are pitlakes a sustainable closure option for south ... · coal mining in rsa started mid 1850’s...
TRANSCRIPT
K5/2577/3
Are pitlakes a sustainable
closure option for South
African Coal Mines?
Andrew Johnstone
Lizel Kennedy
Matsatsi Mpetle
1
2
During Mine
After Mining
Is this environmentally sustainable ?
Background
Coal Mining in RSA started mid 1850’s
Most mines underground until 1960’s
After 1960 more open cast mines – economical to mine
shallow seams to 80m
Open cast mines may result in final voids due to
• Insufficient backfill material
• Roll over mining method
• Lack of closure plans
• Designed to prevent out flow
When a final void filled with water = PITLAKE
4
Pitlake Research
• 1989 Berkley Lake Montana – Davis & Ashenberg
• 1999 Lignite mines Germany – Stottmeister
• 2002 Pit Lake Modelling PitlakQ- Muller
• 2009– Creating lakes from open pit mines Canadian
Research ref 2826
• 2013 Sulphide open pit Mines Spain – Sanchez- Espana
• 2016 Pitlakes and sustainable ecosystems Australia
Lund & Blanchette
There over 200 coal mine pitlakes in South
Africa
5
Motivation for Research
1. Are pitlakes sustainable?
2. Are pitlakes a closure option in South Africa coal
mines
3. What drives the pitlake water balances ?
4. What is the evolution of the water quality?
5. What is the potential environmental impact ?
If they are a solution ?
✓ How should they be designed?
✓ is it acceptable environmental best practise?
✓ What legislative changes are required ?
6
Pitlake Parameters Investigated
• Hydrology of Pitlakes
• Hydrogeology
• Climatic conditions
• Pitlake bathymetry
• Water quality, stratification & mixing
• Water balance
• Geochemistry
• Microbiology
7
Pitlake Water Balance
∆S = (P+ SWin+ GWin) – (E+ (T) + SWout + GWout)* where
∆S - change in storage which is the volume of water in the lake,
P - the precipitation falling onto the pit lake,
SWin - the sum of any surface water inputs which includes runoff and
diverted streams,
GWin - groundwater entering the lake,
E - the evaporation from the lake,
T - plant transpiration
SWout - surface water exiting in the pit lake and includes pumpage,
GWout -is the groundwater leaving the pit lake.
(* Gammons et al 2009)
8
South Africa Rainfall and Evaporation
South Africa :Rainfall > evaporation
Mean Annual Rainfall = 650 mm/a
Mean Annual Evaporation = 1800 mm/a
Mean Net deficit of 1200 mm/a
IOW 1,2 m of net evaporation over the area of the
pitlake surface
If designed correctly = Terminal Sinks
Terminal sink pitlakes = no discharge of water into the
environment
9
Chemical Process in a Pitlake
10
Gammons et al 2009
Are Pitlakes Sustainable ?
Main Factors that determine pitlake sustainability
• Water Balance
• Water Quality
11
Case Studies
Pitlake Coal Field Depth Mining
method
Age of pitlake
(y)
Mafutha Waterberg Single
pitlake 70 m
Opencast bulk
sample
8
Kriel Colliery
Pit 4 (R44,
R42)
Mpumalanga Four Shallow
pitlakes
5 m to 12 m
Opencast strip
mining with
concurrent
rehabilitation
13
Rooikop
KZN
Single
12 m
underground
bord and pillar
linked to a
pitlake
12
KleinfonteinMpumalanga Single
3.5 m
Opencast strip
mining with
concurrent
rehabilitation
27
12
Table 1: Basic information of pitlakes case studies .
Mafutha 2004
13
Mafutha 2011
14
MAFUTHA 2017
Mafutha 2018
16
Monitoring points
17
Mafutha Pitlake Profiles
18
0
10
20
30
40
50
60
70
80
10 15 20 25 30
De
pth
(m
)
Temperature ( C)
150 170 190 210
Electrical conductivity (mS/m)
7 8 9
pH
0 200 400
ORP (mV)
0 2 4 6 8 10 12 14 16
Dissolved oxygen (mg/l)
Mafutha – Water Chemistry and Levels
19
Jan-17, 110
Jan-17, 842
828.00
771.38
Oct
-09
Jan
-10
Ap
r-1
0
Jul-
10
Oct
-10
Jan
-11
Ap
r-1
1
Jul-
11
Oct
-11
Jan
-12
Ap
r-1
2
Jul-
12
Oct
-12
Jan
-13
Ap
r-1
3
Jul-
13
Oct
-13
Jan
-14
Ap
r-1
4
Jul-
14
Oct
-14
Jan
-15
Ap
r-1
5
Jul-
15
Oct
-15
Jan
-16
Ap
r-1
6
Jul-
16
Oct
-16
Jan
-17
720.0
740.0
760.0
780.0
800.0
820.0
840.0
860.0
0.00
50.00
100.00
150.00
200.00
250.00
300.00
Oct
-09
Jan
-10
Ap
r-1
0
Jul-
10
Oct
-10
Jan
-11
Ap
r-1
1
Jul-
11
Oct
-11
Jan
-12
Ap
r-1
2
Jul-
12
Oct
-12
Jan
-13
Ap
r-1
3
Jul-
13
Oct
-13
Jan
-14
Ap
r-1
4
Jul-
14
Oct
-14
Jan
-15
Ap
r-1
5
Jul-
15
Oct
-15
Jan
-16
Ap
r-1
6
Jul-
16
Oct
-16
Jan
-17
Time vs Water Level elevation
Wat
er L
evel
ele
vati
on
(m
amsl
)
[SO
4] (
mg/
L)
Time vs SO4 (mg/l)SO4 (mg/L) Water Level (mamsl) Top of coal elevation (mamsl) Bottom of pit elevation (mamsl)
Mafutha Chemistry
20
Pitlake Chemistry
21
Pit lake A: Time series SO4, HCO3, Cl and TDS
22
0
50
100
150
200
250
300
350
SO4 (m
g/ℓ)
Time
Sulphate
Surface water samples
200
250
300
350
400
Cl (m
g/ℓ)
Time
Chloride
Surface water samples
0
100
200
300
400
500
600
HCO3 (m
g/ℓ)
Time
Bicarbonate
Surface water samples
800
900
1000
1100
1200
1300
1400
1500
TDS (m
g/ℓ)
Time
Total Dissolved Solids
Surface water samples
Kriel Ramp 44
23
Kriel Pitlakes
• Mining by roll-over method, rehabilitated, well-vegetated
• Age = 13 years
• Max depth = 12 m, Relative depth (Zr) = 4.7%
PIT 4PIT 23
N2017 – R42 looking East
2017 – R44 looking North
Kriel Pitlakes
25
Kriel Pitlake Water Chemistry
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Pitlakes: Ca/Na SO4 type
Karoo aquifer: Na-HCO3/Cl
type
Backfill: Ca/Na SO4 type
Rooikop Pitlake
27
ROOIKOP
N
+-64000 +-63500 +-63000 +-62500 +-62000 +-61500 +-61000+-2986800
+-2986300
+-2985800
+-2985300
+-2984800
RGCS1
RGCS2
RGCS3
RUG
RSW1
RSW5
RPOINT5
RPOINT3
RPOINT2
RPOINT1
Underground
Opencast
Seepage
29
After Mining
Summary of Pitlake Chemistry
Pit lakeMafutha Kriel Rooikop
pH 8.4 8.4 7.9
Temperature (°C)
27.2 (epilimnion; summer)
18.6 (hypolimnion, summer)
18.6 (whole pit lake, winter)
20.6 (summer)
13.5 (winter)
20 (summer)
15.3 (winter)
Dissolved oxygen (mg/ℓ)
7.53 (Epilimnion summer)
1.3 (hypolimnion summer)
6.95 (whole pit lake winter)
R44: 2.03 (summer) & 5.7 (winter)
R42: 4.9 (summer) & 8 (winter)
8.3 (summer)
7.9 (winter)
TDS (mg/ℓ) 1000 3443 1208
Total Hardness (as mg/ℓ
CaCO3)186 1210 712
Sodium (mg/ℓ) 301 434 18
Total alkalinity (mg/ℓ) 326 197 118
Sulphate (mg/ℓ) 94 1930 608
Chloride (mg/ℓ) 314 35 2.5
Nitrate-NO3 (mg/ℓ) 9.7 0.24 <0.1
Water type classification Na-Cl Na/Ca-SO4 Ca-SO4
Chlorophyll-a (µg/ℓ) 210.5
3.7
Trophic state classification Oligotrophic Mesotrophic to Eutrophic Oligotrophic to Mesotrophic
Phytoplankton (Dominating
Phylum, Genus)Chlorophyta,Ankistrodesmus
Cryptophyta,Cryptomonas
and
Chlorophyta,Ankistrodesmus
Chlorophyta,Chlorella
Phytoplankton (Dominating
Phylum, Genus)Chlorophyta,Ankistrodesmus
Cryptophyta,Cryptomonas
and
Chlorophyta,Ankistrodesmus
Chlorophyta,Chlorella
Microbes (Dominating
Phylum; Genera)
Proteobacteria;
Acinetobacter, Synechococcus
Proteobacteria;
Hydrogenophaga, Chlorobaculum,
Pseudomonas, Nodularia
Bacteriodetes;
Flavobacterium, Luteolibacter
Stratification & mixing• Strong thermal stratification
(October to March/ April)• Turnover (late autumn/ winter)
• Weak thermal stratification
(October to March/April)• Turnover (late autumn/ winter)
• Weak thermal stratification
(October to March/April)• Turnover (late autumn/ winter)
ClassificationbSlightly alkaline, low TDS, Holomictic,
monomictic
Slightly alkaline, high TDS
Holomictic, monomictic
Circum-neutral, low-TDS
Holomictic, monomictic
30
Comparison of pit lake chemistry, phytoplankton and microbiology (2016-2017)
Water Balance
Pitlake Inflow Outflow
Mafutha Groundwater 90%
Rainfall 8%
Runoff 2%
100% evaporation
Kriel Groundwater 67%
Rainfall 13%
Runoff 20%
100% evaporation
Rooikop Groundwater 58%
Rainfall 11%
Runoff 31%
91% evaporation
9% discharge
31
Water Chemistry and Balance
Chemistry
• Pitlakes are stable and support aquatic life
• Water quality will not comply with catchment water
quality standards
Water Balance
If pitlakes are designed correctly they will not discharge
and will be terminal sinks
Greatest inflow = groundwater
Outflow = evaporation
32
Terminal Sink Pitlakes as a Closure Option
There is no discharge into the catchment
Prevents continual and expensive non sustainable water
treatment
Avoids disposal of water treatment brines: leave brine at
source
Salinity may increase – more research required
Literal zone very important for vegetation and aquatic
life
Will not contain and lethal water quality
33
Legislative changes
Minerals Act:
• Accept pitlakes as a closure option
• Allow for final voids
• Avoid expensive water treatment
Water Act
• Allow for licensing of pitlakes
• Exemption for non compliance with catchment water
quality standards
NEMA
• Trigger some NEMA activities
34
A PRELIMINARY MANUAL FOR THE DESIGN OF COAL MINE PITLAKES AS AN ENVIRONMENTALLY
STABLE CLOSURE OPTION IN SOUTH AFRICAN MINES
35
A PRELIMINARY MANUAL FOR THE DESIGN OF COAL MINE PITLAKES
by
Andrew Johnstone
GCS Water and Environmental (Pty) Ltd,
Project No. K5/2577//3
April 2019
