Temporal geochemical variations in above- and below-drainage coal mine discharge

Applied Geochemistry
By: , and 

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Abstract

Water quality data collected in 2012 for 10 above- and 14 below-drainage coal mine discharges (CMDs), classified by mining or excavation method, in the anthracite region of Pennsylvania, USA, are compared with data for 1975, 1991, and 1999 to evaluate long-term (37 year) changes in pH, SO42−, and Fe concentrations related to geochemistry, hydrology, and natural attenuation processes. We hypothesized that CMD quality will improve over time because of diminishing quantities of unweathered pyrite, decreased access of O2 to the subsurface after mine closure, decreased rates of acid production, and relatively constant influx of alkalinity from groundwater. Discharges from shafts, slopes, and boreholes, which are vertical or steeply sloping excavations, are classified as below-drainage; these receive groundwater inputs with low dissolved O2, resulting in limited pyrite oxidation, dilution, and gradual improvement of CMD water quality. In contrast, discharges from drifts and tunnels, which are nearly horizontal excavations into hillsides, are classified as above-drainage; these would exhibit less improvement in water quality over time because the rock surfaces continue to be exposed to air, which facilitates sustained pyrite oxidation, acid production, and alkalinity consumption. Nonparametric Wilcoxon matched-pair signed rank tests between 1975 and 2012 samples indicate decreases in Fe and SO42− concentrations were highly significant (p < 0.05) and increases in pH were marginally significant (p < 0.1) for below-drainage discharges. For above-drainage discharges, changes in Fe and SO42−concentrations were not significant, and increases in pH were highly significant between 1975 and 2012. Although a greater proportion of above-drainage discharges were net acidic in 2012 compared to below-drainage discharges, the increase in pH between 1975 and 2012 was greater for above- (median pH increase from 4.4 to 6.0) compared to below- (median pH increase from 5.6 to 6.1) drainage discharges. For cases where O2 is limited, transformation of aqueous FeII species to FeIII may be kinetically limited. In contrast, where O2 is abundant, aqueous Fe concentrations may be limited by FeIIImineral precipitation; thus, trends in Fe may not follow those for SO42−. In either case, when the supply of alkalinity is sufficient to buffer decreased acidity, the pH could increase by a step trend from strongly acidic (3–3.5) to near neutral (6–6.5) values. Modeled equilibrium with respect to FeIII precipitates varies with pH and Fe and SO42−reconcentrations: increasing pH promotes the formation of ferrihydrite, while decreasing concentrations of Fe limit the formation of ferrihydrite, and decreasing Fe and SO42−concentrations limit the precipitation of schwertmannite and favor formation of FeIIIhydroxyl complexes and uncomplexed Fe2+ and Fe3+. The analysis of the long-term geochemical changes in CMDs in the anthracite field and the effect of the hydrologic setting on water quality presented in this paper can help prioritize CMD remediation and facilitate selection and design of the most appropriate treatment systems.

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Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Temporal geochemical variations in above- and below-drainage coal mine discharge
Series title Applied Geochemistry
DOI 10.1016/j.apgeochem.2015.02.010
Volume 62
Year Published 2015
Language English
Publisher Elsevier
Contributing office(s) Pennsylvania Water Science Center
Description 12 p.
First page 84
Last page 95
Time Range Start 2012-01-01
Time Range End 2012-12-31
Country United States
State Pennsylvania
Online Only (Y/N) N
Additional Online Files (Y/N) N