1. What’s the origin of high As concentrations in groundwater? Tony Appelo Hydrochemical Consultant Amsterdam

2. Arsenic in Bangladesh groundwater DPHE and BGS, 2001

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

6. A)Reductive dissolution of iron-oxyhydroxides: correlation between As and Fe 2+, Dissolved Organic Carbon (DOC)?

7. A)Reductive dissolution of iron-oxyhydroxides: correlation between As and Fe in sediment?

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

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

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

11. Model it with PHREEQC… Take Ganges water (As = 1 ppb) Equilibrate with Fe(OH) 3 React with CH 2 O?

12. A)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 - HCO 3 in groundwater is higher

14. B) Oxidative dissolution of pyrite which contains As correlation between As and Fe 2+, SO 4 ?

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

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

17. B) Oxidative dissolution of pyrite which contains As Vierlingsbeek, NL, acid rain, agriculture (NO 3 - ) Van Beek & Vd Jagt, 1996, Vienna workshop

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

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

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

21. B) Oxidative dissolution of pyrite which contains As Pro: none in Bangladesh Contra: - pyrite is authigenic, it forms - no correlation among As and SO 4 - As, if released, sorbs on iron-oxyhydroxide

22. C) Displacement of sorbed As by other solutes PZC Sorption of anions on ferrihydrite HCO 3 - PO 4 3-

23. C)Displacement of sorbed As by PO 4 3- correlation between As and PO 4 ?

24. C)Displacement of sorbed As by HCO 3 - correlation between As and HCO 3 ?

25. C)Displacement of sorbed As by HCO 3 - Experiment: add carbonate to Bangladesh sediment (Anawar et al., 2004, CS 54, 743)

26. C)Displacement of sorbed As by HCO 3 - and PO 4 3- Experiment: Inject As in a column filled with FeOOH coated quartz, displace with HCO 3 and PO 4 (Radu et al., 2005, EST 39, 7875)

27. C)Displacement of sorbed As by HCO 3 - and PO 4 3-, model the experiment with PHREEQC:

28. Model it with PHREEQC: Take Rain or Ganges water (A) Equilibrate with CO 2 and calcite in soil + Ganges-derived Fe(OH) 3 (B) React with CH 2 O (C)

30. C) Displacement of sorbed As by HCO 3 -

31. C) Displacement of sorbed As by other solutes (anions HCO 3 -, PO 4 3- ) Pro: + experiments show it + models predict it Contra: - weak correlations of As with HCO 3 - and PO 4 3-

32. A model for Bangladesh groundwater: HCO 3 - naturally displaces As from the sediment, until exhausted (PO 4 3- is naturally low) Natural flow is slow by the small relief Pumping and irrigation return flow mixes groundwaters, probably adds PO 4 3- 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, Fe 2+ 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

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

34. * As originates from iron-oxyhydroxides in the sediment * As is partly in sorbed state, it can be displaced by other solutes * HCO 3 - in groundwater is high, and a displacer * PO 4 3- is a displacer * Reduction of Fe-oxyhydroxides releases sorbed As * Groundwater quality is messy by mixing Summarizing: the origin of high As concentrations in Bangladesh groundwater

35. * 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 HCO 3 - (indicates an effluent reach of the rivers) bank filtration near rivers reduces pathogens * and, water may be treated (surface water as well....) Predicting: where to find low As concentrations in Bangladesh groundwater