Mine Spoil Weathering and
TDS Dynamics
W. Lee Daniels, Zenah Orndorff
and Carl Zipper
Dept. of Crop and Soil Environ. Sciences
Historically, for active surface mines, we have focused our premining analytics on (1) which materials need to be treated/isolated
to prevent AMD and (2) which materials are optimal revegetation
substrates However, we now need to consider (3) what TDS
components will each release?
Where’s TDS come from?
• Acid-base reactions; sulfide oxidation
and carbonate neutralization reactions.
• Background carbonation reactions in
non-sulfidic materials.
• Hydrolysis of primary mineral grains.
• Entrained Cl and SO4 in rocks (minor).
• Other minor weathering reactions like K
release from micas, etc.
All these reduced gray spoils are just waiting to weather!
Oxidized, pH 5.5 overburden over reduced pH 8.0
carbonate (2%) containing overburden at depth.
Sample Collection
• Mine spoils collected from SW Virginia and
Eastern Kentucky (> 25 to date).
• In many cases spoils are “paired” by site to
compare weathered/oxidized materials vs.
unweathered/reduced.
• Composite samples (per shot) collected
from drill cores from two sites in SW VA.
Minimally weathered/reduced
Well-weathered/oxidized
Mixed materials
diverse spoil types represented:
sandstones
siltstones
mudstones
different degrees of weathering
Spoil characterization includes….
• pH and EC (saturated paste)
• Peroxide Potential Acidity (PPA) and Acid-Base
Accounting (ABA; Total S and CCE/Sobek-NP)
• Mehlich I nutrients/metals and hot CaCl2 ext. B
• Total Elemental Analysis – EPA 3050B (acid
digestion) followed by ICPES.
• Sequential Extraction – Modified Tessier (1979)
(Soluble + Exchangeable ions; Carbonates;
Amorph Fe-Mn; Cryst. Fe-Mn Oxides; residual).
COLUMN LEACHING
Capped with
~5 cm sand
• samples air-dried and crushed
to pass a 1.25-cm sieve
• Inside diameter = 7.4 cm
• Length/height = 45+ cm
• Inside bottom of column:
~5 cm sand layer above
Whatman #1 filter above
0.1 mm mesh (glued to endcap)
• 1 cm PVC pipe nipple and
Tygon tubing for drainage
• Sample volume ~1200 cm3
• Each sample run in triplicate under saturated and/or
unsaturated conditions (3 to 6 columns per sample)
• Typically run for minimum of 20 weeks (40 cycles)
Column Leaching
Simulated acid rain (pH ~4.8) applied 2x/week
(Mon/Th.)
Each rainfall event = 100 ml (~2.5 cm – 1 inch)
Samples (100 ml) collected after 24 hrs (Tue/Fri).
Samples analyzed for:
pH, EC, Ca, Fe, Mn, HCO3- , S, Cl, Se, etc….
Several other parameters (i.e. TDS by wt., trace
metals) may be evaluated.
9
Mine Spoil OSM #2 by rep - Saturated vs. Unsaturated
Leachate pH
8
7
Saturated rep 1
Saturated rep 2
Saturated rep 3
Unsaturated rep 1
Unsaturated rep 2
Unsaturated rep 3
6
5
4
0
9
ate pH
8
7
10
20
30
Coarse Refuse OSM #21 by rep - Saturated vs. Unsaturated
40
Spoil characterization
(12 -26 diverse mine spoil samples)
Spoil
pH (2:1)
5.22 – 8.64
pH (paste)
6.28 – 7.85
EC (paste; uS/cm)
200 – 3,480
PPA (Tons/1000 CCE)
0.00 – 3.58
CCE (%)
1.30 – 6.00
S (%)
0.03 – 1.02
9
8
8
7
7
pH
pH
9
6
6
saturated
unsaturated
saturated
unsaturated
5
5
weathered sandstone
unweathered sandstone
4
4
0
5
10
15
20
25
0
30
5
10
15
20
25
30
leach #
leach #
9
8
8
7
7
pH
pH
9
6
6
saturated
unsaturated
saturated
unsaturated
5
5
unweathered mudstone
weathered mudstone
4
4
0
5
10
15
leach #
20
25
30
0
5
10
15
leach #
20
25
30
TDS vs EC
• TDS = Total Dissolved Solids is sum of Ca+K+SO4
etc. in solution. Expressed as mg/L.
Laborious to analyze for!
• Typically estimated by electrical conductance
TDS vs EC - Minespoil Data
(EC) in µS/cm.
TDS ~ 0.7 EC.
5
Minespoil
n = 319 of paired observations
R2 = 0.981
TDS = 0 + 0.981 * EC
• 500 µS/cm ~ 350 mg/L
TDS (g L-1)
4
3
2
1
0
0
1
2
3
EC (dS m-1)
4
5
SANDSTONE: weathered vs unweathered
10 samples
2500
unsaturated
unweathered
weathered
EC (uS/cm)
2000
1500
1000
500
0
0
5
10
15
leach #
20
25
30
unweathered
weathered
SANDSTONE
600
500
400
300
2500
200
EC (uS/cm)
2000
1500
100
5
1000
10
15
20
500
0
0
5
10
15
leach #
20
25
30
25
30
MUDSTONE: weathered vs unweathered
8 samples
unsaturated
unweathered
weathered
EC (uS/cm)
3000
2000
1000
0
0
5
10
15
leach #
20
25
30
unweathered
weathered
MUDSTONE
1200
1000
800
600
EC (uS/cm)
400
3000
200
2000
0
10
15
20
1000
0
0
5
10
15
leach #
20
25
30
25
30
SANDSTONE: saturated vs unsaturated
5 samples
5000
EC uS/cm (log scale)
Filled symbols = saturated
Unfilled symbol = unsaturated
1000
500
300
100
0
5
10
15
leach #
20
25
30
MUDSTONE: saturated vs unsaturated
4 samples
5000
EC uS/cm (log scale)
Filled symbols = saturated
Unfilled symbol = unsaturated
1000
500
300
100
0
5
10
15
leach #
20
25
30
increasing symbol diameter = increasing % sandstone
1200
100% SS
37% SS
66% SS
2% SS
26% SS
43% SS
EC uS/cm
1000
800
600
2% ss (4)
37% ss (2)
26% ss (5)
43% ss (6)
66% ss (3)
100% ss (1)
400
200
0
10
20
30
leach #
6 shots from a drill core
40
water under saturated and unsaturated conditions.
UNSATURATED
SATURATED
303
157 mg L-1
251
-1
Elemental concentration (mg L )
2591
2988
Mine
Spoil
unweath.
OSM # 2
3000
mudstone
2000
1000
1000
0
5
10
15
20
25
30
35
40
45
50
280 mg L-1
403
3000
2000
0
499
Ca
K
Mg
Na
0
0
5
10
15
20
25
30
35
40
45
50
Sulfate
Bicarbonate
211
58 mg L-1
87
635
262
65 mg L-1
53
700
mixed
Mine
Spoil
OSM
#3
materials
-1
Elemental concentration (mg L )
700
437
600
600
500
500
400
400
300
300
200
200
100
100
0
0
5
10
15
20
25
30
35
40
45
50
0
0
5
10
15
20
25
30
35
40
45
50
unsaturated
mg/L
saturated
600
600
500
500
400
400
300
300
200
200
100
100
0
0
0
10
20
30
40
50
0
20
30
leach #
leach #
Weathered
sandstone
10
Ca
K
Mg
Na
sulfate
bicarbonate
40
50
leach #22 (week 13)
400
unsaturated
Ca
K
Mg
Na
sulfate
mg/L
300
200
100
0
weathered
sandstone
unweathered
sandstone
* note: bicarbonate not shown
weathered
mudstone
unweathered
mudstone
Summary Findings
Most samples eluted considerable levels of TDS
over their initial leaching cycles with EC usually
> 500 uS/cm; some were lower.
For all materials, after first pore volumes (7 to 9
cycles) leach, TDS elution drops rapidly.
Samples containing significant reactive sulfides
were most likely to elute high TDS levels for the
duration of the study, regardless of their
leachate pH values.
Summary Findings
Overall, TDS mass was dominated by
six elements/compounds:
Ca, K, Mg, Na, SO4 and HCO3, .
TDS was commonly dominated by…
• sulfate under unsaturated conditions
• sulfate and bicarbonate under saturated
conditions.
Summary Findings
• Many spoils generate drainage with
moderate to high pH and high TDS.
• TDS evolution will be directly related to
the source strata and extent of historic
weathering and oxidation.
“Brown is better!”
• Similar to our historical approach to acidbase-accounting, we need to develop
better predictive tools for TDS release.
How can we do it?
• Assume all pyritic-S will leach as sulfate over
time. Use weak H2O2 to predict S reactivity?
• Measure CCE and assume all will leach as Ca +
bicarbonate over time. But how quickly?
• Use saturated paste EC to predict “first flush
peaks” and ABA parameters + other analytics
via regression to estimate longer term release.
• Predicting the peak of TDS and the shape of the
long-term release slope will take some work!
MUDSTONE: weathered vs unweathered
8 samples
unsaturated
unweathered
weathered
EC (uS/cm)
3000
2000
1000
0
0
5
10
15
leach #
20
25
30
Historical
chart
developed
by Smith et
al. from WV
for ABA vs.
depth
prediction.
We need to
be able to do
the same
thing for
TDS!
Spoil Handling and Placement
• Identify “hot TDS” materials and isolate them in similar
fashion to acid forming strata.
• Avoid durable rock fills where hard/gray unoxidized
rocks (with even moderate TDS potential) are placed
into direct percolation/leaching zones.
• Don’t mix acid-forming and net alkaline materials in
fills to intentionally meet ABA. This is a TDS engine.
• Consider alternative fill designs where surface lifts are
compacted to minimize infiltration.
Regional TDS Prediction Study
I. Obtain a representative regional sample set of mine
spoils and associated weathered overburden from the
central Appalachian region with varying potentials for
TDS release.
II. Fully analyze these spoil materials via a wide range
of laboratory analytical procedures for their potential
to release important TDS components upon leaching
and weathering.
III. Characterize the TDS elution behavior of selected
mine spoil materials via column leaching analyses for
TDS and component ions of interest.
Regional TDS Prediction Study
IV. Determine whether predictive relationships exist
between the various lab procedures employed to
estimate TDS release potential and the actual TDS
flux behavior observed from the leaching columns.
V. Investigate the effect of leaching scale (columns vs.
tanks vs. fills) on the quantity and temporal nature
of TDS release from selected mine spoils.
VI. Relate laboratory TDS predictors to actual field
data sets for coal mining operations dominated by
the spoils tested in this study.
Oxidized, pH 5.5 overburden over reduced
carbonate (2%) containing overburden at depth.
Acknowledgments
Current support provided by industry
cooperators and Powell River Project.
Thanks to OSM for support of original
2007 to 2009 leaching study.
Thanks to Red River Coal, Alpha Natural
Resources, TECO and other industry
cooperators for assistance in sample
collection.