PRINCE OF WALES ISLAND FOREST ROAD 3030
GEOCHEMICAL ACID ROCK DRAINAGE AND
METAL LEACHING ASSESSMENT
PRINCE OF WALES ISLAND, ALASKA
KINETIC TESTING UPDATE
Zenah Orndorff, PhD, Senior Research Associate
and W. Lee Daniels, PhD, Professor
Dept. of Crop and Soil Environmental Sciences
Virginia Polytechnic Institute and State University
Blacksburg, VA, USA, 24061
PSI, Inc. – Herndon, Virginia
USDA Forest Service-Alaska Region
July 09, 2010
This interim report provides a description of methods and a summary of initial
results from kinetic tests evaluating the acid rock drainage (ARD) potential of acid
generating rock material (B5), limestone (B1) and a series of B5/B1 blends provided by
the United States Forest Service (USFS) Alaska Region. As prescribed in our contract
with PSI Inc., the kinetic testing of these materials is accomplished via establishment of
a series of humidity cells and weekly evaluation of the leachate for alkalinity, pH,
sulfate, iron, calcium and magnesium. Samples are also evaluated every five weeks for
additional metal concentrations. This portion of the project was initiated in mid-March of
2010 following completion of the initial static testing phase as reported earlier. In
addition to the specified parameters, electrical conductivity (EC) is measured on a
Ten 5-gallon buckets of B5 material and five 5-gallon buckets of B1 material were
received in October, 2009. All ten buckets of the B5 material were spread out to air-dry,
then the entire volume of material was thoroughly blended and passed through a 1.5”
sieve. Approximately 15% (by volume) of the material required additional crushing to
pass through the sieve. This process was simultaneously completed for the five buckets
of B1 material, although the B1 did not require additional crushing. From the bulk coarse
material, 12 sample blends (10 kg each), were prepared for the mixture ratios given in
Table 1. From each coarse blend, 5 kg was sub-sampled for use in the humidity cells.
Sample preparation for the humidity cells consisted only of size reduction.
This study uses a modified humidity cell design as recommended by colleagues
from West Virginia University and the National Mined Land Reclamation Center. The
basic cell design (Fig. 1) consists of two nested containers, with the inner container
holding the geologic materials tested and the outer container collecting the leachate.
The outer container remains covered and sealed to maintain saturated humidity within
the cell. Examples of the assembled humidity cells as used in this study are shown in
Table 1. Sample blends (by weight)
Sample ID % B5 % B1
AKS-1 100 0
AKS-2, AKS-3 95 5
AKS-4, AKS-5 90 10
AKS-6, AKS-7 85 15
AKS-8, AKS-9 80 20
AKS-10 75 25
AKS-11 70 30
AKS-12 0 100
The cells were constructed with 6 qt inner (sample) plastic containers and 16 qt
outer containers. On the bottom of the inner sample containers, drainage holes (0.25”
diameter) were drilled on a 1” grid; all sample containers were drilled with an identical
pattern. The sample containers were lined with four layers of polyester fleece batting to
minimize loss of solid/sediment material through the drainage holes. Prior to assembly,
all cell construction material weights (inner boxes, outer boxes, PVC rings and batting)
were determined. Each sample container was filled with a 5 kg sample from the
appropriate blend, placed on 3 PVC rings (~1.5” height) within the outer container, and
leached once a week by evenly sprinkling 2.5 L of distilled and deionized water over the
sample. After 24 hours, leachate sampling and analysis includes the following:
1) The inner sample box is removed and weighed, and the outer box with leachate
2) Prior to disturbance of the leachate collected in the bottom of the outer container,
a 50 ml sample of clear supernatant is withdrawn from the container, preserved
with HNO3, and analyzed for Ca, Mg, Fe and S. Every five weeks this sample is
also analyzed for As, B, Ba, Cd, Co, Cu, K, Li, Mn, Na, Ni, Sr, and Zn. Analysis is
completed according to USEPA method SW 846 6010B, revision 2 (USEPA,
1996), using a Spectro CirOS VISION Model FVS12 inductively coupled plasma
3) After collecting the clear sample per #1 above, the remaining leachate within the
container is thoroughly mixed and a sample of approximately 100 ml is collected
to determine the amount of total solids removed/leached from the sample. The
sample is thoroughly mixed, and approximately 40 ml is poured into a pre-
weighed beaker then placed into a 180oC oven overnight to evaporate. The
beakers are cooled in a desiccator for 1 hour, weighed, and the results used to
calculate the total solids of the sample solution. This procedure is completed in
duplicate for each sample.
4) The remaining leachate is thoroughly stirred again and a 200 ml sample is
collected to determine alkalinity (APHA, 1985).
5) The remaining leachate is left in the cell to maintain humidity and the outer box is
closed and sealed. This leachate is then discarded immediately prior to the
Figure 1. Diagram of modified humidity cell (based on design developed by Paul
Ziemkiewicz, National Mined Land Reclamation Center, Morgantown, WV).
Figure 2. Examples of humidity cells utilized for kinetic testing for geochemical acid rock
drainage and metal leaching assessment.
Leachate pH for AKS-1 – 12 over the first 15 weeks of leaching is presented in
Figure 3. As clearly seen in this figure, only AKS1 (B5 material) produced acid drainage
during this period with leachate maintaining a consistent pH of ~2.9. As expected,
sample AKS-12 (100% B1 limestone material) maintained the highest pH, with values
fluctuating between 8.1 and 8.6. For the B5/B1 blends (AKS-2 – 11), pH increased over
the first several leachings eventually achieving a semi-steady state. This response is
typical for these types of materials as acid residues are rinsed away in the presence of
a neutralizing agent. For all of the blended samples (AKS-2 – 11) calcium carbonate
dissolution produced adequate alkalinity to neutralize acidity from pyrite oxidation in the
blended B5 material and generate net alkaline drainage. Overall, the equilibrium
leachate pH values from AKS-2 - 11 ranged from 7.3 to 8.1, and pH regularly increased
with B1 content.
Figure 3. Leachate pH for samples AKS-1 – 12 over the first 15 weeks of reaction and
leaching in humidity cells.
Leachate EC for AKS-1 – 12 over the first 15 weeks of leaching is presented in
Figure 4. For AKS-1 - 11, EC values were initially quite high, ranging from 1.6 to 3.1
dS/m (1600 to 3100 µS/cm), and increased regularly with increasing B5 content due to
pyrite oxidation producing sulfates and Ca loadings from carbonate neutralization
reactions. The EC values decreased noticeably over the first several leachings and
most samples, particularly those with lower B5 content, achieved or appeared to be
approaching a state of equilibrium towards the end of the 15 week period. By the 15th
leaching event, EC levels for AKS-1 – 11 were reasonably low, ranging from 0.21 – 0.31
dS/m. For AKS-12 (pure B1 material), EC levels were low throughout the 15 week
period, with values dropping only slightly from 0.16 to 0.06 dS/m (160 to 60 µS/cm).
Figure 4. Leachate EC for AKS-1 – 12 over the first 15 weeks of leaching in humidity
Leachate alkalinity for AKS-1 – 12 over the first 15 weeks of leaching is
presented in Figure 5. For AKS-1, as expected, alkalinity values were consistently 0
mg/L over the first 15 weeks. For AKS-2 – 12, alkalinity values were initially quite high,
ranging up to 3150 mg/L, and for most samples alkalinity increased with increasing B1
content. Values in the initial leachate (#0) likely were affected by the noticeable
amounts of sediment that initially rinsed off through the filter. Alkalinity values (as well
as sediment loads), decreased sharply over the first few leachings. After leach #3, AKS-
2 – 11 continued to decline slowly with values ranging from 10 – 62 mg/L by leach #15.
During this period, only AKS-3 yielded leachate that did not meet the minimum water
quality criteria of 20 mg/L (note: minimum criteria may be lower if alkalinity of local
natural waters are low; Alaska DEC, 2008). Sample AKS-12 followed the same pattern
through leach #10, but then showed a slight increase and was less consistent for
leachings #11 – 15 with levels fluctuating between 170 – 450 mg/L.
Figure 5. Leachate net alkalinity for AKS-1 – 12 over the first 15 weeks of leaching in
A total of 17 elements were monitored in the humidity cell leachates. Concentrations
of Ca, Fe, Mg, and S were measured weekly, while As, B, Ba, Cd, Co, Cu, K, Li, Mn,
Na, Ni, Sr, and Zn were measured in leachate #’s 0, 1, 5 and 10 (and will continue to be
monitored every five weeks). As expected, all elemental concentrations were highest in
the initial leach and dropped most rapidly over the first few leach events. Table 2
provides a summary of element detection limits, associated Alaska (2008) water quality
criteria levels (MCL), and a summary of samples which may have exceeded those
criteria on certain leaching cycles. A more comprehensive discussion of these values
will be provided in the final repot.
As: Arsenic concentrations were routinely below the detection limit (0.017 mg/L) with
only one exception; AKS-1 yielded a relatively low concentration of 0.038 mg/L in leach
B: With few exceptions, B concentrations were typically near or below the detection limit
(0.007 mg/L). Detectable concentrations (ranging up to 0.023 mg/L) were noted in
AKS-1 (leach #0, 1, 5, 10), AKS-11 (leach #0), and AKS-12 (leach #0, 1).
Ba: Low Ba concentrations (up to 0.011 mg/L) were observed in some samples from
leach #0, but subsequent concentrations were typically below the detection limit (0.003
Ca: Calcium concentrations are illustrated in Figure 6. Concentrations were lowest for
AKS-12 with a value of 27 mg/L in leach #0 and decreased to 15 mg/L by leach #10.
For AKS-1 – 11, Ca concentrations ranged from 432 – 586 mg/L in leach #0, and
decreased to between 26 – 109 mg/L by leach #10.
Cd: Detectable Cd concentrations (up to 0.035 mg/L) were noted for AKS-1 in leach #0
and #1, but for all other samples were routinely near or below the detection limit (0.004
Co: Cobalt concentrations for AKS-1 ranged from 0.154 mg/L in leach #0 to 0.016 by
leach #10. For AKS-2 – 12 low cobalt concentrations (up to 0.052 mg/L) were observed
for some samples in leach #0, but subsequent concentrations were routinely below the
detection limit (0.008 mg/L).
Cu: High Cu concentrations were observed for AKS-1, with an initial value of 4.92 mg/L
in leach #0, dropping to 0.86 mg/L by leach #10. For AKS-2 – 11, detectable Cu
concentrations were observed for several samples in leach #0, but subsequent
concentrations were typically near or below detection level (0.003 mg/L).
Fe: As expected, high Fe concentrations were observed for AKS-1, with an initial value
of 126 mg/L in leach #0 dropping rapidly to 14 mg/L by leach #4 then slowly increasing
to 20 mg/L by leach #10. With few exceptions, Fe concentrations have been relatively
low (<1.0 mg/L) for all other samples.
K: The highest K concentrations were observed in AKS-12 ranging from 0.66 mg/L in
leach #0 to 0.18 mg/L by leach #10. For AKS-1 – 11, concentrations ranged from 0.25
to 0.63 mg/L in leach #0 then decreased to values near or below the detection limit
(0.103 mg/L) for leach #5 and #10.
Li: Low Li concentrations (up to 0.042 mg/L) were observed for most samples in leach
#0, and for AKS-1 in leach #1, but for all other samples were below the detection limit
Mg: Magnesium concentrations are illustrated in Figure 7. Concentrations ranged from
1 mg/L (AKS-12) to 61 mg/L (AKS-1) in leach #0 and subsequently decreased to 0.4
mg/L (AKS-12) to 8 (AKS-1) by leach #10.
Mn: Manganese concentrations ranged from 0.01 mg/L (AKS-12) to 3.9 mg/L (AKS-1) in
leach #0 and decreased to < 0.3 mg/L for all samples by leach #10.
Na: Sodium concentrations ranged from 0.6 – 2.1 mg/L in leach #0 and decreased to
0.11 – 0.42 mg/L by leach #10.
Ni: Nickel elution was most notable for AKS-1 with 1.24 mg/L in leach #0 decreasing to
0.14 mg/L by leach #10. Samples AKS-2-11 also yielded concentrations of 0.09 to 0.41
in leach #0. A few detectable concentrations were observed in subsequent leach
events, but most samples were near or below the detection limit (<0.006 mg/L).
S: Total-S concentrations are illustrated in Figure 8. Concentrations ranged from 7 mg/L
(AKS-12) to 709 mg/L (AKS-1) in leach #0, and decreased to 1 mg/L (AKS-12) to 94
(AKS-1) by leach #10. As expected, these values are directly related to sulfide oxidation
and leaching as sulfate.
Sr: Strontium concentrations ranged from 0.07 (AKS-12) to 0.53 (AKS-1) in leach #0
and decreased to 0.03 (AKS-12) to 0.10 (AKS-1) in leach #10.
Zn: High Zn concentrations were observed for AKS-1 with 3.95 mg/L in leach #0
decreasing to 0.29 mg/L in leach #10. For AKS-2 – 11, Zn concentrations of up to 0.58
mg/L were observed in leach #0, but subsequent samples typically were near or below
the detection limit (0.006 mg/L).
Table 2. Element detection limits, maximum water quality criteria levels (MCL), and samples
which may be exceed criteria indicated by Alaska, (DEC, 2008)
Element1 detection limit acute MCL chronic MCL samples which may exceed MCL
mg/L mg/L mg/L
Leach # AKS Observed
sample # Conc. - mg/L
As 0.017 0.34 0.15 -- -- --
Cd (25)2 0.004 0.0005 0.0001 0 1 0.035
Cd (400)3 0.004 0.0077 0.0006 0 3 0.006
1 1 0.017
10 1 0.005
Cu (25)2 0.003 0.004 0.003 0 1 4.921
Cu (400)3 0.003 0.050 0.029 1 1 3.460
5 1 1.086
5 2 0.005
10 1 0.855
10 2 0.006
10 3 0.018
10 4 0.004
10 5 0.007
10 6 0.005
10 7 0.005
Fe 0.007 nd 1.0 0 1 126
1 1 39
1 5 2
2 1 18
3 1 15
3 3 2
3 9 2
4 1 14
4 6 2
5 1 16
6 1 16
7 1 17
8 1 16
9 1 16
10 1 20
Ni (25)2 0.006 0.145 0.016 0 1 1.237
Ni (400)3 0.006 1.513 0.168 0 2 0.370
0 3 0.411
0 5 0.227
0 6 0.228
1 1 0.681
5 1 0.148
10 1 0.138
10 7 0.025
Zn (25)2 0.006 0.036 0.036 0 1 3.948
Zn (400)3 0.006 0.379 0.382 0 3 0.579
1 1 2.144
5 1 0.339
10 1 0.294
MCL values for B, Ba, Ca, Co, K, Li, Mg, Mn, Na, S, and Sr not defined in Alaska, DEC, 2008.
MCL values are dependent on alkalinity: this value indicates the MCL for hardness = 25 mg/L.
MCL values are dependent on alkalinity: this value indicates the MCL at hardness = 400 mg/L.
Figure 6. Calcium concentrations for AKS-1 – 12 over the first 10 weeks of leaching in
Figure 7. Magnesium concentrations for AKS-1 – 12 over the first 10 weeks of leaching
in humidity cells.
Figure 8. Sulfur concentrations for AKS-1 – 12 over the first 10 weeks of leaching in
The kinetic testing (humidity cell) phase of this program was successfully initiated
in mid-March of 2010 and the data on pH, EC, alkalinity, and ions of concern in the
leachates are presented in this interim progress report. To date, the overall response of
the materials mirrors the behavior predicted by the static testing phase in that the pure
acid generating rock material (B5 – sample AKS 1) produces strongly acidic and metal
rich drainage, but all blends of the limestone (5% and higher) are generating net
alkaline drainage to date. However, the possibility exists that the lower blending rates
of limestone (e.g. 5%) could become less reactive or Fe-coated with time and thus,
these leachates could potentially acidify over longer periods. The primary kinetic testing
phase will continue to a total of 26 weeks with a follow-up 13 week phase if requested
by USFS Alaska.
American Public Health Association. 1985. Standard methods for the examination of
water and wastewater. APHA, Washington, DC.
USEPA. 1996. USEPA test methods. SW-846 manual. USEPA, Washington, DC.
Available at: http://www.epa.gov/region8/water/biosolids/pdf/6010b.pdf (checked
June 29, 2010).
Alaska DEC. 2008. Alaska water quality criteria manual for toxic and other deleterious
organic and inorganic substances. Available at: