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Abandoned underground mine in the 1940's
Photos courtesy of the Office of Surface Mining
and Reclamation
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The same area after reclamation
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Jay W. Hawkins
Office of Surface Mining, Pittsburgh, PA 15220
Introduction
In general, remining is defined as any operation where additional mining
occurs subsequent to the original mining or site abandonment regardless
of the existence or quality of mine discharges. However, the term remining
as it is used in this chapter, primarily refers to surface mining of
abandoned surface or underground mines or reprocessing of coal refuse
piles where preexisting pollution discharges will be affected by remining
under Pennsylvanias Subchapter F and Subchapter G (anthracite)
program. Site-specific effluent standards are established based on loading
rates rather than conventional concentration-based BAT effluent standards.
Mine drainage prediction of remining sites where the discharges are
required to meet conventional effluent standards is covered by standard
prediction techniques discussed in the preceding chapters of this manual.
Prediction of additional mine drainage from remining sites is distinctly
different compared to normal mine drainage prediction at previously
unmined sites. Instead of contaminant concentration (e.g., mg/L) levels
and pH of the effluent, prediction of contaminant load (e.g., lbs/day
or kg/day) levels become the primary objective for remining mine drainage
prediction. Discharge flow rate is used to determine contaminant load
and becomes a primary determinant of the reviewed effluent standard.
There is a direct positive correlation between discharge rate and pollution
load. Smith (1988) and Hawkins (1994a; 1994b) have observed that discharge
flow rate is a major element of contaminant load. Therefore, the physical
hydrology of the mine becomes a larger component of mine drainage prediction
than overburden geochemistry in remining situations compared to mining
virgin sites.
Historical Impacts of Remining
Remining in the bituminous coalfields of Pennsylvania has, at the majority
of sites, resulted in no change or an improvement in the water quality
in terms of contaminant (acidity, iron, and sulfate) loads (Hawkins,
1994b). Analysis of 24 reclaimed western Pennsylvania remining sites
illustrated that a large majority of the sites either did not change
or significantly reduced post-remining acidity, iron, and sulfate loads.
For that study, data were analyzed from remining sites in the bituminous
coal fields of Pennsylvania that had been reclaimed (backfilled to rough
grade) for one year or longer.
The study determined changes in the post-remining contaminant load data
using the methodology employed by the Pennsylvania Department of Environmental
Protection and two other applicable analytical methods (Mann-Whitney
U test and nonparametric upper prediction limits). All three of the
methods indicate that remining either successfully reduced or did not
significantly change the contaminant loads for at least 20 of the 24
sites. More sites (8) exhibited a significant reduction in acidity,
iron, or sulfate load than the number of sites that exhibited a significant
load increase (3 or 4) (Hawkins, 1994b).
There were a few cases where the post-remining water quality was significantly
improved in terms of contaminant load and began to meet the concentration-based
statutory effluent standards (25 PA. Code 87.102). This situation usually
occurred on sites where surface mining daylighted (i.e., remined abandoned
underground mines by surface mining methods) a substantial area of abandoned
underground mines. The water quality improvement appears related to
the presence of significant amounts of alkaline material (e.g., limestone
or calcareous shales) in the overburden. Removal of the coal itself
and redistribution of roof material that collapsed in the open voids
may have contributed to the water quality changes. Before the underground
mine was daylighted, the groundwater had limited and transient contact
with this alkaline material. Groundwater passing through the underground
mine had prolonged exposure to the floor rock, coal and roof rock, all
of which are commonly acid-forming materials. Additionally, roof falls
and pillar weathering continue to add new sources of acidity from the
freshly exposed rock material. Daylighting radically alters the groundwater
flow regime, the rock material contacted, and greatly increases the
rock surface area and groundwater contact time. Therefore, when limestone
in the overlying strata is removed and backfilled, it has the potential
to yield substantially more alkalinity to the groundwater, which can
significantly improve the groundwater quality.
In the cases where the remining increased pollution loads there are
several possible causes. The first reason may be that remining has created
additional pollution. However, short-term changes in flow and/or contaminant
concentration that commonly occur during the initial 1-3 years after
backfilling is another possible cause. The first 1-3 years after backfilling
is a period of substantial physical and chemical fluctuation within
the spoil aquifer. During this period, the water table is reestablishing
and the spoil is undergoing considerable subsidence, piping, and shifting.
The sulfate salts, created by oxidation when the cast overburden was
exposed to the atmosphere during mining, are flushed through the system
(Hawkins, 1995). If the data for that discharge were analyzed after
only 1000 days, the remining would appear to have failed because the
acidity load frequently exceeds the upper bound of the 95% confidence
limits. However, when the post-remining sampling period is extended
to over 1800 days, that conclusion is no longer valid. Therefore, the
true impact of remining on pollution loads may require monitoring beyond
3 years after backfilling and short-term degradation may not be unexpected.
Daylighting of underground mines does not necessarily improve the discharge
water quality. Reed (1980) analyzed the impact of daylighting an abandoned
850 ac (344 ha) underground mine in Tioga County, Pennsylvania. He observed
that the daylighting, still active during his study, was increasing
the acidity concentration of the discharges. A direct relationship between
the amount of daylighting and the acidity concentration increase was
noted. Concentration is frequently a function of discharge rate (an
inverse relationship), therefore load is a better assessment of water
quality improvement. However, the impact of the mining on the acid load
was not determined. The cause of the apparent acidity increases is not
known. However, it is possible the overburden may have had significant
amounts of acid-producing strata, or it may have been a case of temporary
degradation, as previously discussed. Subsequent analysis of the acid
loading data from the three main discharges, after reclamation of the
corresponding recharge area, showed no statistically significant changes
from pre-remining levels. Although, the acid loads may have lowered
from the levels recorded during the active daylighting phase (Meiser,
1982). Daylighting on the same coal seam at a near-by site also resulted
in degraded water quality, due to the substantial high-sulfur strata
and the lack of significant alkaline strata overlying the coal (Naylor,
1989).
Similarly, Ackerman and others (undated) evaluated the impact of daylighting
an abandoned underground mine in Garrett County, Maryland. They observed
that the post-remining pollution loads did not significantly change
from pre-remining levels. However, a slight improvement in pollution
load may have occurred shortly after reclamation. They also observed
that the pollution load seasonal fluctuations were greater than pre-remining
levels.
Proven Track Records and Experience-Based Rules-of-Thumb
Within Pennsylvania there are certain areas, coal seams, and mining
situations (e.g., abandoned underground mines, surface mines, or coal
refuse piles) that are known to mining professionals to have either
a good or bad track record when disturbed by remining. Some regions
and coal seams are known to yield greatly improved water quality after
remining virtually regardless of how the operation is conducted. Other
areas and coals seams are notorious for producing worse quality discharges,
regardless of how well the operation was conducted.
Experience has shown that daylighting of Pittsburgh coal underground
mine workings in Washington, Beaver, and Allegheny Counties, Pennsylvania
substantially yields improved water quality over pre-remining conditions.
When the daylighting is substantial, the discharges change from being
strongly acid to being significantly alkaline. Some acidic mine discharges
improve somewhat after remining, but do not become alkaline. An example
of this situation was an operation in Washington County, where there
were 5 preexisting acidic mine discharges. Some of the discharges went
from being acid to alkaline (acid loads went from 75.6 lb/day (34.3
kg/day) to no acid load), while others exhibited reduced acid loads,
but remained acidic. The degree of change of the discharges appeared
to be related to amount of the recharge area that was daylighted. The
water quality changes appear to be directly related to both the removal
of the coal, which has sufficient sulfur content to be acid producing,
and breakup of the overburden, which possesses a significant amount
alkaline material. Entire streams, such as Potato Garden Run in Beaver
County, have recovered because of nearly complete daylighting of abandoned
underground mine working in that area.
Examples exist where complete daylighting of an underground mine will
eliminate or nearly eliminate the discharges through substantial changes
to the groundwater flow system. At a 43 ac (17 ha) mine in Clinton County,
the underground mine workings were completely daylighted. Subsidence
and collapse features that facilitated recharge to the mine were removed.
The postmining recharge rates through the spoil were significantly below
pre-remining levels (See chapter 3 for a discussion on recharge to mine
spoil). Three years of postmining data seldom showed any measurable
flow at the one discharge point. It is unlikely that the discharge was
completely eliminated, because some of this monitoring was conducted
while the water table was reestablishing. However, the data indicate
that the flow was and will continue to be substantially lower than pre-remining
levels.
There are coal seams in parts of the coalfields where remining is known
to leave discharges unchanged from preexisting levels. Examples of this
are the Freeport coal seams in northern Armstrong County, which are
known to have marginal overburden quality, yet remining seldom increases
the pollution loads. The pre-remining acidity loads (the discharges
are slightly acidic) are generally low and the metals (iron, manganese,
and aluminum) commonly at times meet Best Available Technology (BAT)
(87.102) effluent standards. The overburden is characterized by low
amounts of alkaline material coupled with low sulfur values. Both of
these constituents appear to have been leached from the strata by weathering,
leaving little to react (Michael W. Gardner, personal communication).
There also are certain seams and areas within Pennsylvania where remining
without additional abatement measures such as alkaline addition, typically
increases the pollution load for acidity and/or metals. For example,
commonly remining on the Waynesburg coal seam in Greene County increases
the pollution load. Manganese and iron loads are frequently observed
problems associated with the Waynesburg coal (Michael W. Gardner, personal
communication). The Waynesburg sandstone is thought to be the main AMD
producing unit. Remining on the Lower Kittanning or Clarion coal in
northcentral Pennsylvania generally increases acid loads unless flow
reduction measures are taken and alkaline materials are brought to the
site (Michael W. Smith, personal communication).
Recommendations
Mine drainage prediction for remining sites must be viewed differently
than for virgin sites. Discharge contaminant loads instead of concentrations
are regulated and forecasted. Because of the dominance of flow in contaminant
load determinations, abatement and reclamation plans should stress the
implementation of flow reduction techniques. Abatement practices to
reduce recharge to the spoil aquifer should yield a predictable (within
a range of projected values) decrease in flow using known site conditions
along with standard geologic and hydrologic techniques. The flow reduction
will subsequently yield a predictable contaminant load reduction.
Given the track record in Pennsylvania and the observed benefits that
reducing flow has on contaminant load, remining can be a viable means
of abating or diminishing AMD discharges in many areas. This may be
the only economically viable solution for reducing some of the highly-degraded/high-volume
underground mine discharges. Long-term discharge treatment is typically
not viable and in many instances cost prohibitive.
References
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