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Arsenic in Bangladesh groundwater

What’s the origin of high As concentrations in groundwater? Tony Appelo Hydrochemical Consultant Amsterdam. DPHE and BGS, 2001. Arsenic in Bangladesh groundwater. Contents. Theories, reactions, correlations Pro & Contra, Experiment & model calculations

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Arsenic in Bangladesh groundwater

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  1. What’s the origin of high As concentrations in groundwater?Tony AppeloHydrochemical ConsultantAmsterdam

  2. DPHE and BGS, 2001 Arsenic in Bangladesh groundwater

  3. Contents • Theories, reactions, correlations • Pro & Contra, Experiment & model calculations • A model for Bangladesh groundwater • Summary, predictions

  4. Three theories A) Reductive dissolution of iron-oxyhydroxides, which releases sorbed As B) Oxidative dissolution of pyrite which contains As C) Displacement of sorbed As by other solutes

  5. Reductive dissolution of iron-oxyhydroxides: • correlation between As and Fe2+, Dissolved Organic Carbon (DOC)?

  6. Reductive dissolution of iron-oxyhydroxides: • correlation between As and Fe in sediment?

  7. Reductive dissolution of iron-oxyhydroxides, experiment: • Add acetate (OC) to Bengal sediment (Islam et al., 2004, Nature 430, 68)

  8. Reductive dissolution of iron-oxyhydroxides, experiment: • Add ascorbic acid (OC) to Fe(OH)3 (Pedersen et al., 2006, GCA 70, 4116)

  9. Reductive dissolution of iron-oxyhydroxides, model the experiment with PHREEQC: sorption of As

  10. ?? • Model it with PHREEQC… • Take Ganges water (As = 1 ppb) • Equilibrate with Fe(OH)3 • React with CH2O

  11. Reductive dissolution of iron-oxyhydroxides • Pro: • + high correlation of As and Fe in the sediments • + experiments show the expected reaction • + experiments can be modeled Contra: - weak correlation of As and Fe in groundwater - model pH is higher than observed - HCO3 in groundwater is higher

  12. B) Oxidative dissolution of pyrite which contains As correlation between As and Fe2+, SO4?

  13. B) Oxidative dissolution of pyrite which contains As The (rare) pyrite is framboidal, it forms in the aquifer

  14. Zheng et al., 2005, GCA 69, 5203 B) Oxidative dissolution of pyrite which contains As Groundwater table fluctuations are expected to be small in the Bangladesh lowlands?

  15. B) Oxidative dissolution of pyrite which contains As Vierlingsbeek, NL, acid rain, agriculture (NO3-) Van Beek & Vd Jagt, 1996, Vienna workshop

  16. B) Oxidative dissolution of pyrite which contains As An experiment with Vierlingsbeek sediment by Van Helvoort, MSc thesis VU, UU:

  17. B) Oxidative dissolution of pyrite which contains As, model the experiment with PHREEQC (major ions, pH)

  18. B) Oxidative dissolution of pyrite which contains As, model the experiment with PHREEQC (As)

  19. B) Oxidative dissolution of pyrite which contains As Pro: none in Bangladesh Contra: - pyrite is authigenic, it forms - no correlation among As and SO4 - As, if released, sorbs on iron-oxyhydroxide

  20. PO43- HCO3- Sorption of anions on ferrihydrite PZC C) Displacement of sorbed As by other solutes

  21. Displacement of sorbed As by PO43- • correlation between As and PO4?

  22. Displacement of sorbed As by HCO3- • correlation between As and HCO3?

  23. Displacement of sorbed As by HCO3- • Experiment: add carbonate to Bangladesh sediment (Anawar et al., 2004, CS 54, 743)

  24. Displacement of sorbed As by HCO3- and PO43- Experiment: • Inject As in a column filled with FeOOH coated quartz, displace with HCO3 and PO4 (Radu et al., 2005, EST 39, 7875)

  25. Displacement of sorbed As by HCO3- and PO43-, model the experiment with PHREEQC:

  26. Model it with PHREEQC: • Take Rain or Ganges water (A) • Equilibrate with CO2 and calcite in soil + • Ganges-derived Fe(OH)3 (B) • React with CH2O (C)

  27. C) Displacement of sorbed As by HCO3-

  28. C) Displacement of sorbed As by other solutes (anions HCO3-, PO43-) Pro: + experiments show it + models predict it Contra: - weak correlations of As with HCO3- and PO43-

  29. A model for Bangladesh groundwater: • HCO3- naturally displaces As from the sediment, until exhausted (PO43- is naturally low) • Natural flow is slow by the small relief • Pumping and irrigation return flow mixes groundwaters, probably adds PO43- which additionally displaces As, and labile, reducing, organic matter. It mixes waters which results in weak correlations • The upper aquifers are becoming exhausted of As • In shallow dug wells, Fe2+ oxidizes and precipitates, may sorb As, if still there • The deeper Pleistocene aquifers were flushed during the glacial maxima. As is occluded in recrystallized Fe-Ox and unavailable unless reduced

  30. A model for Bangladesh groundwater: As and Tritium concentrations with depth (Klump et al., 2006, EST 40, 243)

  31. Summarizing: the origin of high As concentrations in Bangladesh groundwater * As originates from iron-oxyhydroxides in the sediment * As is partly in sorbed state, it can be displaced by other solutes * HCO3- in groundwater is high, and a displacer * PO43- is a displacer * Reduction of Fe-oxyhydroxides releases sorbed As * Groundwater quality is messy by mixing

  32. Predicting: where to find low As concentrations in Bangladesh groundwater * As is related to Fe-content in the sediment: As will be flushed quicker where Fe is low find the upper aquifers with low iron content find groundwater near rivers with low HCO3- (indicates an effluent reach of the rivers) bank filtration near rivers reduces pathogens * and, water may be treated (surface water as well....)

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