Transcript As in Bangladesh - Columbia University

Arsenic in groundwaterexamples from US Superfund sites
and Bangladesh
Martin Stute, Barnard College & L-DEO
(with help from A. Keimowitz)
Outline
•
•
•
•
As standards, health effects
Basic geochemistry
As mobilization at a former landfill site in ME
As in Bangladesh
– Causes for As mobilization
– Remediation options
BGS and DPHE (2001)
Health effects of (chronic) arsenic exposure
•
•
•
•
Previous studies in Taiwan, Argentina, and Chile
Cardiovascular disease
Skin lesions (few years of exposure)
Cancers of the skin, lung, liver, and bladder (several
decades of exposure)
• Children’s intellectual function
http://superfund.ciesin.columbia.edu/home.html
Arsenic Overview
• Toxic metalloid dispersed throughout earth’s crust
– average crustal concentration 2mg/kg = 2ppm
• Used in insecticides, herbicides, semiconductors
• Can be released into groundwater
• WHO and US EPA standards: 10 ppb (mg/L)
– Bangladesh standard: 50 ppb
• In South Asia, ~100 million people’s drinking water
exceeds 10 ppb
• In the US, cost estimates of implementing this
standard are $100-$600 million / year
• At US Superfund sites, arsenic is the 2nd most common
contaminant (after lead); present at 718 sites.
Nordstrom, Science 2002.
Gurian et al., ES&T 2001.
http://www.epa.gov/superfund/sites/query/basic.htm.
Arsenic Overview
• Arsenic is stable in multiple oxidation states:
-3, 0, +3, and +5
• This influences As
800
1
H3AsO4 (aq)
Eh (volts)
.5
300
--
HAsO4
H3AsO3(aq)
0
---
-200
AsO4
As
– .5
-
H2AsO 3
pH
25C
0
-700
2
4
6
8
10
12
14
ORP (mV)
-
H2AsO 4
mobility in the
subsurface– both
natural As and
anthropogenic
pollution
Landfill History
1e+6
DMF, MW-210B
DMF, EW-2
1e+5
ppb
1e+4
1e+3
1e+2
1e+1
1e+0
1980
1000
1985
1990
1995
2000
1990
1995
2000
ppb As
As, EW-2
As, MW-210B
100
10
1980
1985
• 1930 Dump operations begin
including local industrial wastes
• 1972 Landfilling begins and
serves adjacent towns
• 1979 Buried drums found at
landfill
• 1982 Landfilling ceases
• 1987 Landfill cover system
installed
• 1995 SVE and P&T remediation
commences
N. Nikolaidis, UCon
As Source: Not the Landfill
140
Arsenic, mg/L
120
100
80
60
40
20
0
0
10
20
30
40
Chloride, mg/L
•Even distribution of
sediment As– no
hotspots
•Typical crustal
abundance
•Dissolved As does not
correlate with Cl – a leachate
tracer– in swamps
•Implies As controlled by a
different transport or
mobilization process
50
Two Distinct Regions
18
mg/kg sediment As
16
Sediment
14
12
Well
Region
10
8
6
4
2
0
Peripheral Region
mg/L dissolved As
0.45
Central Region
Well
Region
Water
0.35
0.25
0.15
0.05
Peripheral Region
Central Region
Reduction Induced by Leachate
Sed As
mg kg-1
Dissolved Species
mean, mg L-1
Cl -
DOC
Sulfide
Fe2+ Iron COD
Water
As
7
1
Central Region
29
6
26
 14
53
 49
24
 15
37
9
35
 23
0.3
0.1
8
2
Peripheral Region
10
2
16
4
5
2
2
4
2
4
1
0
0.01
0.005
Arsenic Mobilization–
Geochemical Explanation
Transport of Reducing Power
70
<0.073 mm
0.185-0.073 mm
60
COD, mg/g
50
40
30
20
10
0
0.0
0.2
0.4
0.6
0.8
1.0
Fraction of fine grained sediments (<0.185 mm)
Coarse
Fine
• Implication: COD high in fast transport
zones.
– [As] and [Fe] too?
– How to measure “fast transport” zones?
As Immobilization via Oxidation
Laboratory Oxidation
30
6
Iron
25
Arsenic
20
4
15
10
As (mmoles)
Fe (mmoles)
8
2
5
(a)
(b)
0
3
4
2
3
1
2
0
1
-1
0
-2
(d)
(c)
-3
-1
0
2
4
6
mmol H2O2 added
8
10
0
2
4
6
8
10
mmol H2O2 added
• Fe & As are both removed from reducing GW in a lab
experiment of progressive oxidation
• Removal is decoupled
ln[As]
ln [Fe]
0
Purposeful in situ Oxidation
• ORC– Oxygen Release Compound
• Contains magnesium peroxide, phosphate and minor
components
• Designed to release O2 over ~6 months
• 1400 kg ORC injected through aquifer thickness
ORC® Pilot Experiment– Results
Well 17
Well 6
0.5
50
0.4
40
0.3
30
0.2
20
0.1
10
Arsenic
Iron
0.0
J
0
an
b
Fe
r
Ma
r
Ap
y n
Ma Ja
b
Fe
r
Ma
r
Ap
y n
Ma Ja
b
Fe
r
Ma
r
Ap
y
Ma
Iron (mg/L)
Arsenic (mg/L)
Well 14
ORC® Pilot Experiment
• 1400 kg ORC released ~330 kg O2
• But what is the subsurface oxygen demand?
COD
 COD
of water satisfied
18 times
 Only 0.1% of COD of
sediments met
BOD
 BOD
of water satisfied
100 times
 Only 20% of BOD of
sediments met
Conclusions– Winthrop, ME
• Leachate induces reducing conditions
• These reducing conditions permit As mobilization
• Hydrogeology has controlled where leachate influence
is strongest; this in turn effects where As is mobilized
• As is removed from solution by oxidation
• In situ oxidation is hampered by redox demand of
sediments
Araihazar
Dhaka
**
Source: M. Steckler, LDEO, based on GTOPO30 digital elevation model (USGS EROS Data Center).
Araihazar
Bangladesh Arsenic
Mitigation and Water Supply Program
5 million wells so far?
http://www.bamwsp.org/
Arsenic in 5,966 wells
As - depth distribution in Araihazar
0
depth (m)
20
40
60
80
100
0
200
400
600
As (mg/l)
800
1000
Wellnests
Wellnests in Araihazar
E
A
C
B
G
H
F
Site A
Residence time
10’s of y
peat
1000’s of y
Site A, Zheng et al., 2005
What’s needed for elevated As
concentrations?
• Iron oxyhydroxides with adsorbed As
– Perhaps other phases?
• reducing conditions (no O2, low ORP)
– natural organic matter
• peat?
– anthropogenic organic matter?
As and groundwater age
4
F
C
E
G
A
B
6
8
depth (m)
10
12
14
16
18
20
0
5
10
3
3
15
20
25
30
H/ He age (years)
35
500m
500m
C
low
F
E
coarse
Low
A
EM
B
high
sediments
fine
recharge rate
high
62
G
63
9
low
13
As
6 cm/y
high
EM conductivity Color Scale (mS/m)
Recharge rate, As and local EM data
Remediation options
Existing wells
Alternative sources
New wells
As removal Well switching
Safi filter @$18
3-kolshi filter @$5
Tube well sand filter
Maintenance
Monitoring
Bacterial growth
Spatial variability
Social resistance
Surface water
Shallow wells Deep wells Pond water Rain collection
Dug wells
$50 for 150 ft
Seasonality
Pathogens
Installation
Distribution
Pond sand filter
Rainwater harverster
50 families @$16 ea. 1 family @ $160/$40 ea.
Bacteria 1/100
Aquaculture
Boiling
Storage-seasonality
Conclusions
• As concentrations are highly variable in
Bangladesh
• As is of natural, but we do not know yet if
there are anthropogenic factors influencing the
As distribution
• Hydrology (groundwater age) is an important
factor
• Deeper wells appear a feasible remediation
option, although we need to keep irrigation in
check