Neutralization of Acidic Discharges From Abandoned Underground Coal Mines by Alkaline Injection

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REPORT OF INVESTIGATIONS/1993
Neutralization of Acidic Discharges
From Abandoned Underground Coal
Mines by Alkaline Injection
- Natlonal1nstitute for
I . ('"j~ety & -Healtb
(l)ccupatlona11
,),.!
Spokane Research Center
By William W. Aljoe and Jay W. Hawkins
B 315 Montgomery Ave •
.S pokane, WA 99201
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United States Department of the Interior
Bureau of Mines
/
U.S. Department o/the Interior
Mission Statement
As the Nation's principal conservation agency, the Department of
the Interior has responsibility for most of our nationally-owned
public lands and natural resources. T11is includes fostering
sound use of our land and water resources; protecting our fish,
wildlife, and biological diversity; preserving the environmental
and cultural values of our national parks and historical places: and
providing for the enjoyment of life through outdoor recreation.
The Department assesses our energy and mineral resources and
works to ensure that their development is in the best interests of
all our people by encouraging stewardship and citizen participa
tion in their care. The Department also has a major responsibility
for American Indian reservation communities and for people who
live in island territories under U.S. administration.
Cover Photograph: Mine poolflow system at
alkaline injection study site.
Report of Investigations 9468
Neutralization of Acidic Discharges
From Abandoned Underground Coal
Mines by Alkaline Injection
By William W. Aljoe and Jay W. Hawkins
UNITED STATES DEPAR"rMENT OF THE INTERIOR
Bruce Babbitt, Secretary
BUREAU OF MINES
Library of Congress Cataloging in Publication Data:
AJjoe, William W.
Neutralization of acidic discharges from abandoned underground coal mines by
alkaline injection / by William W. A1joe and Jay W. Hawkins.
p. em. - (Report of investigations; 9468)
Includes bibliographical references (p. 36).
1. Acid mine drainage. 2. Neutralization (Ol.emistry). 3. Sodium hydroxide. 4.
Abandoned coal mines-Pennsylvania. I. Hawkins, Jay W. ll. Title. ID. Series:
Report of investigations (United States. Bureau of Mines); 9468.
TN23.N43 [TD899.M5] 622 s-dc20 [622' .5] 92-46322 CIP
CONTENTS
Page
Abstract. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Introduction ................................................... , . . . . . . . . . . . . . . . . . . . . 2
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Identification and description of study sites ................................................. 4
Site description-Keystone State Park ................................................... 4
Site description-Friendship IIiIl NHS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Hydrologic evaluation of mine pools-Keystone State Park . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Data from mine sealing report ........................................................ 7
Estimate of mine pool size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Drilling and installation of monitoring wells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Conceptual flow system ............................................................. 9
Mine discharge flow rates and pool levels ................................................ 11
Response of mine pool to precipitation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Water quality considerations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Tracer test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Hydrologic evaluation of mine pools-Friendship IIiIl NHS ..................................... 19
Drilling and installation of monitoring wells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Conceptual flow system ............................................................. 20
Mine discharge flow rates and pool levels ................................................ 21
Response to precipitation ............................................................ 24
Water quality considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Tracer test .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Alkaline injection ........ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
Selection of test site . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
Application of alkaline solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
Water quality in alkaline injection wells. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
Water quality at mine discharge ....................................................... 30
Postinjection efforts ................................................................ 35
Summary and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
ILLUSTRATIONS
1. Study sites for alkaline injection project ................................................ 3
2. Geologic structure of Keystone site ................................................... 4
3. Approximate boundaries of deep mine and mine pool, Keystone site .... . . . . . . . . . . . . . . . . . . . . . . . 5
4. Pipe network at mine discharge, Keystone site ........................................... 5
5. Geologic structure of Friendship Hill site ........ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
6. Approximate locations of mine entries and coal outcrop (1929 mine map), Friendship Hill site. . . . . . . . 6
7. Mine floor contours and pool limit, Keystone site . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
8. Monitoring well locations, Keystone site . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
9. Mine pool monitoring well construction ................................................ 10
10. Cross section of mine pool, Keystone site . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
11. Mine discharge flows and pool elevations, Keystone site .................................... 11
12. Linear regression-mine pool level versus flow, Keystone site ... ',' . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
13. Mine pool response to precipitation, Keystone site ........................................ 12
14. Acidity concentrations at mine discharge, Keystone site. . . . . . . . . . . . . . . . . . • . . . . . . . . . . • • . • . • • . 13
15. Linear regression-flow versus acidity loading, Keystone site ................................. 14
16. Interval sampler used in mine voids ................................................... 14
17. Average water quality in monitoring wells and mine discharge, Keystone site. . . . . . . . . . . . . . . . . . . . . 15
18. Alternate mine pool flow path, Keystone site ............................................ 16
ii
ILLUSTRATIONS-Continued
Page
19. Stratification of water quality in mine voids, Keystone site. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
20. Estimated flow path of bromide tracer, Keystone site ...................................... 17
21. Decline of bromide concentrations at well 3, Keystone site .................................. 18
22. Estimated seat clay contours, Friendship Hill site ......................................... 19
23. Water elevation contours, Friendship Hill site. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
24. Cross section of mine pool, Friendship Hill site .......................................... 21
25. Mine discharge flow rates, Friendship Hill site ........................................... 22
26. Linear regression-flow rate at main discharge versus secondary discharge, Friendship Hill site ....... 22
27. Water elevations in monitoring wells, Friendship Hill site ................................... 23
28. Response of mine pool to precipitation, Friendship Hill site ................................. 24
29. Acidity and sulfate concentrations at main discharge, Friendship Hill site . . . . . . . . . . . . . . . . . . . . . . . . 25
30. Iron concentrations, ferrous percentages, and pH at main discharge, Friendship Hill site ............ 26
31. Average water quality in monitoring wells and mine discharges, Friendship Hill site. . . . . . . . . . . . . . . . 27
32. Comparison of water quality parameters in well 6 and main discharge, Friendship Hill site . . . . . . . . . . . 28
33. Decline of bromide concentrations at tracer injection well and monitoring well, Friendship Hill site .... 29
34. Potential flow paths of injected alkaline solution, Keystone site ............................... 30
35. Sulfate and acidity concentrations in alkaline injection wells, Keystone site. . . . . . . . . . . . . . . . . . . . . . . 31
36. Acidity and total iron concentrations at mine discharge, Keystone site ............ . . . . . . . . . . . . . . 32
37. Aluminum, manganese, and sodium concentrations at mine discharge, Keystone site ............... 32
38. Arrival of injected sodium at mine discharge, Keystone site . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
39. Diagram of hypothetical sodium mixing process, Keystone site ............................... 34
40. Sodium loading at mine discharge after alkaline injection, Keystone site ........................ 35
TABLES
1. Monitoring well data, Keystone State Park .............................................. 9
2. Monitoring well data, Friendship Hill NHS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
3. Rationale for selection of alkaline injection site .......................................... 29
UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT
ft foot gpm gallon per minute
ft2 square foot h hour
ft/d foot per day in inch
ft3/d cubic foot per day kg kilogram
ft/min foot per minute mg/L milligram per liter
gal gallon
NEUTRALIZATION OF ACIDIC DISCHARGES FROM ABANDONED
UNDERGROUND COAL MINES BY ALKALINE INJECTION
By William W. Aljoe1 and Jay W. Hawkins2
ABSTRACT
The hydrologic characteristics of two abandoned underground coal mine sites, near Latrobe, PA, and
Uniontown, PA, were investigated by the U.s. Bureau of Mines for possible implementation of alkaline
injection into the mine pools as a means of abating their acid discharges. The Latrobe site was chosen
for a one-time application of alkaline injection to achieve a partial, short-term neutralization of a limited
portion of~e mine pool. A quantity of alkaline reagent (sodium hydroxide) sufficient for a one-time
neutralization of the water in this portion of the pool was added. Although nearly all of the neutralizing
reagent had moved away from the injection wells within 6 weeks, no evidence of neutralization was
noted at the mine discharge during the study period. This lack of neutralization probably occurred
because the injection wells did not intercept a primary flow path through the mine. Criteria for
successful application of alkaline injection at this or other sites include (1) positive identification of the
primary flow paths through the mine pool and (2) addition of quantities of alkaline reagent sufficient
to neutralize the entire volume of the pool along the primary flow path.
lEnvironmental engineer.
2Hydrologist.
Pittsburgh Research Center, U.S. Bureau of Mines, Pittsburgh, PA.
2
INTRODUCTION
Discharges from abandoned underground coal mines was attempted by at least six mining companies or contrac
contribute 70% to 75% of the acidic drainage from aban tors in the early 1980's (8). None of these attempts was
doned or inactive coal mines in Appalachia (1-2).3 These successful in achieving a significant improvement in spoil
acid discharges degrade the quality of streams and ground water quality; however, the documentation of these efforts
water by decreasing the pH and contributing dissolved was insufficient to derme with certainty the reasons for
metals and sulfate, thereby negatively affecting aquatic life failure. Part of the problem may have been the use of
(3). In addition, acid stream water can add to the cost of concentrated lime slurries as the source of alkalinity,
water resource development. In certain areas, special con coupled with the low ground water flow velocities within
struction techniques, corrosion-resistant materials, and the spoil. Inefficient mixing of the lime slurries with the
special reservoir operation schedules must be used to spoil water may have resulted in the settlement of sus
moderate low-flow acid degradation extremes (4). pended lime, inhibiting the desired neutralization reac
Water treatment generally alleviates the problem of tions. Surface coatings of iron hydroxide precipitates also
acid discharges from active mines, but the cost of building may have prevented the settled lime from reacting further
and maintaining treatment facilities and associated sludge with spoil water.
disposal operations makes this solution impractical for In at least one case (9), acidic discharges were miti
abandoned mines. In an abandoned mine, the acidic water gated by flooding an underground mine with alkaline solu
is derived from dissolution of acid-forming salts formed at tions upon closure. Prior to flooding in 1984, seals were
closure and stored in the mine pool and from ongoing installed at three mine portals to control discharge, and
pyrite oxidation in the mine void, roof fractures, and phosphate rock dust was applied to the physically accessi
underclay. Even if pyrite oxidation was to cease, the large ble entries in the vicinity of the anticipated pool. (A study
quantity of stored acidity in the mine pool could take many by Stiller (10) previously had shown that phosphate can
years to discharge completely (5). remove substantial quantities of ferric iron (Fe3+) from
To control and mitigate the effect of acid mine drain solution as an insoluble precipitate (FeP0 inhibiting the
4),
age, the V.S. Bureau of Mines and other organizations oxidation of pyrite by Fe3+.) A lime slurry then was intro
have conducted research to determine the viability of duced through a pipe in one of the seals, and a sodium
alkaline injection as a method of abating acidic mine pool hydroxide solution was injected through boreholes near the
discharges. Alkaline injection, as its name implies, anticipated updip edge of the mine pool until the mine was
involves injecting an alkaline solution through boreholes partially inundated. Lime slurry injection continued until
from the surface into carefully selected portions of the 1988, and discharge contaminant concentrations remained
mine void. A study by Poissant and Caruccio (6) suggest below effluent limits until the spring of 1989. At that time,
ed that addition of alkalinity to underground mine pools although pH and acidity levels remained acceptable, iron
may have the potential to neutralize stored acidity, pre concentrations rose from less than 1 to about 15 mg/L.
cipitate metals from solution, and reduce further pyrite However, this still represented a considerable improve
oxidation by inhibiting bacterial activity. Also, since ment compared with the original poor water quality. The
the precipitates form and remain in situ, the problem of sealing and flooding effort proved cost-effective, with a to
sludge disposal would be less severe. An earlier study tal cost of $1.3 million compared with a projected 4O-year
by Stoddard (7) found that, under appropriate flow condi treatment cost of $4.7 million.
tions, precipitates that formed during injection of hydrated The success of an alkaline injection system during mine
lime and limestone would settle out and remain in the closure was encouraging. but its potential for use at
mine while neutral or alkaline water discharged. The pre abandoned mines was still questionable. In abandoned
cipitates were also found to seal or reduce flow through a mines, a potentially large pool of acidic water has already
sand barrier in a simulated mine adit. The subsequent formed. Since the water in the pool fluctuates, and can
field study to investigate the sealing capability of the act as both a source of water for pyrite oxidation and a
precipitate showed limited success because the water means for flushing contaminants from pyritic surfaces, it
flow was only stopped for a short time. An important may be necessary to neutralize the pool completely before
byproduct of the investigation, though, was continued the alkaline conditions needed for bacterial inhibition can
neutralization of the mine water. be achieved. More importantly, neutralization minimizes
Injection of alkaline fluids into surface mine spoils as a the adverse effect that mine pools can have on local
means of abating acidic discharges from toe-of-spoil seeps ground water quality. However, physical access to the
mine is usually impossible, and initial estimates of pool
3(talic numbers in parentheses refer to items in the list of references size and location must be based on old, often inadequate
at the end of this report.
3
mine maps. Although boreholes can be used to help de ,.
t
lineate pool limits, their number and location are often
limited by cost and/or surface accessibility. Finally, even
if the mine pool can be located and neutralized, periodic N Pittsburgh
retreatment may be necessary to maintain neutralization
due to continued acid formation in the unflooded mine
sections. Therefore, the cost of implementing alkaline. o
I •
injection at an abandoned site would probably be greater
Scale, miles Latrobe
than at a mine undergoing closure. However, the long
term cost still may be less than that of conventional treat Washington•
ment, especially when considering sludge disposal costs, so
a study directed toward the application of alkaline injec
tion at an abandoned mine appeared to be warranted.
In 1986-87, the Bureau initially investigated the feasibil
ity of alkaline injection at an abandoned underground
.uniontown
mine. Although substantial quantities of water were found
in the mine during a 1983 underground survey, six of the FRIENDSHIP HILL
NATIONAL
seven wells drilled in 1987 failed to intercept a flow path
PENNSYLVANIA HISTORIC SITE
with measurable water levels. The mine-discharge to
WEST VIRGINIA
mine-area ratio was on the order of 10-8, and a single-well
tracer test revealed a flow rate on the order of 0.1 to
0.2 ft/d. Because of this low flow rate, the results would Figure 1.-Study sites for alkaline Injection project.
not have been recognized at the discharge point for 10 to
20 years. Although alkaline injection would most likely
have been effective, the project was discontinued because (1) monitoring mine discharge flow and quality, (2) drilling
it would not be practical to monitor a research project for and installing wells for mine pool monitoring and possible
such a long tilne period. The advantage of a low flow ve alkaline injection points, (3) monitoring water levels and
locity, though, would be achieving long-term neutralization quality in the wells, and (4) performing tracer tests to
and potential inhibition of pyrite oxidation with fewer determine mine pool flow rates, flow paths, and pool vol
injection episodes. ume turnover time. From these investigations, the Key
The study described in this report was designed to stone State Park site was chosen for a one-time application
investigate the hydrologic factors that would control the of alkaline injection to achieve a partial, short-term
implementation of alkaline injection in an abandoned neutralization of the mine pool. Given the high initial cost
underground mine. An initial survey of known abandoned of alkaline injection during mine closure, the results of the
mine discharges was made to locate several sites that had hydrologic investigation at the Keystone site (described in
potential for alkaline injection. Background data on these this report), and the added uncertainty associated with this
sites were then reviewed, and two sites in western Penn abandoned mine, it was recognized that a full-scale field
sylvania subsequently were determined to be worthy of demonstration could not be attempted. However, it was
further hydrologic investigations. These two sites were believed that the results of the initial application at the
at Keystone State Park, near Latrobe, PA, and Friendship Keystone site would provide valuable information that
Hill National Historic Site (NHS), near Uniontown, PA, could be used to evaluate the general feasibility of the
(fig. 1). These hydrologic investigations consisted of technique in the future at this and other sites.
ACKNOWLEDGMENTS
The authors thank the following individuals for co maintenance foreman, Pennsylvania Department of Envi
ordinating field activities and collecting mine discharge ronmental Resources (DER), Keystone State Park; and
water samples and flow rates during the course of this Steve Linderer, site manager, and Del Barton, park ranger,
study: Michael Bucheit, superintendent, and Jay Burns, U.S. National Park Service, Friendship Hill NHS.
4
IDENTIFICATION AND DESCRIPTION OF STUDY SITES
The existence of a distinct, continuous, measurable located beneath park property. Seam thickness ranged
mine pool was the most important factor governing the from 4 to 6 ft.
selection of potential study sites. In abandoned mines, Figure 2 illustrates the general geologic structure in the
such a pool is most likely to be found behind the portal of vicinity of the Keystone site (12) and fJ.gUl"e 3 depicts the
a sealed, updip drift mine. The initial site selection plan overall extent of the mine workings. Most of the mine
was to identify one or more mine sites with pools of this workings were situated on the northwest limb of the
type, conduct preliminary hydrologic studies, and select the Fayette anticline, although southeast portions of the mine
one site at which alkaline injection would be most likely to crossed the anticlinal axis. According to a mine map
succeed. Initial site selection was based on the existence dated 1941, the main mine entries were driven upgradient
of a measurable acidic discharge at a discrete point or from an outcrop at a structural low point of the coal seam.
points, availability of mine maps, a high discharge flow Most of the initial mine development occurred to the east
rate per unit mine area, and a relatively shallow over and southeast of the mains, also slightly upgradient with
burden. In order to identify mines that met these criteria, respect to geologic structure. A few panel headings were
geologic and mine map data were obtained for several driven to the southwest, approximately along strike, but
sites known to contain acidic underground mine dis these were much shorter in length and did not support ex
charges. Mining permit files of the Bureau of Mining and tensive room development. This pattern of mining sug
Reclamation, Pennsylvania DER, Greensburg, PA, office, gests that the entries were laid out such that water would
were also reviewed for potential sites. drain away from the working faces and toward the portals.
The two field sites selected for initial hydrologic evalu When the mains reached the axis of the Fayette anticline,
ation were in Keystone State Park, near Latrobe, PA, and extensive sets of panel entries and rooms could be driven
Friendship Hill NHS, near Uniontown, PA, (fig. 1). These
two sites met all the preliminary site selection criteria, and
had the advantage of being located on public property. At
the Keystone site, the portal seals were installed delib
erately with the intent of flooding the mine. At Friendship
Hill, the portals became sealed as the result of roof col
lapses and emplacement of strip mine spoils. The follow
ing sections describe these study sites in terms of their
geologic settings, mining history, and potential viability for
alkaline injection.
SITE DESCRIPTION-KEYSTONE STATE PARK
One of the advantages of the Keystone site was its rela
tively well-documented history compared with most aban
doned mine sites. The primary source of information was
a report on an attempt to develop and use a gel material
for sealing one of the mine portals (11).
Documentation of subsequent mine reclamation proj
ects was obtained from the Bureau of Abandoned Mine
Reclamation, Pennsylvania DER. Keystone Park person
nel also provided mine maps and valuable information on
previous activities dealing with the abandoned mine.
The mine at Keystone State Park, which was originally
2
known as the Salem No. 2 Mine of the Atlantic Crushed I
Coke Co., was developed in the Upper Freeport Coal Scale, miles
Seam using room-and-pillar methods. According to form
er mine workers, mining began in 1938 and ceased during
the 1950's. Available maps showed that the mined area Figure 2.-Oeologlc structure of Keystone site; structure con
covered approximately 300 acres, most of which was tours on Upper Freeport Coal.