Acidity Decay of Above‐Drainage Underground Mines in West Virginia

Abstract

Acidity of water from abandoned underground mines decreases over time, and the rate of decrease can help formulate remediation approaches and treatment system designs. Th e objective of this study was to determine an overall acidity decay rate for above-drainage underground mines in northern West Virginia from a large data set of mines that were closed 50 to 70 yr ago. Water quality data were obtained from 30 Upper Freeport and 7 Pittsburgh coal seam mines in 1968, 1980, 2000, and 2006, and acidity decay curves were calculated. Th e mean decay constant, k, for Upper Freeport mines was 2.73 × 10 −2 yr −1 , with a 95% confi dence interval of ± 0.0052, whereas the k value for Pittsburgh mines was not signifi cantly diff erent at 4.26 × 10 −2 yr −1 ± 0.017. Acidity from the T&T mine, which was closed 12 yr ago, showed a k value of 11.25 × 10 −2 yr −1 . Th is higher decay rate was likely due to initial fl ushing of accumulated metal salts on reaction surfaces in the mine, rapid changes in mine hydrology after closure, and treatment. Although each site showed a specifi c decay rate (varying from 0.04 × 10 −2 yr −1 to 13.1 × 10), the decay constants of 2.7 × 10 −2 yr −1 to 4.3 × 10 −2 yr −1 are useful for predicting water quality trends and overall improvements across a wide spectrum of abandoned underground mines. We found fi rst-order decay models improve long-term prediction of acidity declines from above-drainage mines compared with linear or percent annual decrease models. Th ese predictions can help to select water treatment plans and evaluate costs for these treatments over time.

Acid mine drainage and stream recovery: Effects of restoration on water quality, macroinvertebrates, and fish

“…It is difficult to evaluate the role of specific reclamation activities in the restoration of Tom's Run because a variety of remediation efforts have been implemented, and because there is some evidence that over time AMD discharges will attenuate without abatement efforts as acid-forming minerals are depleted from exposed overburden. Studies have found significantly lower pollution loads after approximately 40 years in untreated watersheds (Wood et al, 1999), and a reduction in acidity at a rate of about 2% per year (Mack et al, 2010). Thus, it is possible then that the water quality improvements in Tom's Run are at least in part due to the natural depletion of exposed pyrite.…”

Modelling the hydrogeochemical evolution of mine water in a decommissioned opencast coal mine

“…Various analytical methods have been proposed for the quantification of the first flush from underground mines (Demchak et al, 2001;Frost, 1979;Gzyl and Banks, 2007;Mack et al, 2010). The approaches in these studies relied mainly on the contaminants' time dependent decay rates in mine water fitted with decay curves.…”

“…By extrapolation, the further course of the contaminants was then predicted. Mack et al (2010) report that most of the investigated underground mines had an annual contaminant decay rate of approximately two percent per annum. Additionally, Mack et al (2010) highlighted that due to changing hydrogeological conditions in the mines, the decay rates are likely to be dynamic with higher decay rates in the initial stages of the first flush, equilibrating to lower ones at later stages.…”

“…Mack et al (2010) report that most of the investigated underground mines had an annual contaminant decay rate of approximately two percent per annum. Additionally, Mack et al (2010) highlighted that due to changing hydrogeological conditions in the mines, the decay rates are likely to be dynamic with higher decay rates in the initial stages of the first flush, equilibrating to lower ones at later stages. Similarly, the temporal concentrations of sulfate and chloride in discharging mine water were fitted with exponential decay curves to confirm the first flush phenomenon (Gzyl and Banks, 2007).…”

Water quality changes of a closed underground coal mine in Korea

“…As described in Demchak et al (2004), the type of underground mine (above-drainage versus below-drainage) is an important factor in the degree and rate of change in water chemistry over time and the prediction of future changes (Mack et al 2010).…”