Transcript Ph.D. Defense A geochemical study of arsenic release

Geochemical study of arsenic
release mechanisms in the
Bengal Basin groundwater
Carolyn B. Dowling, Robert J. Poreda,
Asish R. Basu, and Scott L. Peters
Sampling
Sixty-eight groundwater samples
•Bangladesh
•West Bengal (India)
•3He/3H groundwater ages
•Major elements by Ion
Chromatography
•Trace elements by Inductively
Coupled Plasma – Mass
Spectrometry (ICP-MS)
Sediments from drill core
•Trace elements by ICP-MS
Compare adsorbed vs. dissolved
trace element concentrations
Groundwater Depth Profile of
Dissolved As
Figure 2. Depth profile of dissolved As. The groundwater arsenic concentrations above the
WHO standard (0.01ppm; 0.13 μM) are concentrated in the shallow wells (<60m). The higher
levels of As in the deeper wells ( > 60m) are most likely from the wells screening both deep,
As-free and shallow, As-rich groundwater.
Dissolved As versus Fe, CH4, and NH4
Figure 3. Dissolved As versus Fe, CH4, and NH4. Dissolved arsenic levels from all the wells in the
Bengal Basin are plotted against iron, methane, and ammonia. There are only weak to modest
correlations between arsenic and iron, methane, and ammonia.
Dissolved As versus CH4, Fe, and NH4
in Faridpur and Laxmipur
Figure 4 . Dissolved As versus CH4, Fe, and NH4 in Faridpur and Laxmipur. Dissolved arsenic levels from wells located in Faridpur and
Laxmipur are plotted against methane, iron, and ammonia. There are reasonable correlation (r2 = 0.8-0.9) between arsenic and methane, iron,
and ammonia. The As- CH4 and As-NH4, correlations indicate that there are active microbes in the anoxic groundwater. The As, CH4, and NH4
associations combined with the Fe data support bacterial reduction of FeOOH as the main release mechanism of As into the groundwater. The
samples experiencing low levels of microbial activity (based on CH4 levels) are excluded from the As-Fe correlation.
Watershed Hydrology
Figure 5. Schematic cross section of groundwater flow in the Bengal Basin along the NW-SE (A – A’)
line. All wells are projected to the A-A’ line in Figure 1. There are many municipal wells (up to 150 m)
that contain tritium and have a 3He/3H groundwater age. Shallow wells at Laxmipur, Faridpur, and Kustia
are tritium dead with high biogenic methane. Deeper wells in the upper aquifer, with no tritium and
moderate amounts of radiogenic 4He, are more than 100 years old. In the lower aquifer, the groundwater
has no tritium, elevated methane and helium concentrations, and residence times of greater than 1000
Laxmipur Groundwater and Sediment Oxalate
Extraction vs Depth
Figure 6. Groundwater and sediment oxalate extraction data plotted against depth at Laxmipur, Bangladesh. Select dissolved and adsorbed data
from the water and sediment oxalate extraction analyses (arsenic and iron) are plotted against depth at Laxmipur. The concentrations of
dissolved As and Fe are 5 times less than the adsorbed bulk fraction. Adsorbed As and Fe display a similar trend. Grain size plays an important
role in controlling the concentrations of trace metals dissolved in the groundwater and adsorbed onto sediment. The drop in the adsorbed metals
seen at 39 m corresponds to a sandier interval in the drill core.
Sediment As-Fe
Figure 7. Adsorbed Fe versus adsorbed As in the sediment fractions.
There is an overall correlation (r2 = 0.72) between adsorbed Fe and As
indicating that Fe and As come from the same source. It also points out
the importance of grain size on trace metals because the bulk sediment
has the lowest concentration of As and Fe while the fine-grained fraction
has the highest levels.
As/Fe Ratios with Depth
Figure 8. Adsorbed As-Fe molar ratios plotted against depth. There
is an overall decrease in the ratios with depth, resulting from more
groundwater flushing the lower sediments and removing some of
the As from the system.
Groundwater Age Dating
Research Questions?
Why do we care about Arsenic in groundwater?
Is it a problem in the Bengal Basin?
Which wells are contaminated by Arsenic?
Where are the wells located? What are their depths?
Does As correlate with other elements?
What are the sources of As?
Sediments? Industrial pollution? Agricultural
pollution?
Why is it a problem in the Bengal Basin?
Some Answers
Arsenic contamination is a real issue
Source is natural
Bulk sediments supplies As to the
groundwater
Microbial mediated reduction of iron oxyhydroxides
a.k.a. the microbial breakdown of FeOOH
Time Line
World Health Organization (WHO)
Until 1970s, population used polluted rivers
Drilled 2 million groundwater wells
Most wells are contaminated with arsenic (As)
Levels are greater than WHO maximum contaminant
level (MCL) of 0.01 ppm or 0.13 mM
Symptoms of Arsenic poisoning develop slowly
30-60% of the population is affected
Arsenic Geochemistry
Species
As(V), Arsenate, AsO43As(III), Arsenite, As2O42
30-60X toxic and 5-10X mobile
As strongly adsorbs onto iron oxy-hydroxides (FeOOH)
As-laden FeOOH are deposited in estuaries and wetlands
Existing Theories of As Release
Oxidation of pyrite (Rarely used anymore)
Requires oxic water
Competitive exchange with phosphorus
Phosphate (PO43-)
Dissolved As and P exchange for one another
Dissolution of iron oxy-hydroxides (FeOOH)
FeOOH strongly adsorb As
Correlation between Fe and As
Anaerobic microbes
Adsorption of selected anions on hydrous
ferric oxide as a function of pH
Figure 5-9. Drever, The Geochemistry of Natural Waters 3rd Edition
Background
^^^^^^^ Himalayas^^^^^^^^
Brahmaputra
Bangladesh and West Ganges
Bengal State, India
Quaternary deposits
Ganges-Brahmaputra
Himalayas
Sea level changes
and river migration
Complex stratigraphy
of coarse and finegrained sediment.
India
Bangladesh
Bay of Bengal
(Modified from http://www.geoexplorer.co.uk)
Sampling
Where is the Arsenic located?
Groundwater chemistry
Is the Arsenic coming from the
sediments?
Sediment chemistry
What is the watershed hydrology?
Groundwater flow
sta
Ti
R.
O
Brahmaputra R.
26°
Bangladesh
XO
Farakka
Shibganj
Ishurdi
Nawabganj
A
Kustia
Dhaleshwari R.
O
O
Dhaka
X
24°
Bangladesh
West Bengal
(India)
23°
Sediment
Drill core
River
Rajbari
Meherpur
Jhenidah
R.
Sixty-eight 25°
groundwater
samples
ges
R.
Me
gh
na
Sampling
G an
Faridpur
Sonargaon
Madaripur
Samta
West
Bengal
Barisal
Barasat
X
Calcutta
Laxmipur
O
Noakhali
X
Burir Char
Baruipur
22°
Bay of Bengal
21°
87°
88°
89°
90°
91°
92°
Groundwater Depth Profile
Is As a problem?
More than 60% of
samples above 0.13 mM
Where are the wells?
Throughout the country
What are the depths?
Highest levels of As at
shallow depths (< 60 m)
Does As correlate with others?
Iron (Fe)
Previous studies
link As and Fe
Weak correlation
between As and Fe
(r2=0.37)
Methane (CH4) &
Ammonia (NH4)
Microbial activity
Weak to modest
correlation
(r2: 0.39-0.55)
Correlations with Arsenic
Faridpur and Laxmipur
As-rich areas
Faridpur
Laxmipur
Strong
correlations with
CH4, Fe, & NH4
(r2: 0.8-0.9)
Are microbes involved?
As-CH4 and As-NH4 correlations
As microbes are oxidizing organic matter,
they are breaking down FeOOH
Microbes converting As(V) to As(III)
Microbes



Shewanella alga BrY
MIT-13
Geospirillum barnesii SES-3
Groundwater Age Dating
3H/3He
Age Dating Technique
Tritium (3H) is formed
Above ground nuclear testing
Cosmogenic reactions (14N + n = 3H + 12C)
Component of water molecule (3H2O)
3H decays to 3He
t1/2=12.4 yrs
Groundwater residence time:
t=(1/l)ln{1+(3He*/3H)}
Groundwater Age Dating
Variations in groundwater velocities
~0.4 m/yr
~3 m/yr
Complicated
stratigraphy
Complex distribution
of As
Watershed Hydrology
What is the source of As?
Sediments influence
groundwater
Mineralogy
Grain size
Adsorption/desorption
Dissolved As and Fe have
similar patterns
Adsorbed As and Fe have
comparable patterns
Bulk capable of supplying
As to groundwater
Sediment As-Fe
Modest correlation at
any depth
r2=0.7
Sources of As and Fe in
all solid phases may be
the same
Microbial dissolution of
FeOOH
Grain size plays an
important role
As/Fe Ratios
with Depth
As-Fe ratios
decrease with depth
More groundwater
has flowed through
the deeper
sediments
Removed As from
deeper aquifer
system
Overview of As Release
Rain
Water
Rain
Vadose Zone (unsaturated)
Phreatic Zone (saturated)
Ground Level
Water enters the ground and
equilibrates with calcium
carbonate and particles in the soil.
Water Unsaturated
Vadose
Zone(vadose) Zone
Recharge
Water Table
~ 6m
Phreatic
Zone
Water Saturated
(phreatic) Zone
Oxygen Present
Mica
Weathering
As and
Fe
Oxygen
Present
Release
Bug Zone #1
Water Velocity = 0.4 m/yr
No Oxygen Present
No Oxygen Present
Microbial
Activity
Adsorption
Bug Zone #2
Fe-Oxyhydroxide
FeOOH
Biological Release
~150 m
Adsorption
Arsenic
As
Aerobic organisms
consume O2
Anaerobic microbes
reduce FeOOH
Biological activity consumes
oxygen and produces carbon
dioxide into the groundwater.
Biological activity breaks down
Fe oxyhydroxide particles,
producing As and Fe ions. Some
As is adsorbed back onto the Fe
oxyhydroxides.

Releases Fe and As
Dissolved As levels


Biological activity
Adsorption reactions
Summary
As in groundwater
30-60% population is affected
60% of the samples above WHO MCL
(0.13 mM)
Depth less than 60 m
Anoxic groundwater greater than 60 yrs
Complicated distribution of As in
groundwater
Summary
Source of As
The As-laden sediments
Released from the sediments through
microbes
Bulk sediments are capable of supplying all
of the arsenic to the groundwater
Present
The Bad News
Groundwater will have high arsenic levels
for a very long period of time.
The Good News
The drinking supply wells can be drilled to
deeper depths.
Future Research?
Universal Problem??
Rapid accumulation of
sediments from
Himalayas



Ganges
Brahmaputra
Sea level changes and
river migration
Mekong Delta, Vietnam


(Modified from http://www.central.k12.ca.us)
Yangtze River
Irrawaddy River
Mekong River

Dong The Nguyen
Feb. 2004 AAAS travel
proposal
Future NSF proposal