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OCR for page 3455
Proc. Natl. Acad. Sci. USA
Vol. 96, pp. 3455-3462, March 1999
Colloquium Paper
This paper was presented at the National Academy of Sciences colloquium "Geology, Mineralogy, and Human Welfare, "
held November 8-9, 1998 at the Arnold and Mabel Beckman Center in Irvine, CA.
Negative pH, efflorescent mineralogy, and consequences for
environmental restoration at the Iron Mountain
Superfund site, California
D. KIRK NORDSTROM*T AND CHARLES N. ALPERST
*United States Geological Survey, 3215 Marine Street, Boulder, CO 80303-1066; and tUnited States Geological Survey, Placer Hall, 6000 J Street, Sacramento,
CA 95819-6129
ABSTRACT The Richmond Mine of the Iron Mountain
copper deposit contains some of the most acid mine waters
ever reported. Values of pH have been measured as low as
-3.6, combined metal concentrations as high as 200 g/liter,
and sulfate concentrations as high as 760 g/liter. Copious
quantities of soluble metal sulfate salts such as melanterite,
chalcanthite, coquimbite, rhomboclase, Voltaire, copiapite,
and halotrichite have been identified, and some of these are
forming from negative-pH mine waters. Geochemical calcu-
lations show that, under a mine-plugging remediation sce-
nario, these salts would dissolve and the resultant 600,000-m3
mine pool would have a pH of 1 or less and contain several
grams of dissolved metals per liter, much like the current
portal effluent water. In the absence of plugging or other
at-source control, current weathering rates indicate that the
portal effluent will continue for approximately 3,000 years.
Other remedial actions have greatly reduced metal loads into
downstream drainages and the Sacramento River, primarily
by capturing the major acidic discharges and routing them to
a lime neutralization plant. Incorporation of geochemical
modeling and mineralogical expertise into the decision-
making process for remediation can save time, save money,
and reduce the likelihood of deleterious consequences.
Mining and Water Quality
Mining of metallic sulfide ore deposits (primarily for Ag, Au,
Cu. Pb, and Zn) produces acid mine waters with high concen-
trations of metals that have harmful consequences for aquatic
life and the environment. Deaths of fish, rodents, livestock, and
crops have resulted from mining activities and have been noted
since the days of the Greek and Roman civilizations. Mining
and mineral processing have always created health risks for
miners and other workers. In addition, mining wastes have
often threatened the health of nearby residents by exposure to
emissions of sulfur dioxide and oxides of As, Cd, Pb, and Zn
from smelter stacks and flues, metal-contaminated soils, and
waters and aquatic life with high concentrations of metals. As
with most forms of resource extraction, human health risks
accompany mineral exploitation.
In 1985, the U.S. Environmental Protection Agency (EPA)
estimated that 50 billion tons (45 x 10~2 kg; 1 ton = 907 kg)
of mining and mineral processing wastes had been generated
in the United States and about 1 billion tons would continue
to be generated each year (1~. More recently, the EPA has
described 66 "damage cases" at their web site (www.epa.gov,
search for Mining and Mineral Processing Wastes, accessed
Sept. 9, 1998) in which environmental injuries from mining
PNAS is available online at www.pnas.org.
activities in the U.S. are detailed. Government records indicate
that many millions, perhaps billions, of fish have been killed
from mining activities in the U.S. during this century (2~.
Incidents of arsenic poisoning in residents of Thailand result
from arsenic contamination of the shallow groundwaters be-
cause of weathering of mine wastes (3~. A mine flood disaster
in Spain occurred in April 1998 in which about 6 million m3 of
acid water and sulfide tailings escaped from a breached
impoundment and covered about 6,500 acres of farmland and
river banks along a 70-km reach of the Guadiamar River with
fine-grained sulfides (details at www.csic.es). Numerous rivers,
estuaries, and reservoirs throughout the world have been used
as dumping grounds for the large volumes of waste produced
during mineral extraction and processing. Mineral processing,
in addition to fossil fuel and metal utilization, has increased the
concentration of selected metals and nonmetals in the atmo-
sphere. The emissions of As, Cd, Cu, Pb, Sb, and Zn from
anthropogenic sources are all greater than emissions from
natural sources, sometimes several times higher (4, 5~.
Acid mine drainage is produced primarily by the oxidation
of the common iron disulfide mineral pyrite. Pyrite oxidation
is a complex process that proceeds rapidly when this mineral
and other sulfides are exposed to air. A simplified represen-
tation of this chemical process is given by the reaction of nYrite
with air and water,
- r~-
FeS2(s) + 7/202(g) + H2O(1) > Fe(2aq) + 2S°4(aq) + 2H(aq)
in which the product is a solution of ferrous sulfate and sulfuric
acid. The dissolved ferrous iron continues to oxidize and
hydrolyze when the mine water is no longer in contact with
pyrite surfaces,
Fe(aq) + 1/4°2(g) + s/2H2O(I) ~ Fe(OH)3(S) + 2H(aq) [21
producing additional acidity. Iron- and sulfur-oxidizing bac-
teria, especially Thiobacillus ferrooxidans, are known to cata-
lyze these reactions at low pH, increasing reaction rates by
several orders of magnitude (6~. These processes occur natu-
rally and, indeed, natural acidic drainage is well known from
many locations (7~. Mining has the overall effect of dramati-
cally increasing the oxidation rates by providing greater ac-
cessibility of air through mine workings, waste rock, and
tailings, by creating greater surface area exposure through
blasting, grinding, and crushing, and by concentrating sulfides
in tailings. The overall rates of sulfide oxidation and metal
Abbreviation: EPA' Environmental Protection Agency.
tTo whom reprint requests should be addressed. e-mail: dkn@
usgs.gov.
3455
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3456 Colloquium Paper: Nordstrom and Alpers
release in areas affected by mining are estimated to be orders
of magnitude faster than natural rates.
Another process, sometimes overlooked, plays an important
role in the environmental consequences of mining: the for-
mation of soluble, efflorescent salts. Acid ferrous sulfate
solutions often become so enriched through rapid pyrite
oxidation and evaporation that soluble salts form. These often
appear as white, blue-green, yellow to orange or red efflores-
cent coatings on surfaces of waste rock, tailings, and in
underground or open-pit mines. Acidity and metals, formerly
contained in the acid mine water, are stored in the salts, which
can quickly be dissolved by a rising groundwater table or be
dissolved when exposed to rain and flowing surface waters, and
then infiltrate to groundwaters. The Iron Mountain Mine
Superfund site is an extreme example of how the formation of
soluble efflorescent minerals can make certain remediation
alternatives much more risky and potentially disastrous than
might otherwise be imagined.
Iron Mountain
Iron Mountain is located in Shasta County, California, ap-
proximately 14 km northwest of the town of Redding (Fig. 1),
in the southern part of the Klamath Mountains. "Iron Moun-
tain Mine" is really a group of mines within Iron Mountain that
include Old Mine, No. 8 Mine, Confidence-Complex, Brick
Flat Open Pit Mine, Mattie Mine, Richmond and Richmond
Extension Mines, and Hornet Mine. Ag, Au, Cu. Fe, Zn, and
pyrite (for sulfuric acid production) were recovered at various
times beginning in the early 1860s and ended with the termi-
nation of open-pit mining in 1962. Iron Mountain was once the
largest producer of Cu in the state of California, and now it
produces some of the most acidic waters in the world. Prior to
the late 1980s when major remediation efforts began, approx-
imately 2,500 tons of pyrite weathered every year from one
mine alone (the Richmond Mine) and water containing about
300 tons per year of dissolved Cd, Cu. and Zn drained from the
site into the Sacramento River. During periods of high runoff,
sudden surges of acid mine waters into the Sacramento River
have caused massive fish kills, which state and federal agencies
have investigated since 1939. More than 20 fish-kill events have
occurred in Sacramento River receiving waters since 1963,
with at least 47,000 trout killed during a single week in 1967 (8~.
40O45'
22O30' 122°1 5.
1 1
~ ~ Lake
Mountain 3(SI~asta
Bout
~-~> r spring Creek
/N Amoebas Dam
Slickrock ~ I
Or be, Keswick
Dam
~~~ i, <\Redding
Map
Area
FIG. 1. Location of Iron Mountain Mine, California (adapted from
ref. 15~.
Proc. Natl. Acad. Sci. USA 96 (1999)
Furthermore, the town of Redding (with approximately
lOO,OOO residents) receives its drinking water from the Sacra-
mento River, downstream from the Iron Mountain site. Large,
metal-rich sediment deposits containing toxic porewaters have
built up in Keswick Reservoir, where the Spring Creek drain-
age from Iron Mountain empties into the Sacramento River.
A brief history of mining, water management, and environ-
mental action at Iron Mountain is outlined in Table 1.
The mineral deposits are primarily massive sulfide lenses as
much as 60 m thick containing up to 95% pyrite, variable
amounts of chalcopyrite and sphalerite, and averaging about
1% Cu and about 2% Zn. Some disseminated sulfides occur
along the south side of the mountain. The deposits at Iron
Mountain and elsewhere in the West Shasta mining district are
Devonian in age and have been classified as Kuroko type,
having been formed in an island arc setting in a marine
environment (9~. The country rock is the Balaklala Rhyolite,
a keratophyric rhyolite that has undergone regional metamor-
phism during episodes of accretion of oceanic crust to the
continent. The brittle, fractured nature of the altered volcanic
bedrock gives rise to a hydrologic conditions dominated by
fracture-flow at Iron Mountain. The mineral composition of
the rhyolite is albite, sericite, quartz, kaolinite, epidote, chlo-
rite, and minor calcite; consequently it has little buffering
capacity. Kinkel and others (10), Reed (11), and South and
Taylor (12) have documented the chemical and isotopic com-
positions of ore, gangue, and country-rock minerals in the
West Shasta mining district. Weathering of massive sulfide
deposits at and near the surface has given rise to large gossan
outcrops, enriched in Ag and Au. The 10 million tons of gossan
in place prior to mining is the residue from at least 15 million
tons of massive sulfide that weathered naturally. A total of 7.5
Table 1. Brief chronology of Iron Mountain mining and
envrionmental activities
Year
1860s
1879
1897
1902
1907
1928
1939
1943
1950
196:
1983
Activity
Discovery of massive gossan outcropping
Silver discovered in gossan and mining begins
Mountain Copper Co. acquires property and under
ground mining begins
U.S. Forest Reserve sues company for vegetation
damage from smelting activities
Smelting ends and ore is transported to Martinez,
CA, for processing
California Fish and Game Commission files
complaint regarding tailings dam
State initiates water quality and fish toxicity studies
Shasta Dam, upstream from Iron Mountain
outflows, is completed
Keswick Dam, downstream from Iron Mountain
outflows, is completed
1955-1962 Open-pit mining of pyrite at Brick Flat for sulfuric
acid production
Spring Creek Debris Dam is completed, regulating
outflow of acid mine waters to the Sacramento
River
1967 Stauffer Chemical Co. acquires property
1976 Iron Mountain Mines, Inc., acquires property
1976-1982 State of California fines company for unacceptable
releases of metals
Iron Mountain listed on National Priorities List
(NPL) for EPA Superfund, ranking as the
third-largest polluter in the State of California
1986-1998 Four Records of Decision by EPA have instituted
several remedial activities that include partial
capping, surface-water diversions, tailings
removal, and lime neutralization of the most
acidic, metal-rich flows, reducing copper and zinc
loads by 80-90~o
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Representative terms from entire chapter:
geological survey
Colloquium Paper: Nordstrom and Alpers
million tons of sulfide ore was mined at Iron Mountain, and
remaining reserves are estimated at approximately 15 million
tons (13), so the overall size was at least 37.5 million tons prior
to weathering. Preliminary paleomagnetic data on iron oxides
in the gossan show portions with reversed polarity, indicating
the gossan began forming at least 780,000 years ago. Secondary
enrichment in the upper zones of the massive sulfides resulted
in high concentrations of Cu (5-10~o) and Ag (about 1 oz/ton).
This enrichment took place at or near the water table during
gossan formation.
Three main massive sulfide ore bodies, the Brick Flat, the
Richmond, and the Hornet, include most of the oxidizing
sulfides causing the current water-quality problems. These ore
bodies are thought to be parts of a single massive sulfide body
about 0.8 km long, over 60 m wide, and over 60 m thick that
was offset by two normal faults (Fig. 2~. All three of these
bodies have been mined, and the consequences include large
changes in the hydrogeology, resulting in highly contaminated
waters from tunnels, tailings, and waste rock piles.
Acid Effluent from the Richmond Mine
Conditions at Iron Mountain are nearly optimal for the
production of acid mine waters, and this mine drainage is some
of the most acidic and metal-rich reported anywhere in the
world (14, 15~. In the Richmond Mine, about 8 million tons of
massive sulfide remain (13~. At current weathering rates it
would take about 3,200 years for the pyrite in the Richmond
ore body to fully oxidize. The massive sulfide deposit is about
95% pyrite and is excavated by tunnels, shafts, raises, and
slopes which allow rapid transport of oxygen by air advection.
The sulfides are at or above the water table so that moisture
and oxygen have ready access. Airflow is driven by the high
heat output from pyrite oxidation. About 1,500 kJ of heat is
released per mole (120 g) of pyrite. Air enters the main tunnel,
heats up in the mine, then travels up through raises and shafts
to the surface. The average flux of acid mine drainage from the
Richmond portal indicates that about 2,400 mol of pyrite is
oxidized every hour, producing about 1 kW of power or almost
9,000 kW per year. Water temperatures as high as 47°C have
been measured underground, and the amorphous silica geo-
thermometer (16, 17) would suggest temperatures of at least
50°C in the subsurface. In the early days of mining at Iron
Mountain, fires were frequent during underground excavation,
and temperatures of 430°F (221°C) were recorded at the ore
surface (18~.
A considerable amount of historical data exist for effluent
composition and discharge from the Richmond Mine because
it is the largest single source of dissolved metals (both in terms
of concentration and in terms of flux) in the Iron Mountain
district. The Richmond ore body was discovered about 1915
but it was not mined on a large scale until the late 1930s and
the war years (1940-1945~. Regular monitoring of the Rich-
mond Mine effluent by the California Regional Water Quality
Control Board in cooperation with the EPA began in 1983. A
summary of the data for discharge, pH, and Cu and Zn
concentrations for 1983-1991 is shown in Table 2. Further
_ East
~ 3,500
At: 3,300
~ 3,1 00 Richmond Richmo~~
,, 2, 700 Lawson \ /\ deposit
is 2,500
To
c 2,300
2,100- _
FIG. 2. Cross-section of Iron Mountain (adapted from ref. 154.
Proc. Natl. Acad. Sci. USA 96 (1999' 3457
Table 2. Richmond Mine portal effluent
characteristics, 1983-1991
Mean
Discharge, liter/e
pH
Zinc, mg/liter
Copper, mg/liter
Data are from ref. 19.
Range
4.4 0.5-50
0.8 0.02-1.5
1,600 700-2,600
250 120-650
compilation and details of Richmond portal effluent compo-
sition and discharge can be found in Alpers et al. (~19~.
The variability in the Richmond effluent with time can be
seen quite clearly for the 1986-1987 monitoring period. Fig. 3
covers the time period of late November 1986 to April 1987
and shows the rainfall (at Shasta Dam), and the consequent
changes in Richmond Mine discharge and copper and zinc
concentrations. One explanation for the large increase in
copper concentrations is the dissolution of underground sol-
uble salts from the flushing effect of meteoric recharge (see
below). An observed increase in temperature with increased
discharge may be the result of the dilution of concentrated
sulfuric acid, the dissolution of soluble salts, and increased
pyrite oxidation.
One of the obvious options for remediation of the Richmond
Mine was to plug it. Many mines have been plugged, but the
consequences have not been consistently favorable. The EPA
wanted to know what the consequences of plugging the
Richmond Mine might be; for example, what would the
composition of the resultant mine pool be? There was, how-
ever, no basis on which to speculate without some idea of the
underground conditions. Hence, one of the activities of the
Second Remedial Investigation Phase (1986-1992) under the
Superfund Program was an underground survey of the Rich-
mond tunnel and part of the mine workings. Prior to under-
ground renovations in 1989-1990, the last underground tour,
to the best of our knowledge, was in 1955 (Don White, U.S.
Geological Survey, personal communication, 1989~. The last
mining had occurred in the late 1940s. Other than an occa-
sional inspection by a company employee, there had been no
recorded observation of the underground workings for 35-40
years. After underground renovations, entry was safe, and on
September 10-12, 1990, water and mineral samples were
2,000
z == ~ ,000
O
600
. ~ 400
t _
, G200
, ~
0 of
cat
a ~200
~L
~5
OCR for page 3458
3458 Colloquium Paper: Nordstrom and Alpers
collected. They revealed extremely acidic seeps with pH values
as low as -3.6 and total dissolved solids concentration of more
than 900 g/liter.
The chemical compositions of five of the most acidic waters
found underground in the Richmond Mine during 1990-1991
are shown in Table 3. These concentrations are the highest ever
recorded for As, Cd, Fe, and SO4 and nearly the highest for Cu
and Zn in groundwater. The high subsurface temperatures
have induced considerable evaporation, which, in addition to
pyrite oxidation, has caused the high concentrations of dis-
solved metals and sulfate.
The reporting of negative pH values has been controversial,
and for several good reasons. The conventional definition of
pH based on the former National Bureau of Standards criteria
and defined buffer systems limits the range of definable and
measurable pH values to that of 1 to 13. Outside this range, the
concept and measurement of pH are difficult at best. Further-
more, a new definition of pH must be used that is consistent
with the conventional definition, different buffers must be
used and electrode performance and interferences must be
determined. The most acceptable model for activity coeffi-
cients at present for defining pH below 1.0 is the Pitzer
ion-interaction approach (20, 21~. Acid mine waters are solu-
tions of sulfuric acid, so the Pitzer model applied to sulfuric
acid (22, 23) could serve as a definition for pH. Standardized
sulfuric acid solutions would then serve as buffer solutions for
calibration and the remaining question is the performance of
standard glass membrane electrodes under these extreme
Proc. Natl. Acad. Sci. USA 96 (1999)
conditions. Several Orion Ross glass membrane electrodes and
a Sargent-Welch glass membrane electrode all performed well
and could be calibrated up to a sulfuric acid concentration of
about 8 molal. Another difficulty facing the definition of pH
below 0.0 is scaling of individual ion activity coefficients. There
is no generally accepted procedure for defining individual ion
activity coefficients without some arbitrary assumptions. Two
common methods with the Pitzer approach include "unscaled"
Pitzer equations, and "MacInnes scaled," using the MacInnes
assumption (24~. The MacInnes assumption is simpler, more
flexible for a wide range of complex chemical compositions,
and is more consistent with conventional speciation models
applied to natural waters (24~. It could be argued that the
MacInnes assumption becomes less defensible at high concen-
trations where the unscaled approach should be more appro-
priate, but there is no obvious justification for using one
approach over the other and the choice remains arbitrary. In
the present investigations, the MacInnes scaling was used
primarily because geochemists who have applied the Pitzer
method to the interpretation of brines and saline waters find
the MacInnes assumption more consistent with conventional
practice. If the unscaled approach is used, the resultant pH
values begin to differ significantly from MacInnes scaling for
sulfuric acid solutions with pH values below -0.5. For exam-
ple, at a sulfuric acid concentration of about 5.0 molal a scaled
pH would be -2, whereas the unscaled pH would be notably
higher, about -1.2.
Some of these negative-pH mine waters were in apparent
equilibrium with prominent soluble salts. For example, a
Table 3. Compositions of five extremely acid mine water samples from the Richmond Mine
Concentration of element in sample, mg/liter
90WA103 90WA109 90WAllOA 90WAllOC 91WA111
34.8°C 38°C 42°C 46°C 28°C
Element pH 0.48 pH -0.7 pH -2.5 pH -3.6
Aluminum 2,210 6,680 1,420 6,470
Antimony 4.0 16 29 15
Arsenic(III) 8.14 38 32 - 74
Arsenic (total) 56.4 154 340 850
Barium 0.068 0.1 0.2 <0.1
Beryllium 0.026 0.1 0.2 <0.1
Boron 1.5 2.5 17
Cadmium 15.9 48.3 211 - 370
Calcium 183 330 279 443
Chromium 0.12 0.75 0.6 2.6
Cobalt 1.3 15.5 5.3 3.6
Copper 290 2,340 4,760 9,800
Iron(II) 18,100 79,700 34,500 9,790
Iron (total) 20,300 86,200 111,000 16,300 68,100
Lead 3.6 3.8 11.9 8.3
Magnesium 821 1,450 437 2,560
Manganese 17.1 42 23 119
Molybdenum 0.59 1.0 4.2 2.3
Nickel 0.66 2.9 3.7 - 6.3
Potassium 261 1,170 194 11.1
Selenium 0.42 2.1 4.2 - <2.8
Silicon (as sio2) 170 34 35
Silver 0.16 0.65 2.4 0.70
Sodium 251 939 416 44
Strontium 0.25 0.49 0.90
Sulfur (as S04) 118,000 360,000 760,000
Thallium 0.44 0.15 0.39 1.6
Tin 1.6 15 41
Titanium 5.9 125 1.0
Vanadium 2.9 11 15 28
Zinc 2,010 7,650 23,500 49,300
Associated mineral~s)
Melanterite Rhomboclase, romerite Rhomboclase
A dash indicates no determination was made.
Colloquium Paper: Nordstrom and Alpers
FIG. 4. Growth of cuprian melanterite in a manway of the Rich-
mond Mine with stalactite dripping pH = -0.7 water into plastic
beaker. (Photo by D.K.N. and C.N.A.)
stalactite of zincian-cuprian melanterite had water dripping
from the tip that had a pH of -0.7 (Table 3 and Fig. 4~.
Enormous quantities of highly soluble iron sulfate salts were
found as efflorescences and precipitates, coating walls, ceil-
ings, and floors of the mine and growing out of muck piles in
colorful assemblages. Identification of these soluble salts made
it possible to estimate what the composition of a mine pool
formed by mine plugging might be.
Soluble Salts and Consequences of the Mine-Plugging
Scenario
Ten soluble iron sulfate salts plus gypsum and chalcanthite
were identified in the Richmond Mine. These minerals and
their idealized formulae are listed in Table 4, with the iron salts
in approximate sequence downward from the early formed to
the later formed. Rhomboclase was found as stalactites and
stalagmites (Fig. 5), and clusters of coquimbite, romerite,
copiapite, and Voltaire crystals were common throughout the
mine (Fig. 6~. Rhomboclase was rarely found without Voltaire
crystals.
As long as an acid mine water is in contact with pyrite, the
dissolved iron will remain in the ferrous state because of the
strong reducing capacity of the pyrite. Rapidly flowing mine
water will still maintain a high proportion of ferrous iron
because the oxidation rate is often slow enough relative to the
Table 4. Idealized formulae of sulfate minerals found in the
Richmond Mine
Mineral
Melanterite
Rozenite
Szomolnokite
Copiapite
Romerite
Coquimbite
Kornelite
Rhomboclase
Voltaite
Halotrichite-bilinite
Proc. Natl. Acad. Sci. USA 96 (1999J 3459
FIG. 5. Stalagmite of rhomboclase (white) and coquimbite (pur-
ple) in the Richmond Mine. (Photo by C.N.A. and D.K.N.)
flow rate of the water. Consistent with this expectation, the
only iron sulfate salts containing exclusively ferrous iron,
melanterite, rozenite, and szomolnokite, are found close to
pyrite sources and associated with more rapidly flowing wa-
ters. Ferric-bearing minerals are found to form in more
stagnant conditions and can be considered to be hydrologic
"dead-ends," where much of the Fell has had time to oxidize
to Fells. Additional evidence for this mineralogical evolution is
the observation that melanterite is the first-formed mineral
when typical acid mine water is allowed to evaporate under
ambient conditions and rhomboclase and Voltaire are the last
formed (25~.
A copper-zinc partitioning study of melanterite demon-
strates that melanterite prefers copper over zinc (15~. The
consequences of this partitioning are that portal effluents will
tend to have higher ratios of Zn/Cu during the dry season
when melanterite is forming underground and lower Zn/Cu
ratios in the wet season when these salts are dissolved and
flushed from the mine workings. This trend is seen in the
historical data on the Richmond Mine effluent (15~.
Dissolution of these soluble, iron sulfate salts (with variable
amounts of copper, zinc, cadmium, and aluminum substituting
Idealized formula
Fei~SO4 7H20
Fe~SO4 4H20
Fei~SO4 H2O
Fei~Fe4~(SO4~6~0H)2 20H20
Fe~Fe2~(SO4~4 14H20
Fe2~(SO4~3 9H2O
Fe2~(SO4~3 7H2O
(H3O)Fe~(SO4~2 3H2O
K2Fes~Fe4~(SO4~2 18H2O
Fe~(Al,Fe~2(SO4~4 22H2O
CaSO42H2OFIG. 6. Cluster of coquimbite, Voltaire, and copiapite from the
CuSO45H20Richmond Mine. (Photo by G. Robinson, Canadian Museum of
3460 Colloquium Paper: Nordstrom and Alpers
for the iron) can generate acidic solutions with high concen-
trations of dissolved metals. During the rising limb of a stream
discharge in central Virginia after the onset of rain, Dagenhart
(26) showed that rapid increases in the concentrations of Cu.
Zn, Fe, and Al resulted from the dissolution of efflorescent
salts found on upstream tailings and waste rock piles. This
phenomenon must be common at mine waste sites and is likely
to be an important cause of fish kills associated with periods
of high runoff, especially after prolonged dry periods. Now we
consider the consequences of dissolution of the enormous
quantity of salts in the Richmond Mine in a mine-plugging
scenario.
The chemical composition of the mine pool created by
plugging the Richmond Mine can be estimated by allowing
these salts to dissolve in a volume of water equivalent to the
void space created by the underground workings. The exact
proportion of the different type of salts is not known, but the
results of the calculations are not particularly sensitive to this
factor. The amount of salts stored underground is a more
critical factor, and so that was considered a variable. Compu-
tations were made by inputting the mineral compositions to the
PHREEQE program (ref. 27, now superseded by PHREEQC, ref.
28) for a range of salt volumes. PHREEQE can calculate the
speciation and chemical equilibrium for mass transfer pro-
cesses such as precipitation, dissolution, oxidation-reduction
reactions, ion exchange, and gas addition or removal (29~. The
results are shown in Fig. 7, where the resultant pH in the mine
pool is plotted against the volume of added salts under two
scenarios: active infiltration (actively injecting clean water)
and passive infiltration (letting the groundwater naturally fill
the void spaces). The latter scenario gives a worse picture
because passive infiltration would allow more pyrite oxidation
and the buildup of more acid waters. The salts probably occupy
about 1% of the volume of the mine workings based on visual
inspection from the limited subsurface survey. As can be seen
in Fig. 7, however, an error in this value makes little difference.
The consequences are that a mine pool of about 600,000 m3
with a pH at or below 1, with many grams of dissolved metals
per liter (much like the current portal effluent), would likely
form at or near the top of the groundwater table, in a rock with
almost no neutralization capacity, and in which the hydrologic
flow is governed by fractures, excavations, and drill-holes.
Thus, plugging presents a remediation scenario that has a high
degree of risk with potentially dangerous results.
It has been common engineering practice to plug abandoned
or inactive mines without monitoring, modeling, or even
considering the physical and chemical consequences. Major
leaks or failures at plugs, widespread and disseminated seeps
of enriched acid mine waters, and increases in subsurface head
pressures of more than 100 m have occurred. For some mine
3
2
pH
" " 1 " " 1 " " 1 " " 1 " "_
Active infiltration
Passive infiltration
· Sample 90WA103
1
,, 1,, 1 1 1,,,
4 6 8 10
0 2
VOLUME PERCENT SALT DISSOLVED
FIG. 7. PHREEQE simulation of water composition for mine pool
after plugging the Richmond Mine.
Proc. Natl. Acad. Sci. USA 96 (1999)
sites, plugging may ultimately prove to be successful, but more
careful planning and peer review are essential to lessen the
probability of disastrous results.
Regulatory Investigations and Remediation
Several investigations and regulatory actions at Iron Mountain
have been initiated by California State agencies over the last
few decades. These are too lengthy to summarize here. Since
the original listing of Iron Mountain on the National Priorities
List in 1983, the EPA has authorized four Records of Decision
(RODs) and has considered numerous options for remedia-
tion. A condensed version of the main remedial alternatives is
as follows:
No action
Surface-water diversion
Lime neutralization
Capping (partial or complete capping of the mountain to
prevent infiltration)
Enlargement of Spring Creek Debris Dam (acid water
storage and release structure)
· Ground-water interception
· Air sealing
· Mine plugging
· On-site leaching and solution extraction
· Continued mining under environmentally safe conditions
· Combined alternatives
Surface-water diversions have been installed to divert clean
headwater streams around contaminated areas. The waters
that are the largest sources of metal loadings have been
captured and diverted to a lime neutralization plant. In the late
1980s, an emergency lime neutralization plant with a capacity
of about 60 gallons per minute (gpm; 1 gallon = 3.8 x 10-3 m3)
was installed to handle the worst flows from the Richmond and
Hornet portals. By December of 1992, this plant had been
expanded to handle 140 8pm, but was operated only 4 months
per year during highest flows. In July of 1994 a new plant with
a capacity of about 1,400 8pm began operation at the Minne-
sota Flats tailings site. In 1996 it was upgraded to 2,000 8pm,
and high-density sludge treatment was added. Now it accepts
drainage from Slickrock Creek (pumped from Old Mine and
No. 8 Mine workings) as well as the Richmond and Hornet
Mine portal effluents.
The decision to build the larger treatment plant and to treat
discharges from the Lawson tunnel (Hornet Mine) was also
influenced by geochemical modeling. Opinion was divided as
to whether the flow of acid mine water from the Lawson tunnel
originates from the Richmond Mine by spillage or leakage or
whether the Hornet ore body produces its own contaminant
effluent. An ore chute and a raise that connected the two
mines were identified from the old mine maps (Fig. 2). Because
of its proximity to the surface and the collapsed nature of the
mine workings, it was generally agreed that the Hornet Mine
itself could not be effectively plugged. However, consultants
proposed that plugging the Richmond Mine would stop or
greatly reduce the flow from the Lawson tunnel. There was
also reason to believe that during the intervening years since
mining ceased, cave-ins and other ground failures had largely
cut off direct connections from the Richmond to the Hornet.
Alpers and others (19) studied the historical data on rain-
fall-discharge relationships between the two mines, Zn/Cu
ratios as a signature of reactions within each mine site, and
mass balance calculations for the two portal effluents. The
most definitive method of determining the possible influence
of the Richmond Mine water on the Lawson tunnel effluent
was a mass balance approach. Using the known water com-
positions discharging from each mine and knowing the com-
position of the minerals that are reacting to form the effluent
Colloquium Paper: Nordstrom and Alpers
waters, it is possible to calculate the mass amounts of minerals
dissolved or precipitated to produce these waters by using the
BALANCE program (30~. Mineral reaction signatures were
developed for each mine effluent separately, and then Rich-
mond effluent was mixed with clean ground water and allowed
to precipitate and dissolve additional minerals to determine if
it was possible to derive the Lawson effluent from the Rich-
mond. No version of this mass balance model produced a water
that matched the Lawson effluent. Next, Richmond effluent
was also mixed with Lawson effluent, and geochemical reac-
tion was allowed, to see how much effluent each mine could
be contributing to the Lawson. The model results indicated
that not more than about 2% of the Richmond effluent could
be present in the Lawson effluent. Therefore, the Hornet Mine
is producing its own effluent independently of the Richmond
Mine. Even if the Richmond Mine were successfully plugged,
water from the Hornet would continue to be a significant
problem and it would have to be treated.
The fourth Record of Decision, issued in September of 1997,
selected the construction of a dam on Slickrock Creek. This
structure will capture the largest remaining loads of Cu and Zn
and divert them to the neutralization plant for treatment. The
remaining remediation is now focused on Boulder Creek,
lower Spring Creek, Spring Creek Reservoir, and the metal-
enriched sediments that formed in Keswick Reservoir from the
neutralization of acid mine waters for nearly 50 years.
The EPA and the potentially responsible parties remain in
legal contention over the appropriate final remediation ap-
proaches to be used at Iron Mountain and the costs. Both the
U.S. Government and the potentially responsible parties have
funded a considerable number of investigations, remediation
efforts, legal fees, and oversight management. The loads of
copper, zinc, and cadmium into the Sacramento River have
been reduced by 80-90~o, and further remediation is in
progress or being planned. The main challenge that remains is
how to find a permanent (and passive) treatment solution in
light of the fact that the mine drainage will continue for
approximately 3,000 years unless the sulfide ore is mined out.
Conclusion
Prevention and control of contamination at mine sites is a
challenging task, and remediation of large inactive mine sites
such as Iron Mountain has proven to be extraordinarily
difficult, complex, and expensive, not to mention litigious. The
physical and chemical nature of the site makes it difficult to
assess the effectiveness of remediation and the relative risks
and costs of various alternatives and their contingencies. There
are no easy solutions to these types of environmental problems,
but several important points can be made about cleanup of
mine waste sites on the basis of our experiences at Iron
Mountain.
First, there is tremendous value to having a technical
advisory team of multidisciplinary professionals, without an
obvious conflict of interest, to advise the regulatory agencies,
to review data, and to make recommendations. Mine sites and
their contaminants are complex functions of the geology,
hydrology, geochemistry, pedology, meteorology, microbiol-
ogy, and mining and mineral processing history, and their
remediation is subject to considerations of economic limita-
tions, available technology, and potential land use. Further-
more, the risks of failed remediation or no action are often
poorly known. Assessing such risks involves toxicology, epi-
demiology, wildlife biology, and dealing with public percep-
tion. To ignore professionals in these areas, who can contribute
both to the wisest choice of remediation strategies and to
public awareness and education is to invite mistakes.
Second, the effectiveness of a remedial alternative usually
cannot be easily quantified or predicted. Hence, we must admit
that remediation is experimental. Research is required to
Proc. Natl. Acad. Sci. USA 96 (1999) 3461
effect the best and most appropriate remediation available at
a given time for a given site. Both long-term and short-term
remediations are needed. For the short term, we need to fill in
the knowledge gaps, especially as they pertain to a particular
site. For the long term, we need to continue to develop better
remediation techniques and mining and processing techniques
that can utilize mine wastes and mineral deposits of lower
grade. Mineralogical and geochemical knowledge make it
possible to foresee the potential consequences of a remedial
option and to plan a remediation strategy. The results of
long-term research by the U.S. Geological Survey provided
technical tools (computer programs for geochemical modeling
and procedures for measuring pH) that could be used to
answer important questions regarding remediation scenarios.
Third, it would seem prudent to proceed on mine waste
cleanup in a phased, iterative approach. Our natural inclina-
tion is to identify the worst part of a hazardous waste site and
attempt to clean it up. For Iron Mountain, there is no single
remedial solution that would clean up 90% of the problem on
a permanent and maintenance-free basis (with the exception
of completely mining the mountain). There are, however,
several options (most of which have been exercised) that are
low risk and low cost and should reduce the discharge of acid
mine waters. These options can be instituted while delibera-
tions and research continue to find the long-term solution.
Fourth, mine waste sites commonly contain low-grade re-
sources that are potentially mineable it requires the right
technology to make resource recovery economic. In an age of
increasing recycling, recycling strategies should be applied to
mine sites. Many mine wastes have already undergone further
metals extraction and others could be stockpiled or tested for
new uses. Additional research into metal recovery from acidic
solutions could also provide economic incentive to recycling
metals from mine drainage waste streams.
Finally, Iron Mountain has been an extraordinary and
extreme environment in which to study and document the
processes of acid mine water production and efflorescent
mineral formation, the value of which goes far beyond just the
immediate remediation needs. The processes and properties
found at Iron Mountain are probably commonplace at metal
sulfide mine and mineral processing sites, but usually on a
smaller scale. We now have some direct observations of the
composition of water that produces efflorescent minerals. We
have some idea of the consequences of efflorescent mineral
dissolution when a mine is plugged. We can estimate the
geochemical consequences of various remediation scenarios
for mine sites with better confidence. Unraveling the dynamic
processes that affect water-mineral interactions is often crit-
ical to solving hazardous waste problems in the hydrogeologic
environment.
We are grateful to personnel of Region 9 of the U.S. EPA, especially
Rick Sugarek, for their continued support of our investigations on this
project and to personnel of CH2M Hill for their help and assistance in
our efforts to answer technically challenging questions. We thank the
California Regional Water Quality Control Board in Redding and all
the state agencies that have worked on Iron Mountain for their
cooperation and support. Roger Ashley and Katie Walton-Day (U.S.
Geological Survey) and James Hanley and Carol Russell (EPA)
provided helpful reviews. We also acknowledge Rick Sugarek for
making helpful suggestions on the manuscript.
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