Carbon Dynamics and Redox Changes in an Arsenic Contaminated Aquifer in Bangladesh

Transcription

1 Carbon Dynamics and Redox Changes in an Arsenic Contaminated Aquifer in Bangladesh Mason Stahl, Ph.D. MIT Department of Civil and Environmental Engineering BUET 1

2 Presentation Overview Background Research Motivation and Relevance Bangladesh Study Site (Field/Lab Experiments) Implications of our Findings Conclusions and Future Work 2

3 Groundwater Arsenic in S-SE Asia 3

4 Groundwater Arsenic in S-SE Asia 50 million 4

5 Groundwater Arsenic in S-SE Asia 50 million > 1.5 million 5

6 Groundwater Arsenic in S-SE Asia 50 million > 1.5 million > 0.6 million 6

7 Groundwater Arsenic in S-SE Asia 30 million 50 million > 1.5 million > 0.6 million 7

8 Mechanism of Arsenic Release Arsenic mobilized by reductive dissolution of Fe-oxides Diagram adapted from Ravenscroft,

9 Research Motivation Arsenic mobilized by reductive dissolution of Fe-oxides How current levels of dissolved arsenic reached present state is unclear 9

10 Research Motivation Arsenic mobilized by reductive dissolution of Fe-oxides How current levels of dissolved arsenic reached present state is unclear In particular, need to characterize organic carbon sources driving reactions 10

11 Research Motivation Arsenic mobilized by reductive dissolution of Fe-oxides How current levels of dissolved arsenic reached present state is unclear In particular, need to characterize organic carbon sources driving reactions - What is the organic carbon source driving arsenic mobilization? - Surface derived? - Co-deposited with the aquifer? 11

12 Research Motivation Arsenic mobilized by reductive dissolution of Fe-oxides How current levels of dissolved arsenic reached present state is unclear In particular, need to characterize organic carbon sources driving reactions - What is the organic carbon source driving arsenic mobilization? - Surface derived? - Co-deposited with the aquifer? If surface derived High As may only be possible with surface derived inputs Changes in delivery of surface sources of OC will affect As levels If sediment derived High As is possible without inputs of surface sourced OC However, introduction of surface derived OC could further increase As 12

13 Research Motivation Arsenic mobilized by reductive dissolution of Fe-oxides How current levels of dissolved arsenic reached present state is unclear In particular, need to characterize organic carbon sources driving reactions - What is the organic carbon source driving arsenic mobilization? - Surface derived? - Co-deposited with the aquifer? If surface derived High As may only be possible with surface derived inputs Changes in delivery of surface sources of OC will affect As levels If sediment derived High As is possible without inputs of surface sourced OC However, introduction of surface derived OC could further increase As Research Goal Improve our understanding of the OC sources driving arsenic mobilization 13

14 Research Relevance Research Goal Improve our understanding of the OC sources driving arsenic mobilization Research Relevance How will OC and arsenic cycling within aquifers respond to perturbations such as: - Changes in land use - Intensive groundwater pumping - Climatic/hydrologic shifts What do our findings imply for past, present, and future contamination of aquifer systems? 14

15 Munshiganj, Bangladesh 15

16 Ponds in Bangladesh 16

17 Ponds in Bangladesh Ponds are ubiquitous and provide a source of OC rich recharge to the aquifer Intensive irrigation pumping draws pond recharge into the aquifer 17

18 Ponds in Bangladesh Ponds are ubiquitous and provide a source of OC rich recharge to the aquifer Intensive irrigation pumping draws pond recharge into the aquifer What are the relative roles of ponds/surface sources vs. sediment OC sources in driving arsenic concentrations in Bangladesh? 18

19 Ponds in Bangladesh Ponds are ubiquitous and provide a source of OC rich recharge to the aquifer Intensive irrigation pumping draws pond recharge into the aquifer What are the relative roles of ponds/surface sources vs. sediment OC sources in driving arsenic concentrations in Bangladesh? Approach: Construct a pond and study OC cycling and redox changes beneath the pond and throughout the aquifer 19

20 May 15, 2011 Pond Construction (May June, 2011) 20

21 May 27, 2011 Pond Construction (May June, 2011) 21

22 Pond Construction (May June, 2011) 22

23 Pond Construction (May June, 2011) 23

24 Pond Construction (May June, 2011) 24

25 Pond During Monsoon Flooding 25

26 Pond During Dry Season 26

27 Pond Hydrology 27

28 Site Overview 28

29 Site Cross-section 29

30 Site Cross-section 30

31 Aquifer Arsenic Levels WHO Limit WHO Limit 31

32 Shallow Zone Beneath Pond 32

33 Shallow Zone Beneath Pond Is arsenic released in near-surface during recharge? 33

34 Arsenic remains low beneath the pond 34

35 Arsenic remains low beneath the pond Investigate carbon cycle beneath the pond to: 1) Determine OC source driving redox processes 2) Identify if this OC is advected to depth Diagram adapted from Ravenscroft,

36 Pond Carbon Balance: Quick Primer on OC and IC Organic Solid SOC Plant matter Bacteria Dissolved DOC OC that passes through 0.2um filter Inorganic SIC Carbonate minerals (e.g., calcium carbonate) DIC CO 2 H 2 CO 3 HCO 3 CO 3 36

37 Pond Carbon Balance: Quick Primer on OC and IC Solid Dissolved Organic SOC Sorption/desorption, degradation/dissolution DOC Consumption by microbes Inorganic SIC Precipitation/dissolution DIC 37

38 Shallow Zone Beneath Pond Investigate carbon cycle beneath the pond to: 1) Determine OC source driving redox processes 2) Identify if this OC is advected to depth 38

39 Pond Carbon Balance: Aqueous Carbon 39

40 Pond Carbon Balance: Aqueous Carbon 1) DIC Increase >> DOC Inflow - Solid-phase source of OC required 2) DIC generated in < 50 cm - Redox reactions occur in shallow zone 40

41 Pond Carbon Balance: Aqueous and Solid-phase Carbon 41

42 Pond Carbon Balance: Aqueous and Solid-phase Carbon 1) DIC Increase >> DOC Inflow - Solid-phase source of OC required 2) DIC generated in < 50 cm - Redox reactions occur in shallow zone 3) DIC Age << DOC Age << SIC Age SOC Age - Young solid-phase OC required DIC generated from oxidation of pond muck 42

43 Pond Carbon Balance: Aqueous and Solid-phase Carbon 43

44 Pond Carbon Source: Age Depth 14 C DIC DIC Age DIC m yr mm Pond > Modern DIC Input Prod

45 Pond Carbon Source: Age Depth 14 C DIC DIC Age DIC m yr mm Pond > Modern DIC Input Prod DIC produced is much younger than DOC, SOC, SIC Young labile OC generated the DIC 45

46 Pond Carbon Source: Age Depth 14 C DIC DIC Age DIC m yr mm Pond > Modern DIC Input Prod What is the age of labile OC deeper? -Young surface OC? - Old sediment OC? 46

47 DOC Reactivity Experiments: Overview Sample Acidified immediately and analyzed for DOC 14 C Inoculate w/ unfiltered water Duplicate Transferred back to MIT Duplicate Reacted for 75 days and then analyzed DOC 14 C 47

48 DOC Reactivity Experiments: Age Change 48

49 DOC Reactivity Experiments: Consumed Age 49

50 DOC Reactivity Experiments: Shallow Samples Reactive DOC is older than bulk DOC and Younger than SOC Mix of sediment and surface source 50

51 DOC Reactivity Experiments: Deep Samples 51

52 DOC Reactivity Experiments: Deep Samples Reactive DOC is older than bulk DOC Sedimentary Sourced 52

53 Organic Carbon Mineralized: Deep Zones 53

54 Organic Carbon Mineralized: Deep Zones 54

55 Organic Carbon Mineralized: Deep Zones Depth 14 C DIC DIC Age DIC m yr mm > Modern DIC Input Prod

56 Organic Carbon Mineralized: Deep Zones Depth 14 C DIC DIC Age DIC m yr mm > Modern DIC Input Prod DIC Input is older than DOC at these depths 56

57 Organic Carbon Driving Arsenic Mobilization: Summary As Labile OC 0.3 kyr 2.7 kyr 3.5 kyr Pond muck consumed Labile OC older than Pond muck 9.9 kyr Labile OC Age Sediment Age 4.1 kyr Mix of sediment and surface source 12.2 kyr Labile OC Age Sediment Age 57

58 Organic Carbon Driving Arsenic Mobilization: Summary As Labile OC 0.3 kyr 2.7 kyr 3.5 kyr Pond muck consumed Labile OC older than Pond muck Arsenic mobilization appears to be driven primarily by sedimentary OC 9.9 kyr 4.1 kyr Labile OC Age Sediment Age Mix of sediment and surface source 12.2 kyr Labile OC Age Sediment Age 58

59 Explaining Patterns of Arsenic Contamination 59

60 Explaining Patterns of Arsenic Contamination Modeled groundwater ages from Klump et al

61 Explaining Patterns of Arsenic Contamination 61

62 Explaining Patterns of Arsenic Contamination Variability in arsenic concentrations appear to be explained by variability in water age 62

63 Explaining Patterns of Arsenic Contamination Decreased flushing 63

64 Explaining Patterns of Arsenic Contamination Decreased flushing Increased flushing 64

65 Explaining Patterns of Arsenic Contamination Young recharge Labile OC 65

66 Explaining Patterns of Arsenic Contamination Recharge with Labile OC Recharge with No labile OC 66

67 Explaining Patterns of Arsenic Contamination Variability in arsenic concentrations appear to be explained by variability in water age Further supports central role of sedimentary OC in driving arsenic mobilization 67

68 Summary and Conclusions Redox processes at depth appear to be predominantly driven by sedimentary OC High arsenic can occur absent reactive surface-derived OC High arsenic likely widespread prior to human perturbations However, introduction of labile OC could shift dominant source from sedimentary to surface-derived Arsenic is released at relatively constant rate within the aquifer Arsenic variability largely explained by differences in water age Increased flushing may lead to decrease in arsenic Decreased flushing may lead to increase in arsenic Low arsenic beneath pond at present Arsenic mobilization due to OC rich pond recharge may potentially be confined to shallow zone beneath pond 68