Applied Geochemistry 16 (2001) 947±961
www.elsevier.com/locate/apgeochem
Geochemical models of the impact of acidic groundwater
and evaporative sulfate salts on Boulder Creek at
Iron Mountain, California
David C. Keith a, Donald D. Runnells a,*, Kenneth J. Esposito a,
John A. Chermak a, David B. Levy a, Steven R. Hannula a,
Malcolm Watts b, Larry Hall c
a
Shepherd Miller, Inc., 3801 Automation Way, Suite 100, Fort Collins, CO 80525, USA
b
Zeneca, Inc., 3411 Silverside Road, Hanby 1, Wilmington, DE 19897, USA
c
Zeneca, Inc., 1391 South 49th Street, Richmond, CA 94804, USA
Received 11 February 2000; accepted 30 June 2000
Editorial handling by R. Fuge
Abstract
During dry season base¯ow conditions approximately 20% of the ¯ow in Boulder Creek is comprised of acidic
metals-bearing groundwater. Signi®cant amounts of e‚orescent salts accumulate around intermittent seeps and surface
streams as a result of evaporation of acid rock drainage. Those salts include the Fe-sulfates Ð rhomboclase
2‡
3‡
(…H3 O†Fe3‡ …SO4 †2 3H2 O), ferricopiapite (Fe3‡
5 …SO4 †6 O…OH†20H2 O), and bilinite (Fe Fe2 …SO4 †4 22H2 O); Al-sulfates Ð
alunogen (Al2 …SO4 †3 17H2 O) and kalinite (KAl…SO4 †2 11H2 O); and Ca- and Mg-sulfates Ð gypsum (CaSO4 2H2 O),
and hexahydrite (MgSO4 6H2 O). The dissolution of evaporative sulfate salt accumulations during the ®rst major storm
of the wet season at Iron Mountain produces a characteristic hydrogeochemical response (so-called ``rinse-out'') in
surface waters that is subdued in later storms. Geochemical modeling shows that the solutes from relatively minor
amounts of dissolved sulfate salts will maintain the pH of surface streams near 3.0 during a rainstorm. On a weight
basis, Fe-sulfate salts are capable of producing more acidity than Al- or Mg-sulfate salts. The primary mechanism for
the production of acidity from salts involves the hydrolysis of the dissolved dissolved metals, especially Fe3+. In
addition to the lowering of pH values and providing dissolved Fe and Al to surface streams, the soluble salts appear to
be a signi®cant source of dissolved Cu, Zn, and other metals during the ®rst signi®cant storm of the season. # 2001
Elsevier Science Ltd. All rights reserved.
1. Introduction
Some of the most perplexing issues facing environmental scientists and regulators involve estimating the
impacts of di€erent sources of contamination to
groundwater and surface streams in areas that have
been disturbed by mining. An extensive data set has
* Corresponding author. Tel.: +1-970-223-9600; fax: +1-970223-7171.
E-mail address: [email protected] (D.D. Runnells).
been collected to quantify natural background concentrations versus the residual e€ects of mining at the
historic Iron Mountain Mine in Shasta County, California. This investigation focuses on utilizing that data
set to distinguish the relative contributions of dissolved
e‚orescent sulfate salts, acidic groundwater, and surface runo€ to metal loads in Boulder Creek at Iron
Mountain.
Boulder Creek is one of three perennial streams at
Iron Mountain which ultimately drain into the Sacramento River, and ultimately into San Francisco Bay
(Fig. 1). The pH at the mouth of Boulder Creek, where it
0883-2927/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved.
PII: S0883-2927(00)00080-9
948
D.C. Keith et al. / Applied Geochemistry 16 (2001) 947±961
Fig. 1. Location map of study area.
enters Spring Creek (Fig. 1), is typically around 3 during
dry summer base ¯ow conditions and rises to above 4
during storms in the late wet season. The acidic conditions
in Boulder Creek maintain elevated concentrations of dissolved trace metals compared to other natural streams.
The primary, or ultimate source, of acidity to
groundwater and surface streams in Boulder Creek is
from the oxidation of pyrite and other sul®de minerals
in mineralized areas of the watershed. However, as
shown in this investigation, e‚orescent sulfate salts are
an important intermediate source of acidity and metals.
The investigations presented below were designed to
understand how these intermediate sources of acidity
in¯uence the acidity and metal loads in Boulder Creek
D.C. Keith et al. / Applied Geochemistry 16 (2001) 947±961
during base¯ow and storm conditions. The ®ndings of
these investigations should be relevant to other sites
impacted by acid rock drainage (ARD).
2. Hydrogeochemical setting
The Boulder Creek drainage basin is located
approximately 16 km NW of Redding, California (Fig.
1), draining the northeastern side of the Iron Mountain.
Iron Mountain contains massive sul®de deposits and is
part of the West Shasta mining district. The ore deposits
consist of massive pyrite with minor amounts of Cu and
Zn sul®des. The ore host rocks in this district are
hydrothermally altered Devonian volcanic rocks
including the keratophyric Balaklala rhyolite and the
spilitized Copley Greenstone (Kinkel et al., 1956). An
E±W cross-section through Iron Mountain (Fig. 2)
shows two major sul®de deposits, the Richmond and
Hornet deposits, that were originally a continuous lens
of massive sul®de cut later by normal faults (Kinkel et
al., 1956). The Hornet portal lies upstream of the Richmond portal, and the ®rst signi®cant appearance of
ARD in Boulder Creek is a short distance downgradient
from the original outcrop of the Hornet ore body and
the historic Hornet mine workings. The majority of the
remaining sul®de-bearing waste rock and bedrock is
located on the southwestern side of the creek, and ARD
impacts to groundwater are generally limited to these
areas of the watershed.
The area of the Boulder Creek drainage basin is
approximately 700 hectares. Headwaters originate at an
elevation of approximately 1060 m, and at its discharge
point into Spring Creek the elevation of Boulder Creek
is approximately 430 m. The elevation drop of 630 m
occurs along a 6100 m run, which is equivalent to a 10%
949
grade. The mean annual precipitation of the area varies
from 140 cm (55 inches) at lower elevations to 200 cm
(80 inches) at higher elevations (US Environmental
Protection Agency, EPA, 1992). The mean annual precipitation based on the long-term data from a weather
station at nearby Shasta Dam is approximately 150 cm
(60 inches) and is representative of precipitation that
can be expected for lower elevations of the Boulder
Creek watershed (EPA, 1992). About 85% of the annual
precipitation falls between November and April. Snow
is common above elevations of 610 m between November
and March, but has a minimum contribution to Boulder
Creek. Flows in Boulder Creek range from less than 34
m3/h (150 gpm) in the dry summer and fall months to
over 79,500 m3/h (350,000 gpm) during ¯ash ¯oods
which primarily occur in the winter wet season (Fig. 3).
During the ¯ash ¯oods it is extremely hazardous to collect samples or make ®eld measurements.
3. Methods
For this investigation, a network of stream monitoring
stations and wells that exists along and within Boulder
Creek was utilized to obtain periodic ¯ow measurements
and water samples. The location of these stations and
other relevant surface features within the study area are
shown in Fig. 4. The primary stream monitoring stations
are located at weirs within Boulder Creek and are designated using the pre®x BCW. At the primary stations water
samples are collected automatically using ISCOTM samplers. In addition to the permanent stations shown in
Fig. 4, Boulder Creek water quality was monitored
using grab samples along the monitored reach (Fig. 5).
Water quality parameters that were measured in the
®eld included: pH, electrical conductivity, temperature,
Fig. 2. An E±W cross section through Iron Mountain showing the Richmond and Hornet deposits, Richmond adit, and Lawson
tunnel (modi®ed from Alpers et al., 1994b).
950
D.C. Keith et al. / Applied Geochemistry 16 (2001) 947±961
Fig. 3. Stream hydrograph of Boulder Creek Mouth (BCMO) for April 1995 to April, 1996.
Fig. 4. Location of primary stream monitoring stations and other relevant surface features within the Boulder Creek watershed.
D.C. Keith et al. / Applied Geochemistry 16 (2001) 947±961
951
Fig. 5. Variation in Cu concentration with distance downstream from BC Falls in Boulder Creek.
and oxidation/reduction potential (Eh). Laboratory
measured constituents include dissolved Al, As, Ca, Cd,
Cu, Fe, K, Mg, Mn, Na, Pb, SiO2, Zn (EPA Method
200.7), SO4 (EPA Method 300), and total dissolved
solids (EPA Method 160.1). Chloride and alkalinity are
known to be very low and are not routinely measured.
All samples collected for laboratory analysis of dissolved metals were ®ltered (<0.45 mm) and preserved
with HNO3 in the ®eld.
In order to determine the mineralogic character of
secondary phases associated with the mixing of acidic
groundwater and Boulder Creek water, orange-red
coatings from cobbles and sediments within the creek
were examined by X-ray di€raction (XRD) and scanning
electron microscopy (SEM). Several size fractions were
analyzed, including < 4 mm and < 1 mm separates.
All XRD scans were conducted using Cu-Ka radiation
from 2 to 65 degrees 2-theta. SEM examination was
conducted on rock chips that had particularly thick coatings of the orange-red stream precipitate. Chemical
analyses of 0.1 g splits of the precipitate were performed
by digesting the samples in a heated bath with HCl±
HNO3±HF (7 ml) at a ratio of 3.5:2:1.5, and adding 93
ml 1.5% H3BO3 prior to ICP analysis. Sulfate -S was
determined based on total S analyses using a LECOTM
furnace.
The mineralogic character of samples of surface salts
collected at di€erent locations throughout the watershed
was determined by XRD, and the chemical composition
(including determination of Al, Cu, Fe and Zn concentrations) of each sample was determined by dissolution and ICP. In addition, the distribution of surface salt
accumulations at Iron Mountain was determined in a
mapping program conducted in October of 1995. Notable
salt accumulations identi®ed in the ®eld reconnaissance
and mapping within the Boulder Creek watershed are
shown in Fig. 4.
In order to assess the e€ects of adding dilutive water
and dissolved salts to Boulder Creek in early wet season
storms, the geochemical baselines of the stream conditions in the late dry season were estimated by performing geochemical mixing calculations using EQ3/6
(Wolery and Daveler, 1992). Fig. 4 shows water sampling locations that were utilized in the modeling. Three
sequential simulations, involving the titration of ARDimpacted groundwater into Boulder Creek water, were
completed to represent three stream reaches of interest.
Those simulations included: (1) model titration of Hornet03 well water into BCFalls water, (2) model titration of
Mattie-01 well water into the result of simulation 1, and
(3) model titration of seep groundwater (BCD) into the
result of simulation 2. The seep water used in simulation
3 was actually a composite of waters collected from
several locations during a temporary diversion of that
section of the creek (Hall and Ekoniak, 1997). The chemical analyses of groundwater and stream water from
952
D.C. Keith et al. / Applied Geochemistry 16 (2001) 947±961
BCFalls used in the modeling exercise are shown in
Table 1. All of the water analyses that were utilized in
the geochemical modeling were charge balanced by
adjusting the concentrations of SO4 (shown in Table 1).
An initial simulation of BCFalls water was charge
balanced using HCO3 and mixed with Hornet-03 water
to evaluate the importance of potential HCO3 bu€ering
in that stream reach. The results indicated that alkalinity in the upstream portion of Boulder Creek was not
an important factor in the mixing calculations.
The simulated titration end points of the creek water
were determined by comparing the predicted pH values
in the simulations to the observed pH values for each
reach under consideration. The titration for a speci®c
reach was stopped when the pH predicted in the calculation equaled the observed pH in that reach at base
¯ow conditions. The simulations allowed goethite (aFeOOH), Fe(OH)3, schwertmannite (Fe8O8(OH)6SO4),
and gibbsite (Al(OH)3) to precipitate. Thermodynamic
data for schwertmannite (from Bigham et al., 1996) were
added to the original EQ3/6 database for the simulations.
All other thermodynamic data were utilized as they exist
in the EQ3/6 database. Oxygen fugacity in Boulder Creek
water was set equal to the partial pressure of atmospheric
O2 at the temperatures shown in Table 5.
4. Results
4.1. Stream water
Stream sampling shows that there is a step-wise
increase in metal concentrations, and a decrease in pH,
within three speci®c sections of Boulder Creek. An
example of step-wise increases in Cu concentrations
with downstream distance is shown in Fig. 5. The
reaches are de®ned as: (1) the Hornet Portal to BCW10,
(2) BCW10 to BCW15, and (3) BCW15 to BCW19
(Fig. 4). The lengths of these reaches correspond to
approximately 55, 490, and 100 m, respectively.
The chemical conditions observed within Boulder
Creek vary with season and ¯ow conditions. During
base ¯ow conditions (November/December 1995 sampling events) the ®rst signi®cant decrease in pH within
the creek was observed just below the Hornet Portal
near Waste Rock Pile-10 (WR 10C, Fig. 4); the pH
declined from 6.1 to 3.3 in this reach. Downstream
between BCW10 and BCW15, the pH dropped to 3.2,
and in the area between BCW15 and WR19 the pH
dropped to approximately 2.9. Concentrations of major
cations, SO4, and trace metals all showed corresponding
increases with distance downstream.
Table 1
Water quality data used in geochemical mixing calculations
Sample point
Associated stream reach
BC Falls
Non-impacted water
Hornet-03
Hornet portal to BCW10
Mattie-01
BCW10 to BCW15
BCD
BCW15 to BCW19
Date sampled
9-Nov-95
(Base¯ow)
1-Nov-95
(Monitoring Well)
1-Nov-95
(Monitoring Well)
21-May-95
(Base¯ow seepage)
Field parameters
pH, Field
Temperature ( C)
Elec. cond. (mS/cm)
Eh (mV)
6.1
10.3
86
Not availablea
2.31
14.3
5850
650
3.2
17.6
3030
732
2.5
15
1510
Not available
Lab parameters (dissolved mg/l)
Aluminum
Arsenic
Calcium
Cadmium
Copper
Iron
Potassium
Magnesium
Manganese
Sodium
Lead
Silica
Sulfate
Zinc
0.211
0.044
5.71
<0.0019
0.0253
0.0306
<0.47
3.79
0.0278
4.54
<0.032
19.8
20 (37.6)b
0.03
191
<0.4
8.8
0.115
29.9
746
0.93
63.5
1.1
5.53
<0.4
100
3730 (3519)
16.2
296
0.113
65.1
0.379
20.1
4.5
1.23
175
9.47
7.12
0.112
110
2730 (2634)
55.8
81.6
<0.001
21.4
0.0787
6.38
10.4
0.69
32
1.42
4.06
0.035
58.5
742 (892)
11.5
a
b
Log fO2= 0.69 assumed for geochemical simulations: redox sensitive elements were speciated accordingly.
Sulfate concentrations shown in parentheses are modeled concentrations utilized to achieve a charge balanced solution.
D.C. Keith et al. / Applied Geochemistry 16 (2001) 947±961
During the ®rst major storm of the wet seasons of
1995 and 1996, concentrations of metals increased
above concentrations observed at base¯ow conditions in
Boulder Creek even as the discharge increased several
orders of magnitude (Figs. 6, 7 and 8). In other words,
there was no observable dilution of the baseline concentrations by the storm water. In the storms, the pH of
the creek remained low (approximately 3.0), and the
953
concentration of Fe, Al, Cu, and Zn were signi®cantly
higher than concentrations that were predicted considering simple dilution of the creek by rainwater. These
results indicate that a near-surface source of metals and
acidity exists. By comparison, the increase in the concentrations of metals in later wet season storms was not
apparent relative to the increase in the concentration of
metals in early season storms.
Fig. 6. Dissolved Fe and Al concentrations at the mouth of Boulder Creek, December, 1995 storm.
Fig. 7. Dissolved Cu and Zn concentrations at the mouth of Boulder Creek, December, 1995 storm.
954
D.C. Keith et al. / Applied Geochemistry 16 (2001) 947±961
4.2. Mineralogy
Table 2
Chemical analysis of orange-red precipitates in Boulder Creeka
Determination
Result (mg/kg)
Less than 1 m size fraction
Sulfate
Aluminum
Copper
Iron
Zinc
29,400
33,600
550
263,000
85
Less than 4 m size fraction
Sulfate
Aluminum
Copper
Iron
Zinc
99,900
7860
470
300,000
60
4.2.1. Stream sediment precipitates
The results of XRD and SEM analyses show that the
orange-red coatings on rocks and sediments within the
creek are primarily poorly crystalline goethite. The
results of several chemical analyses of the precipitates
show that the < 1 mm fraction contains approximately
26% Fe by weight and elevated concentrations of Al
(3.4%) and SO4 (3%) (Table 2). The larger size fraction
materials contain more Fe (30%) and SO4 (10%),
and less Al (0.8%) than the < 1 mm fraction. Signi®cant amounts of Cu and Zn were also associated
with both size fractions of these precipitates.
4.3. Surface salt accumulations
The e‚orescent salt mapping and characterization
program revealed that salt accumulations in the Boulder
a
Note conversion of metals to metal oxides accounts for 91
wt.% of the <1 mm fraction and 99% of the <4 mm fraction.
Table 3
Color, mineralogy, and chemical analysis results of salts in Boulder Creek watershed
Sample I.D.
Color description
Mineralogya
Copper (mg/kg)
Zinc (mg/kg)
Iron (mg/kg)
Aluminum (mg/kg)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
White, trace yellow
Reddish yellow
White
White to olive yellow
White to yellow
White to pale yellow
White to yellow
White
White
White to pale yellow
White to pale yellow green
White to reddish yellow
White, trace pale yellow
White
White to olive yellow
White
White
Yellow
White to olive yellow
White to yellow
Yellow
Yellow
Yellow
Yellow
White
White to yellow
White to yellow
White to yellow
Yellow
White to yellow
K, G
Fc, Rh, Al, H
Al, G, K
Al, K, G, H
Fc, Rh, H
Fc, Rc, G, K
Fc, Rh, Al, K
Al, K, H
K, Bi
Rh, K, H, G
Fc, Rh, G, K
K, G, H, Al
Al
Al, K, H
Fc, Al, H
K, G
Rh, Al, K, H
Fc, Rh, Al, K
Fc, Al, Rh, G,
Fc, Rh, Al
Fc, Rh, G, K
Fc, Rh, G
Fc, Rh
Fc, Rh, G
Fc, Rh
Fc, Rh
Fc, Rh
Fc, Rh
Fc, Rh
K, G, H, Al
1830
1430
1600
3360
4090
2090
1130
3140
1040
878
3660
1610
2370
2450
3400
845
293
1760
5320
872
234
125
781
491
662
221
505
159
95
0
5070
7060
4970
6500
17,200
9930
12,200
10,300
6450
574
4350
7900
9250
8070
17,000
6680
13,000
4920
5450
3150
660
280
52
1580
10
124
53
420
26
3
6200
22,500
1770
11,700
4500
42,600
11,700
9280
45,100
33,700
10,800
4560
537
472
59,500
657
3700
89,700
91,100
131,000
142,000
105,000
126,000
16,000
132,000
160,000
178,000
167,000
163,000
19
58,900
49,100
43,900
43,600
43,500
41,700
40,900
39,100
37,700
37,000
35,400
35,200
34,400
27,500
26,500
25,500
25,000
23,000
18,900
14,900
7730
6720
6630
5690
2490
2040
1870
1670
831
211
1550
5440
59,000
24,600
Average values
a
Al=alunogen; Bi=bilinite; Fc=ferricopiapite; G=gypsum; H=hexahydrite; K=kalinite; Rh=rhomboclase.
Geochemical model mixing simulations indicate that
a groundwater in¯ux of approximately 20% of the total
¯ow in Boulder Creek is adequate to explain the
95.3
33.3
7.1
Ferricopiapite
Rhomboclase
346
43.1
6.6
Ferricopiapite
Rhomboclase
301
23.2
5.2
Rhomboclase
137
35.2
4.3
Ferricopiapite
Rhomboclase
33
28.2
6.5
Hexahydrite
Rhomboclase
51.5
24
5.8
Ferricopiapite
Rhomboclase
574
27.8
5.9
Ferricopiapite
Rhomboclase
70.2
22.9
6.8
Ferricopiapite
Rhomboclase
2920
3780
16.2
Gypsum
Hexahydrite
Kalinite
Gossan salt 8
Gossan salt 7
Gossan salt 6
Gossan salt 5
Gossan salt 4
Gossan salt 3
Gossan salt 2
Gossan salt 1
955
Cu (mg/kg)
Zn (mg/kg)
% Moisture
Minerals identi®ed
by XRD
4.4. Model mixing simulations
Table 4
Chemical analysis of salts associated with gossan at Iron Mountain
Creek watershed generally occur as thin crusts where
intermittent seeps and surface streams exist. Salt accumulations also occurred in areas where surface materials
contained signi®cant concentrations of sul®de minerals,
even where obvious seeps were not present. The most
notable examples of the e‚orescent salt accumulations
in the absence of groundwater seeps were on residual
natural sul®de lenses and nodules within the massive
gossan outcrops at the top of Iron Mountain (area not
shown on Fig. 4).
The results of XRD and chemical analyses of 31 salt
samples collected from seeps, intermittent surfaces
streams, and residual waste rock piles, are presented in
Table 3. These results show that Fe-sulfates, including
rhomboclase ((H3O)Fe3+(SO4)2.3H2O), ferricopiapite
2+
.
.
(Fe3+
Fe3+
2 (SO4)4
5 (SO4)6O(OH) 20H2O), bilinite (Fe
22H2O), and the Al-sulfates, including alunogen (Al2
(SO4)3.17H2O) and kalinite (KAl(SO4)2.11H2O) were
the dominant phases in the crusts. Accessory minerals
included gypsum (CaSO4.2H2O) and hexahydrite
(MgSO4.6H2O). All of these minerals are characterized
as soluble sulfate minerals by Aplers et al. (1994a),
indicating that their existence on the surface is likely
transitory depending on seasonal weather conditions.
Alpers et al. (1994a) suggest that non-ferrous metal sulfate
minerals precipitate after Fe2+ is removed from solution either by precipitation, or oxidation to Fe3+ and
precipitation as Fe oxide or hydroxide phases. The
locations of non-ferrous and ferrous-bearing salts relative
to acid-generating source rocks in this study support the
Alpers et al. (1994a) conclusion. The concentrations of Fe,
Al, Cu, and Zn of each sample are listed in Table 3. These
results show that more Cu and Zn are associated with
Al-sulfates than with Fe-sulfates from the same area.
The results of XRD examination and the Cu and Zn
contents of nine samples of salts from natural gossan
outcrops at Iron Mountain are shown in Table 4. These
salts, which formed on residual sul®de lenses within the
gossan, were primarily composed of ferricopiapite and
rhomboclase, and had signi®cantly higher concentrations
of Cu than Zn, presumably as solid-state substitution in
the crystal lattice. Similarly, Alpers et al. (1994b)
showed that seasonal precipitation and dissolution of
melanterite (Fe2+,Zn,Cu)SO4.7H2O) in the mine workings at Iron Mountain exerts a signi®cant control on the
Zn/Cu ratios in portal e‚uent. The ¯uctuation of portal
water chemistry was attributed to preferential partitioning of Cu compared to Zn in the melanterite solid
solution as melanterite dissolves and precipitates during
wet and dry seasons, respectively.
Gossan salt 9
D.C. Keith et al. / Applied Geochemistry 16 (2001) 947±961
956
D.C. Keith et al. / Applied Geochemistry 16 (2001) 947±961
Table 5
Observed Boulder Creek water chemistry versus water chemistry predicted in geochemical mixing calculations
Sample point
BC Falls
BCW10
BCW10
BCW15
BCW15
BCW19
BCW19
Date sampled/conditions
9-Nov-95
(Observed
base¯ow)
9-Nov-95
(Observed
base¯ow)
Predicted
9-Nov-95
(Observed
base¯ow)
Predicted
9-Nov-95
(Observed
base¯ow)
Predicted
Field parameters
pH, Field
Temperature(C)
Elec. cond. (mS/cm)
6.1
10.3
86
3.28
11.1
383
3.3
10
3.18
11.8
560
3.18
10
±
2.9
12.6
1090
2.94
10
±
Lab parameters (dissolved mg/l)
Aluminum
Arsenic
Calcium
Cadmium
Copper
Iron
Potassium
Magnesium
Manganese
Sodium
Lead
Silica
Sulfate
Zinc
Reach % acidic groundwater
Cumulative % acidic groundwater
0.211
0.044
5.71
<0.0019
0.0253
0.0306
<0.47
3.79
0.0278
4.54
<0.032
19.8
20
0.03
0
0.0
2.37
<0.042
7.32
0.0048
0.246
2.82
<0.47
5.3
0.122
5.12
<0.032
20.7
97.4
0.469
NAa
NA
2.57
±
5.75
0.0015
0.39
<0.01
<0.47
4.53
0.14
4.55
<0.032
22
81
0.23
0.6
0.6
14.9
<0.042
11.1
0.023
1.02
1.21
<0.47
12.2
0.465
5.21
<0.032
30.3
213
3.14
NA
NA
54.2
±
12.3
0.057
3.57
<0.01
<0.47
28
1.4
4.9
<0.032
34
429
6.9
8
8.6
39.4
<0.042
25.6
0.0457
3.07
4.24
<0.47
28.4
1.17
6.14
<0.032
46.5
485
7.19
NA
NA
58.4
±
14
0.06
4
<0.01
0.12
28
1.38
4.8
<0.032
40
558
8.7
10.5
19.1
a
NA=not applicable.
observed pH values in Boulder Creek below the impacted reaches. For the speci®c reaches, a simulated starting mixture of 99% BCFalls water (pH=6.1) and <
1% Hornet-03 water (pH=2.3) produces a water with a
pH of 3.3. An additional 8% of groundwater from the
reach between BCW10 and BCW15 (represented by
Mattie-01 well water, pH=3.2) lowers the stream pH to
3.2. Finally, an additional 10.5% of groundwater from
the reach between BCW15 and BCW19 (Boulder Creek
Diversion seepage [BCD], pH=2.5) lowers the stream
pH to 2.9.
As part of a larger ®eld experiment, most of the surface water in the reach between BCW15 and BCW19
was diverted into a ¯ume in the spring and summer of
1995 (Hall and Ekoniak, 1997). Flow measurements
taken during the diversion showed that the amount
of remaining groundwater recharge into the original
stream bed was approximately 10% of the original total
¯ow of the creek. These observed results verify that the
geochemical modeling results for that reach (modeled as
10.5% in¯ux) under base ¯ow conditions are accurate
relative to the observed 10% in¯ux.
The predicted chemistry and the observed chemistry
for each reach of Boulder Creek considered in the
simulations are shown in Table 5. In general, the predicted concentrations of each element compare well to
the observed concentrations, but predicted concentrations of Al and SO4 in the stream water were consistently higher than the observed values. The higher
concentrations of Al and SO4 in the simulations likely
resulted from a lack of consideration of surface complexation/coprecipitation reactions in the EQ3/6 program. Consistent with the simulation results, the
elevated concentrations of Al (up to 3.4%) and SO4 (up
to 10%) in the stream precipitates (Table 2) indicate
that surface reactions and coprecipitation of metals are
signi®cant in Boulder Creek. Neither Al or SO4 behaves
as a conservative component in Boulder Creek water.
Goethite was predicted to precipitate in all of the
simulations. Other mineral saturation states, as predicted in the calculations, show that sections of Boulder
Creek which are impacted by ARD are oversaturated
with respect to nontronite, hematite, pyrolusite, and
several phases of silica (Table 6). Saturation indices
indicate that the stream is near saturation with respect
to amorphous silica (Table 6). If goethite precipitation is
suppressed, the resultant solutions become oversaturated with respect to schwertmannite and jarosite.
D.C. Keith et al. / Applied Geochemistry 16 (2001) 947±961
957
Table 6
Saturation indices for select minerals in geochemical modeling of the 3 stream reaches of Boulder Creek. All saturated and oversaturated minerals are shown
Stream reach
Fe-compounds
Fe(OH)3 (amorphous)
Goethite (FeOOH)
Hematite (Fe2O3)
Jarosites (KFe3(SO4)2(OH)6; ideal formula)
Schwertmannite (Fe8O8(OH)6SO4)
Al-compounds
Alunite (KAI3(SO4)2(OH)6)
Boehmite (AlOOH)
Gibbsite (Al(OH)3)
Hornet portal to BCW10
BCW10 to BCW15
BCW15 to BCW19
Log Q/Ka
Log Q/K
Log Q/K
5.3
0.0
0.9
11.3 to
19.0
5.3
0.0
0.9
9.8 to
18.1
5.3
0.0
0.9
8.7 to
17.6
11.7
4.9
3.9
2.1
10.2
1.9
3.4
1.6
9.1
2.4
4.1
2.3
Clays (ideal formulas)
Beidellites (R+
0.33Al2(Si3.67Al0.33)O10(OH)2)
Illite (K0.6Al1.8Mg0.25(Al0.5Si3.5)O10(OH)2)
Kaolinite (A14Si4O10(OH)5)
Montmorillonites (R+
0.33(Al1.67Mg0.33)Si4O10(OH)2
3+
Nontronites (R+
0.33Fe2 (Si3.67Al0.33)O10(OH)2)
3.9 to-5.0
8.6
2.0
4.7 to 5.3
2.6 to 4.2
1.4 to-3.0
6.7
0.5
2.8 to 4.0
3.5 to 5.1
2.8 to 4.3
8.0
1.8
4.0 to 4.9
3.6 to 5.1
Silica compounds
Chalcedony (SiO2)
Quartz (SiO2)
SiO2 (amorphous)
0.6
0.9
0.5
0.8
1.1
0.3
0.9
1.2
0.2
Other compounds of interest
Chalcanthite (CuSO4:5H2O)
Gypsum (CaSO4:2H20)
Pyrolusite (MnO2)
5.8
2.7
0.7
4.4
2.0
1.2
4.4
1.9
0.7
a
Q is the empirical activity product and K is the theoretical equilibrium constant for a dissolution reaction.
5. Discussion and interpretation of modeling results
The measured increase in metals and decrease in pH
comprise a step-like function in Boulder Creek and are
interpreted to be the result of the addition of groundwater from three identi®able reaches along the southwestern (mineralized) bank of the creek. The addition of
groundwater within each reach exerts an incremental
impact on the composite water quality at the mouth of
Boulder Creek and the water quality is modi®ed by
storm runo€ in the wet season.
X-Ray di€raction results indicate that the initial solid
products from the hydrolysis of Fe3+ in Boulder Creek
are poorly crystalline, metastable phases such as microcrystalline goethite and ferrihydrite (nominally
Fe5HO8.4H2O) (Chukhrov et al., 1973; Russell, 1979).
The metastable phases form because the more stable
phases, goethite and hematite, have slow growth kinetics at low temperatures (Chukhrov et al., 1973; Russell,
1979). Bigham et al. (1996) suggest that precipitates of
Fe3+ formed at pH 6.5 or higher are generally composed of ferrihydrite or a mixture of ferrihydrite and
goethite, whereas those precipitated from waters having
pH values in the range of 2.8±4.5 are predominately
schwertmannite with trace to minor amounts of goethite.
However, as mentioned above, schwertannite was not
identi®ed in Boulder Creek samples analyzed for this
investigation. Nor were silica phases identi®ed, despite
the apparent states of oversaturation (Table 6). Slow
growth kinetics for silica phases are well documented
(Krauskopf, 1956) and likely account for the oversaturation of the ¯uids with respect to those phases.
Nordstrom and Ball (1986) showed that aluminum
can behave either conservatively or non-conservatively
in acidi®ed stream waters, with a transition from nonconservative to the conservative as the pH falls below
4.5±5.0. In agreement with the observations of Nordstrom and Ball (1986), Boulder Creek waters, with pH
values generally in the range of 3±4, are undersaturated
with respect to Al-bearing minerals such as gibbsite and
alunite, KAl3(SO4)2(OH)6 (Table 6). These results indicate that the formation of distinct Al mineral phases is
not an important contributing factor in controlling Al
concentrations or pH values in the creek.
958
D.C. Keith et al. / Applied Geochemistry 16 (2001) 947±961
As an alternative explanation for the observed loss of
Al and SO4, Chapman et al. (1983) and Rampe and
Runnells (1989) showed that adsorption or coprecipitation of Al and SO4 often accompanies precipitation of
Fe3+ hydroxide in acid mine drainage streams. Aluminum has been observed to substitute into goethite and
hematite to maximum concentrations of 33 mol%
(AlOOH) and 14 mol% Al2O3, respectively (Yapp,
1983; Schultze and Schwertmann, 1984; Schwertmann,
1985; Tardy and Nahon, 1985). The chemical analysis of
the stream precipitate (Table 2) con®rms this behavior and
shows that the observed loss of Al and SO4 from Boulder
Creek water is associated with the precipitation of the
poorly crystalline Fe-oxyhydroxides, oxyhydroxysulfates,
Fig. 8. Dissolved Fe and Al concentrations at the mouth of Boulder Creek, February, 1996 storm.
Fig. 9. Predicted pH values in reaction path simulations of ground water from monitoring well Hornet-03 mixing with Boulder Creek
water.
D.C. Keith et al. / Applied Geochemistry 16 (2001) 947±961
and goethite, either as adsorbed metals or coprecipitated
compounds. As mentioned earlier, adsorption and solidsolution reactions were not considered in the EQ3/6
simulations.
The precipitation of Fe-oxyhydroxides generates signi®cant acidity. Fig. 9 shows the predicted decrease in
pH in a mixing simulation of the reach between BCFalls
and BCW10 where all mineral formation was suppressed in the model, compared to the predicted
decrease in pH in a simulation where goethite precipitation was allowed in the model. This ®gure shows
that precipitation of goethite (or other Fe oxyhydroxide
precipitation) substantially lowers the pH in Boulder
Creek as illustrated in the reaction:
Fe3‡ ‡ 2H2 O <ˆˆˆˆ> FeOOH…solid† ‡ 3H‡ :
6. Dilution of creek water
The e€ect of simple dilution by rain on the water
quality of Boulder Creek was assessed using the results
of the geochemical modeling presented above. The base
¯ow prior to the initial storm of the 1995/1996 wet season was approximately 59 m3/h (260 gpm) compared to
approximately 29,500 m3/h (130,000 gpm) of ¯ow at
BCMO during the peak of the storm (a 500 dilution).
A reaction path simulation mixing 500 parts of clean
water (represented by water from BCFalls) into water at
BCW19 shows that simple dilution would produce pH
values of approximately 5.1 (Fig. 10). This pH is a full 2
959
units higher than the stream pH values observed in the
initial storm of the 1995/1996 wet season. These results
indicate that chemical reactions, in addition to simple
dilution, must be important in controlling the chemistry
of Boulder Creek.
6.1. E€ect of sulfate salt dissolution
Other than soluble e‚orescent surface salts, there are
no obvious surface sources of metals and acidity that
could respond almost instantaneously with discharge in
Boulder Creek, as is observed. The in¯uence that these
minerals can have on the chemistry of storm runo€ was
previously discussed by Bayless and Olyphant (1993).
The entrainment of acidic pore water within the unsaturated soils and colluvium could also serve as a minor
source, but later wet season data indicate this source is
insigni®cant compared to the surface salt accumulations
that accumulate during the prolonged dry season at
Iron Mountain.
A geochemical modeling exercise was performed to
evaluate the potential amount of acidity that each
e‚orescent salt phase could release to Boulder Creek
upon dissolution. These calculations where performed
by simulating the titration of end-member compositions
of each salt phase into the previously diluted Boulder
Creek water. The results of EQ6 calculations show that
small amounts of Fe-bearing salts can lower the pH to
below 3.0, as is observed in the creek. The modeling
results presented here show that rhomboclase dissolution causes the most rapid decrease in pH per gram of
dissolved salt, followed by ferricopiapite, bilinite, and
Fig. 10. Predicted change in pH at BCW19 as Boulder Creek is diluted by rainwater during a storm.
960
D.C. Keith et al. / Applied Geochemistry 16 (2001) 947±961
Fig. 11. Predicted change in pH as SO4 salts are titrated into diluted Boulder Creek water.
jarosite (Fig. 11). The additional acidity due to rhomboclase dissolution results from the release of the
hydronium ion (H3O+) within its crystal structure. In
contrast, it was not possible to lower the pH to the
observed stream values with any amount of dissolution
of the Al- or Mg-bearing minerals hexahydrite, alunogen, kalinite, or gypsum. These results are due to the
di€erence in the hydrolysis reactions for these elements.
The reaction path calculations predict that goethite
precipitation (or its micro-crystalline equivalent) will
maintain Fe concentrations in Boulder Creek below 1
mg l 1 at pH values around 3.0; however, Fe concentrations during the December, 1995 storm were as
high as 90 mg l 1. Some of this discrepancy is likely due
to passage of colloidal particles of Fe precipitates
through the 0.45 mm ®lters; LeGendre and Runnells
(1975) showed that only about 70% of the Fe in a contaminated mountain stream was removed by a 0.45 mm
®lter, relative to nearly 100% removal by a 0.01 mm ®lter. However, the high Fe concentrations observed in
Boulder Creek during the December, 1995 storm (Fig.
6) indicate that there is likely also a kinetic barrier to
Fe-oxyhydroxide precipitation within the creek that was
not considered in the reaction path modeling.
7. Conclusions
Studies of mine drainage have historically emphasized
the oxidation of sul®de minerals as the predominant
source of acidic solutions in ARD impacted areas. The
present results show that e‚orescent sulfate minerals
are a potent intermediary source of acid and metals.
Over the long term, weathering of metal-sul®de minerals
is certainly the ultimate source of ARD; however, this
long-term e€ect in part is transmitted to water by the
formation and dissolution of secondary by-products
such as hydrated sulfate minerals. These minerals are
generally most abundant on the ground surface and in
the vadose zone, and are concentrated by evaporation
of SO4-rich waters. The chemistry of Boulder Creek
during storms following prolonged dry seasons at Iron
Mountain indicates that these salts add signi®cant acidity
and dissolved metals to surface runo€ upon dissolution.
The dissolution of evaporative sulfate salts during the
®rst major storm of the wet season at the historic Iron
Mountain Mine produces a characteristic hydrogeochemical response in surface waters that is not
apparent in subsequent storms. Geochemical modeling
shows that the incorporation of solutes from relatively
small amounts of dissolved Fe-bearing sulfate salts will
maintain the pH of surface streams below values predicted considering dilution e€ects during a rainstorm.
The primary mechanism for the release of acidity from
these salts is the hydrolysis of Fe3+. In addition to the
lowering of the pH values and providing dissolved Fe
and Al to surface streams, metal-sulfate salts are a signi®cant source of dissolved Cu, Zn, and other metals.
Additional work would be helpful to understand cation
substitution patterns within this group of minerals.
Although the data, processes, and models discussed in
this paper speci®cally pertain to Boulder Creek, the
D.C. Keith et al. / Applied Geochemistry 16 (2001) 947±961
®ndings of this investigation should be relevant to most
other sites in arid or temperate regions that are impacted by natural and mining-related ARD.
Acknowledgements
The authors are grateful to Stau€er Management
Company for their ®nancial support of this research and
for their permission to publish these results. We also
would like to thank SVL Laboratories, Kellogg, Idaho
for contributing chemical analyses of the Boulder Creek
stream precipitates.
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