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Assessment of Characteristics
and
Remedial Alternatives for Abandoned Mine Drainage:
Case Study at Staple Bend Tunnel Unit
of
Allegheny Portage Railroad National Historic Site,
Cambria County, Pennsylvania, 2004
By Charles A. Cravotta, III
U.S. Department of the Interior
U.S. Geological Survey
Open-File Report 2005-1283
U.S. DEPARTMENT OF THE INTERIOR
Gale A. Norton, Secretary
U.S. GEOLOGICAL SURVEY
P. Patrick Leahy, Acting Director
U.S. Geological Survey, Reston, Virginia: 2005
_____________________________________________
Abstract
This report
describes field, laboratory, and computational methods that could be
used to assess remedial strategies for abandoned mine drainage (AMD).
During April-June, 2004, the assessment process was applied to AMD from
bituminous coal deposits at a test site in the Staple Bend Tunnel Unit
of Allegheny Portage Railroad National Historic Site (ALPO-SBTU) in
Cambria County, Pennsylvania. The purpose of this study was (1) to characterize
the AMD quantity and quality within the ALPO-SBTU test site; (2) to
evaluate the efficacy of limestone or steel slag for neutralization
of the AMD on the basis of reaction-rate measurements; and (3) to identify
possible alternatives for passive or active treatment of the AMD. The
data from this case study ultimately will be used by the National Park
Service (NPS) to develop a site remediation plan. The approach used
in this study could be applicable at other sites subject to drainage
from abandoned coal or metal mines.
During April 2004, AMD from 9 sources (sites1, 1Fe, Fe, 2, 3, 3B, 5,
6, and 7) at the ALPO-SBTU test site had a combined flow rate of 1,420
gallons per minute (gal/min) and flow-weighted averages for pH of 3.3,
net acidity of 55 milligrams per liter (mg/L) as CaCO3, and concentrations
of dissolved sulfate, aluminum, iron, and manganese of 694 mg/L, 4.4
mg/L, 0.74 mg/L, and 1.2 mg/L, respectively. These pH, net acidity,
sulfate, and aluminum values exceed effluent criteria for active mines
in Pennsylvania.
During April-June 2004, limestone and steel slag that were locally available
were tested in the laboratory for their composition, approximate surface
area, and potential to neutralize samples of the AMD. Although the substrates
had a similar particle-size distribution and identical calcium content
(43 percent as calcium oxide), the limestone was composed of crystalline
carbonates and the slag was composed of silicate glass and minerals.
After a minimum of 8 hours contact between the AMD and limestone or
steel slag in closed containers (cubitainers), near-neutral effluent
was produced. With prolonged contact between the AMD and limestone or
steel slag, the concentrations of iron, aluminum, and most dissolved
trace elements in effluent from the cubitainers declined while pH was
maintained greater than 6.0 and less than 9.0. The cubitainer testing
demonstrated (1) lower alkalinity production but higher pH of AMD treated
with steel slag compared to limestone, and (2) predictable relations
between the effluent quality, detention time, and corresponding flow
rate and bulk volume for a bed of crushed limestone or steel slag in
an AMD passive-treatment system.
The process for evaluating AMD remedial strategies at the ALPO-SBTU
test site involved the computation and ranking of the metal loadings
during April 2004 for each of the AMD sources and a comparison of the
data on AMD flow and chemistry (alkalinity, acidity, dissolved oxygen,
ferric iron, aluminum) with published criteria for selection of passive-treatment
technology. Although neutralization of the AMD by reaction with limestone
was demonstrated with cubitainer tests, an anoxic limestone drain (ALD)
was indicated as inappropriate for any AMD source at the test site because
all had excessive concentrations of dissolved oxygen and (or) aluminum.
One passive-treatment scenario that was identified for the individual
or combined AMD sources involved an open limestone channel (OLC) to
collect the AMD source(s), a vertical flow compost wetland (VFCW) to
add alkalinity, and an aerobic wetland to facilitate iron and manganese
oxidation and retention of precipitated solids. Innovative passive-system
designs that direct flow upward through submerged layers of limestone
and/or steel slag and that incorporate siphons for automatic flushing
of solids to a pond also may warrant consideration. Alternatively, an
active-treatment system with a hydraulic-powered lime doser could be
employed instead of the VFCW or upflow system. Now, given these data
on AMD flow and chemistry and identified remedial technologies, a resource
manager can use a publicly available computer program such as “AMDTreat”
to evaluate the potential sizes and costs of various remedial alternatives.
____________________________
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