Pyrite, or iron sulfide (FeS2), is a mineral that is commonly found within Indiana coal seams and the adjacent rock strata. When coal beds and surrounding rock units are disturbed during mining, the associated pyrite is exposed to oxygen and water, and chemical reactions produce highly mineralized acidic mine drainage (AMD). This AMD contaminates and degrades nearby groundwater and surface water quality.
The oxidation of pyrite is a step-wise process as described below using a series of reactions that have been documented in numerous academic publications on AMD formation. In these reactions active acidity is defined as the concentration of hydrogen ions (H+), which is measured as pH (with lower pH values corresponding to higher hydrogen ion concentrations). The initial chemical reaction occurs when pyrite reacts with oxygen and water to release H+ and ferrous iron (Fe2+):
FeS2 + 7/2O2 + H2O -> Fe2+ + 2SO42- + 2H+ (1)
Ferrous iron (Fe2+) is then oxidized to ferric iron (Fe3+) in the presence of oxygen and hydrogen ions:
Fe2+ + 1/4O2 + H+ -> Fe3+ + 1/2H2O (2)
Reaction 2 proceeds slowly in the absence of bacteria but is accelerated rapidly when catalyzed by iron-oxidizing bacteria. Ferric iron (Fe3+) can generate even more hydrogen ions through the following reaction, which is prevalent at pH > 5:
Fe3+ + 3H2O -> Fe(OH)3 + 3H+ (3)
Iron sulfate [Fe2(SO4)3] forms when ferric iron (Fe3+) combines with sulfate (SO42-) and this compound reacts in the presence of pyrite and water to form sulfuric acid (H2SO4), a strong acid that contributes a significant amount of mineral acidity:
FeS2 + 7Fe2(SO4)3 + 8H2O -> 15FeSO4 + 8H2SO4 (4)
Reaction 4 does not require oxygen to induce pyrite weathering as shown in reaction 1. This allows additional pyrite not directly exposed to oxygen-enriched surface waters to continue to degrade, producing even more AMD. The combined result of these reactions is to generate a cascading effect of increasingly acidic mineralized waters. These chemical reactions generate both active acidity, by increasing hydrogen ion concentrations, and mineral acidity, which is associated with strong acids such as sulfuric acid. These chemical processes will continue to produce acidity unless 1) water and/or oxygen are restricted from interacting with the pyrite and reaction products; 2) alkalinity (the acid-neutralizing ability of a solution typically owing to the presence of carbonates or hydroxides) is introduced into the system; or 3) the pyrite has been completely depleted.
The preceding reactions occur at abandoned mine land (AML) sites wherever oxygenated water interacts with pyritic deposits. Spoil (overburden removed during surface mining) can contain enough pyrite to generate AMD, but these reactions are propagated to a greater extent within and around waste deposits generated by cleaning processes where coal is separated from mineral matter at coal-preparation facilities. These waste deposits concentrate pyrite to levels much higher than occurs in unmined coal or overburden rock and are known as tailings (fine-grained refuse from coal preparation) and gob (coarse-grained refuse). In addition to these surface and near-surface sources of AMD, water-filled voids (or mine aquifers) associated with abandoned underground mines may contain significant volumes of acidic water. Therefore, AMD can be manifested as 1) groundwater within refuse deposits and underground mine voids, 2) seeps and springs issuing from mine voids as well as spoil and refuse deposits, 3) storm runoff that dissolves sulfate minerals present on the surface of pyrite-bearing deposits, and 4) within ponds and streams receiving each of these sources during baseflow (low-flow) conditions or storm events.
Typical water-quality indicators of AMD include pH, total dissolved solids (TDS), sulfate (SO42-), and total iron (Fe). Data for these parameters are shown in Table 1 for samples collected from selected AML sites where relatively extreme cases of AMD have been noted.
Table 1. Water chemistry data for several AML aquifers and surface water features where AMD has been detected. Many of the sites were reclaimed to some extent following sampling (see noted reference for details).
| AML site (monitoring location)* | Data reference | AMD source | pH (# of samples) | TDS (mg/L) | Sulfate (mg/L) | Total Fe (mg/L) |
|---|---|---|---|---|---|---|
| EPA Secondary Drinking Water Standards | Electronic Code of Federal Regulations Title 40, Part 143 | 6.5-8.5 | 500 | 250 | 0.3 | |
| Midwestern (MW7) | Naylor and others, 2010 | Refuse aquifer | 1.1-1.8 (3) | 12,000-35,000 | 8,200-17,500 | 2,800-5,700 |
| Blackhawk (6 sites) | Harper and others, 2011^ | Underground mine voids | 3.5-9.2 (148) | NA | 123-15,594 | 1-3,289 |
| Friar Tuck (#81) | Branam and others, 1994 | Seep from a gob deposit | 1.8-2.6 (22) | 28,800-63,900 | 22,000-35,300 | 2,300-6,000 |
| Friar Tuck (Sheetwash) | Olyphant and others, 1991 | Runoff from a refuse deposit | 2.0-2.6 (2) | 150-8,260 | 122-5,970 | 13-1,970 |
| Midwestern (SW4) | Naylor and others, 2010 | Stream receiving drainage from a gob deposit and mine aquifer | 2.8-3.1 (4) | 3,300-3,900 | 2,280-2,500 | 190-330 |
| Friar Tuck (#108) | Branam and others, 1994 | Impoundment / pond receiving drainage from a gob deposit seep | 1.8-4.5 (8) | 1,560-7,710 | 1,140-5,240 | 190-1,400 |
| Midwestern (SW2) | Naylor and others, 2010 | Highwall pond adjacent to a gob deposit | 2.6-3.1 (4) | 600-970 | 370-690 | 6-80 |
^This publication is currently in preparation but is scheduled to be released in 2011.
The values shown in Table 1 are well within the spectrum of AMD contaminant concentrations. However, there are more extreme examples of AMD concentrations in addition to numerous occurrences of minimal AMD contaminants that have been observed at various AML sites in southwestern Indiana. Unpublished water-quality data for a coarse refuse aquifer at the Green Valley AML site in Vigo County included TDS values between 11,000 and 111,000 mg/L, sulfate concentrations from 7,530 to 80,200 mg/L, total iron from 2,400 and 22,000 mg/L, and pH values between 2.3 and 4.3. Alternatively, samples collected from an underground mine void at the Midwestern site (Hartwell No. 1 Mine) had TDS values between 490 and 1,000 mg/L, sulfate concentrations from 35 to 270 mg/L, total iron from 1 to 10 mg/L, and pH values between 6.1 and 7.3 (Harper and others, 2011), indicating that portions of the mine void contain water with a chemistry that is more typical of regional groundwater in southwestern Indiana.
References
Branam, T. D., and Harper, D., 1994, Tabulated analytical data for water samples from the Friar Tuck site: Indiana Geological Survey Open-File Report 94-13, 169 p., 7 fig.
Harper, D. H., Branam, T. D., and Olyphant, G. A., 2011 (in review), Characterization of groundwater in the coal-mine aquifers of Indiana: Indiana Geological Survey Special Report.
Naylor, S., Olyphant, G. A., and Branam, T. D., 2010, Hydrochemical effects of using coal combustion byproducts as structural fill and capping material at an abandoned mine lands reclamation site, southwestern Indiana, in Barnhisel, R. I., ed., Proceedings of the 2010 National Meeting of the Society of Mining and Reclamation, Bridging Reclamation Science and the Community, June 5-11, 2010, Pittsburgh, Pennsylvania, p. 672-690; related slide presentation available at IUScholarWorks repository, <http://hdl.handle.net/2022/8997>, date accessed, August 30, 2010.
Olyphant, G. A., Bayless, R. E., and Harper, D., 1991, Seasonal and weather-related controls on solute concentrations and acid drainage from a pyritic coal-refuse deposit in southwestern Indiana, U.S.A.: Journal of Contaminant Hydrology, v. 7, p. 219-236.