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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) 3 He/ 3 H groundwater ages Major elements by Ion Chromatography

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Geochemical study of arsenic release mechanisms in the Bengal Basin groundwater

Carolyn B. Dowling, Robert J. Poreda, Asish R. Basu, and Scott L. Peters



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 ofdissolved 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.



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.


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

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.

research questions
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
Some 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
time line
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
Arsenic 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
existing theories of as release
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


^^^^^^^ ^^^^^^^^



  • 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.




Bay of Bengal

(Modified from

  • Where is the Arsenic located?
    • Groundwater chemistry
  • Is the Arsenic coming from the sediments?
    • Sediment chemistry
  • What is the watershed hydrology?
    • Groundwater flow
  • Sixty-eight groundwater samples
    • Bangladesh
    • West Bengal (India)
  • Sediment
    • Drill core
    • River
groundwater depth profile
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
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
Correlations with ArsenicFaridpur and Laxmipur
  • As-rich areas
    • Faridpur
    • Laxmipur
  • Strong correlations with CH4, Fe, & NH4 (r2: 0.8-0.9)
are microbes involved
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
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 dating1
Groundwater Age Dating
  • Variations in ground-water velocities
    • ~0.4 m/yr
    • ~3 m/yr
  • Complicated stratigraphy
  • Complex distribution of As
what is the source of as
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
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 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
Overview of As Release


  • 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


Phreatic Zone

Oxygen Present

No Oxygen Present






  • 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
  • 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
  • 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
Future 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



(Modified from