Page 300 - Advances In Productive, Safe, and Responsible Coal Mining

276 Advances in Productive, Safe, and Responsible Coal Mining

accounting procedures have been refined over time as more stringent effluent limits
have been implemented [16].
Material characterization is essential to development of mining practices that will
reduce pollution in waters discharged from disturbed areas. Where neutralization
potential is sufficient to overwhelm any acids that will be produced, then no special
handling plan is needed for the purpose of controlling acidity. In such situations, the
entire overburden profile can be blasted and mixed together when constructing
the mine backfill if the primary goal is minimizing discharge acidity. However, if
the acid-producing potential is greater than the amount of neutralizers, then special
handling plans or alkaline addition is required to control pollution [17]. Skousen
et al. [14] found that overburden analysis by acid-base accounting was >90% accurate
in predicting postmining drainage quality from 56 surface mines in West Virginia.
However, it can be expected that the common practice of blending net acid-producing
spoils with alkaline spoils or reagents, so as to attain a net-alkaline discharge, in
hydrologically active locations would accelerate the release of TDS compared to
the alternative procedure of isolating acid-forming materials [18]. Concerns with
other pollutants such as Se can also be evaluated by analysis of whole rocks where
solids are extracted with a strong acid and the concentrations of Se in the rock are
measured. Thresholds can be established where rock materials with certain contents
1
(such as 20mg Sekg ) are specially handled so as to reduce their contact with envi-
ronmental waters for leaching.
Leaching tests are an alternative method for overburden characterization. Tests are
conducted by breaking or grinding rocks into smaller pieces, subjecting those rock
fragments to repeated leaches with defined water volumes, and analyzing leachates
for chemical composition. The most common leaching tests are humidity cells,
soxhlet extractors, and column-leaching procedures [12,19,20]. Leaching tests pro-
vide information in addition to that provided by acid-base accounting [14,21].
Orndorff et al. [22] and Daniels et al. [18] conducted leaching tests to characterize
mine overburdens for TDS release. They showed that unweathered rock materials typ-
ically release waters with higher SCs than do weathered rock materials and that fine-
textured sedimentary rocks (siltstones and shales) typically release waters with higher
SCs than do more coarsely textured sedimentary rocks (sandstones). In this context,
the term “weathered” refers to those rock materials that occur in the upper portion of
the overburden column, typically extending for depths of 10–50ft (3–15m) directly
beneath the soil. Weathered rock materials have been affected by percolating rainwa-
ter, and therefore differ both visually and chemically from unweathered rocks
below them.
Orndorff et al. [22] and Daniels et al. [18] also showed that tested geologic
materials typically release waters with the highest SCs early in the leaching process,
with subsequent leaching producing lower SCs. Daniels et al. [18] found that
common laboratory procedures can be conducted with less time and effort than
leaching studies, such as short-term saturated-paste or static SC measurements of
water:spoil mixtures that correspond with peak SC levels in leaching waters; and
SC levels in waters produced by column leaching tests that correspond with field
observations.