Title: Ph.D.%20Defense%20%20A%20geochemical%20study%20of%20%20arsenic%20release%20mechanisms%20%20in%20the%20Bengal%20Basin%20groundwater
1Geochemical study of arsenic release mechanisms
in the Bengal Basin groundwater
Carolyn B. Dowling, Robert J. Poreda, Asish R.
Basu, and Scott L. Peters
2Sampling
- 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
3Groundwater 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 (lt60m). The higher levels of
As in the deeper wells ( gt 60m) are most likely
from the wells screening both deep, As-free and
shallow, As-rich groundwater.
4Dissolved 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.
5Dissolved 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.
6Watershed 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 years.
7Laxmipur 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.
8Sediment As-Fe
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.
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.
9Groundwater Age Dating
10Research 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?
11Some Answers
- Arsenic contamination is a real issue
- Source is natural
- Bulk sediments supplies As to the groundwater
- Microbial mediated reduction of iron
oxy-hydroxides - a.k.a. the microbial breakdown of FeOOH
12Time 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
13Arsenic Geochemistry
- Species
- As(V), Arsenate, AsO43-
- As(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
14Existing 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
15Adsorption of selected anions on hydrous ferric
oxide as a function of pH
Figure 5-9. Drever, The Geochemistry of Natural
Waters 3rd Edition
16Background
Himalayas
Brahmaputra
- Bangladesh and West Bengal State, India
- Quaternary deposits
- Ganges-Brahmaputra
- Himalayas
- Sea level changes and river migration
- Complex stratigraphy of coarse and fine-grained
sediment.
Ganges
India
Bangladesh
Bay of Bengal
(Modified from http//www.geoexplorer.co.uk)
17Sampling
- Where is the Arsenic located?
- Groundwater chemistry
- Is the Arsenic coming from the sediments?
- Sediment chemistry
- What is the watershed hydrology?
- Groundwater flow
18Sampling
- Sixty-eight groundwater samples
- Bangladesh
- West Bengal (India)
- Sediment
- Drill core
- River
19Groundwater 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 (lt 60 m)
20Does As correlate with others?
- Iron (Fe)
- Previous studies link As and Fe
- Weak correlation between As and Fe (r20.37)
- Methane (CH4) Ammonia (NH4)
- Microbial activity
- Weak to modest correlation (r2
0.39-0.55)
21Correlations with ArsenicFaridpur and Laxmipur
- As-rich areas
- Faridpur
- Laxmipur
- Strong correlations with CH4, Fe, NH4 (r2
0.8-0.9)
22Are 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
23Groundwater 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
- t1/212.4 yrs
- Groundwater residence time
- t(1/l)ln1(3He/3H)
24Groundwater Age Dating
- Variations in ground-water velocities
- 0.4 m/yr
- 3 m/yr
- Complicated stratigraphy
- Complex distribution of As
25Watershed Hydrology
26What 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
27Sediment As-Fe
- Modest correlation at any depth
- r20.7
- Sources of As and Fe in all solid phases may be
the same - Microbial dissolution of FeOOH
- Grain size plays an important role
28As/Fe Ratios with Depth
- As-Fe ratios decrease with depth
- More groundwater has flowed through the deeper
sediments - Removed As from deeper aquifer system
29Overview of As Release
Rain
- Vadose Zone (unsaturated)
- Phreatic Zone (saturated)
- Aerobic organisms consume O2
- Anaerobic microbes reduce FeOOH
- Releases Fe and As
- Dissolved As levels
- Biological activity
- Adsorption reactions
Vadose Zone
Recharge
Phreatic Zone
Oxygen Present
No Oxygen Present
Microbial Activity
FeOOH
As
Adsorption
30Summary
- 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
31Summary
- 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
32Present
- 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.
33Future Research?
- Universal Problem??
- Rapid accumulation of sediments from Himalayas
- Yangtze River
- Irrawaddy River
- Mekong River
- Sea level changes and river migration
- Mekong Delta, Vietnam
- Dong The Nguyen
- Feb. 2004 AAAS travel proposal
- Future NSF proposal
Ganges
Brahmaputra
(Modified from http//www.central.k12.ca.us)