine discharges in the Uniontown Syncline have remained acidic since 1974-1975. Factors responsible for the acidic water with high sulfate concentrations discharging from one of the unflooded mines in the Uniontown Syncline were evaluated with the aid of a simple tank reactor model. Application of this model to a particular unflooded mine indicated that in-mine acid production is correlated with recharge rate, and that acidic conditions in the mine have been maintained in the mine for more than 25 years because of the sustained acid production rates due to the presence of oxygen. Simulating the flow and chemical dynamics for the case study unflooded mine highlighted the importance of the infiltration water on the discharge water quality. Without the incoming alkalinity, the discharge water would be more acidic and have a lower pH. The need for complete understanding of the infiltration water chemistry and occurrences in the mine was made evident through the modeling.

The usefulness of the tank reactor modeling approach in helping with interpretation of data for abandoned coal mines in the Uniontown Syncline suggests that this type of model is potentially useful for a wide range of abandoned, flooded, and unflooded mine sites. The model does not require lots of data for calibration, an advantage for study of abandoned mine sites that typically are not well characterized or monitored. Insight into the processes occurring within the mine voids and their relative rates can lead to improved predictions of mine water quality evolution, treatment planning, and a better understanding of the discharge impacts on receiving streams.

The results suggest that mine flooding has substantial benefits for reduction of mine drainage contaminant load over time periods less than 25 years after mine abandonment, with the time dependent on the volume of mine voids, and that mine development strategies and mine drainage restoration strategies should promote mine flooding when possible. When mine flooding is not achievable, little improvement in mine water quality within this time frame can be expected. Further, the results point to the importance of infiltration water quality in determining the ultimate quality of AMD discharges. Mine development and AMD restoration strategies should aim to utilize overburden materials that yield alkaline leachate. Given the limited areal regions where recharge occurs for some mines, especially deep mines, consideration should be given to installing alkalinity-yielding materials in recharge areas if they do not exist there naturally. In addition, the limited extent of recharge areas for deep mines suggests that, for some mines, it may be feasible to consider installation of less permeable materials in recharge areas to reduce recharge and hence discharge flows.


Journal Articles on this Report : 3 Displayed | Download in RIS Format

Other project views: All 17 publications 3 publications in selected types All 3 journal articles
Type Citation Project Document Sources
Journal Article Lambert DC, McDonough KM, Dzombak DA. Long-term changes in quality of discharge water from abandoned underground coal mines in Uniontown Syncline, Fayette County, PA, USA. Water Research 2004;38(2):277-288. R825794 (Final)
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  • Journal Article McDonough K, Lambert D, Mugunthan P, Dzombak D. Hydrologic and geochemical factors governing chemical evolution of discharges from an abandoned, flooded, underground coal mine network. Journal of Environmental Engineering 2005;131(4):643-650. R825794 (Final)
  • Full-text: ACSE Library
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  • Abstract: ACSE Abstract
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  • Journal Article Mugunthan P, McDonough KM, Dzombak DA. Geochemical approach to estimate the quality of water entering abandoned underground coalmines. Environmental Geology 2004;45(6):769-780. R825794 (Final)
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  • Supplemental Keywords:

    coal, ecosystem, EPA Region 3, geochemistry, groundwater, land, metals, mid-Atlantic, mining, natural attenuation, reclamation, restoration, sulfates, watersheds., RFA, Scientific Discipline, Geographic Area, Water, Ecosystem Protection/Environmental Exposure & Risk, Water & Watershed, Wastewater, Environmental Chemistry, Ecosystem/Assessment/Indicators, Ecosystem Protection, Restoration, State, Ecological Effects - Environmental Exposure & Risk, Ecological Risk Assessment, Ecology and Ecosystems, Aquatic Ecosystem Restoration, Watersheds, Ecological Indicators, aquatic ecosystem, ecological exposure, watershed, Pennsylvania, underground coal mine, Appalachia, limestone drains, acid mine drainage, aluminum, geochemistry, ecological recovery, iron, manganese, aquatic ecosystems, water quality, acid mine discharge, ecosystem restoration, water treatment, ecosystem response , ecological response, metals, watershed restoration, PA

    Relevant Websites:

    http://www.ce.cmu.edu/~acidmine Exit
    http://www.ce.cmu.edu/~dzombak Exit

    Progress and Final Reports:

    Original Abstract
  • 1998
  • 1999 Progress Report
  • 2000 Progress Report