1. Environmental Tracer Applications in Aquifers with Elevated Arsenic Concentrations in Bangladesh
Stute, M, Horneman, A, Schlosser, P, Zheng, Y, van Geen, A, Santella, N, Smethie, W, Ho, D, Ahmed, KM, and
Hoque, MA
Columbia University, Lamont-Doherty Earth Observatory, Barnard College, Queens College, Dhaka University
Abstract for the 2006 annual Geological Society of America meeting in Denver, 7-10 November. Session on tracers in hydrogeology.
Distributions of 3H/3He, atmospheric noble gases, CFCs, and SF6 were determined in aquifers in
Bangladesh with elevated dissolved Arsenic concentrations. Tracer distributions provide
evidence for degassing during recharge and rapid recharge caused by flooding. Comparison of
groundwater 3H+3He data with 3H concentrations in precipitation suggests that some samples
are affected by mixing with pre-bomb groundwater. CFC 11 and 12 concentrations in this
reducing environment decline rapidly as a function of 3H/3He age at rates of ~0.4 to ~6 yr-1 and
Publish... command tutorial
2 of 2 7/11/ :22 PM
from ~0.25 to ~5 yr-1, respectively. SF6 ages were generally higher than 3H/3He ages, most
likely due to interactions with subsurface gas phases. SF6 was found in 3H-free deeper
groundwater indicating the occurrence of natural SF6 in Bangladesh aquifers. 3H/3He-derived
recharge rates are consistent with hydraulic estimates. A tight relationship between arsenic and
groundwater age implies that hydrogeology plays an important role in determining the variability
of Arsenic concentrations in Bangladesh.
Supported by: USEPA/NIEHS SBRP 1 P42 ES10349

2. [As] – a global problem http://www.bgs.ac.uk/arsenic/Bangladesh/
BGS and DPHE (2001)

4. As drinking water standards
World Health Organization (WHO) guideline: 10 mg/L
US standard mg/L
Bangladesh standard for drinking water: 50 mg/L
Range in Bangladesh: <1 to over 1000 mg/L

5. Health effects of (chronic) As exposure
Early studies in Taiwan, Argentina, and Chile
Cardiovascular disease
Skin lesions (few years of exposure)
Cancers of the skin, lung, liver, and bladder (several decades of exposure)
Children’s intellectual function

6. Arsenic in Bangladesh
25 million people > 50µg/L (Bangladesh standard)
51 million people > 10µg/L
(US/WHO standard)
BAMWSP (Bangladesh Arsenic
Mitigation and Water Supply Program)

7. Why is there [As] in Bangladesh?
Need:
In Bangladesh:
As in groundwater
=> 1000 ug/L
As(III)
As in sediment
1-10 mg/kg
As in sediments (not much)
Organic matter
Reducing conditions (no oxygen)
BGS and DPHE (2001)

8. Bangladesh is a flat place
Well As (ug/L)
WHO guideline
Bangladesh standard
Zheng et al., 2004
BGS/DPHE (2001)

9. …and complicated (heterogeneous)
Surface, last glacial maximum
Fig. 2 Simplified hydrogeological section through north-central
Bangladesh (after Ravenscroft 2003). The lithological section is
derived from several hundred individual logs reported by BADC
(1992) that have been averaged as the most probable lithology in
each 3-m depth slice within the local administrative unit (union)
through which the line of section passes. Lower case ‘g’ (grey) and
‘b’ (brown) denote the dominant sand colour. The bold dotted line
shows the inferred position of the land surface during the last
glacial maximum. Large arrows show the general direction of
regional groundwater flow. The depth of the section represents the
varying average depth of wells in each area, which is governed by
the greater thickness of clays and lower permeability of sands
beneath the Madhupur Tract. Permeability and EC data are also
from BADC (1992). Iron data are from Davies and Exley (1992)
Local, intermediate,
and regional flow systems
Ravenscroft et al., 2005

10. Araihazar One of 464 thanas (counties) in Bangladesh
Population: 300,000
Size: 170 km2
Population density:
1800 people/km2
Sources: C. Small, LDEO, Landsat 7 image from USGS EROS Data Center, Sioux Falls, SD, 1991 National Census.

13. High spatial variability
Arsenic in 5,966 wells

14. As - depth distribution in Araihazar
6000 wells
Van Geen et al., 2002

15. Wellnests

16. Wellnests in AraihazarC E G F B H

17. Stute et al, 2007
Pickens et al., 1978

18. Hydraulic heads show large variationB
Zheng et al., 2005

19. Conceptual model dry => wet season wet => dry season
BGS and DPHE, 2001

20. …but hydraulic gradients are still small
G5/CW G3 F5 F3 E5 E3 C5 C4
BP6
BP3
AP6
AP2
Banana
Bridge -4 -3 -2 -1 1 2
hydraulic head relative to local river (m a.s.l.)
A B C E F G
river
Lower heads in depper aquifer indicates downward flow, shallow aquifer drains into local stream

21. Shallow Depth Profiles

23. 4 types of samples: Precipitation (A) Surface waters (B)
Groundwater (C)
Rice fields C)

28. Precipitation (mm/month)
-12
-10 -8 -6 -4 -2 2 4 6 8
Sep-03
Dec-03
Mar-04
Jun-04
Sep-04
Dec-04
Mar-05
date
d18O (‰) 50
100
150
200
250
300
350
400
Precipitation (mm/month) 1 3
5 river
6 river 7 9 10
river
Precipitation

29. d18O over time

30. Time series, SW8 )

31. Cl- over time
150
100 50
150

32. Groundwater ‘dating’ with 3H and 3He1 10
100
1000
1950
1960
1970
1980
1990
2000
Shillong
Yangoon
Allahabad
Dhaka
Groundw. 3
H (TU)
year b
3H He
T1/2 = 12.43y
3H/3He age t:
3H, 3H+3He as dye
3H/3He as radioactive clock
Use of:

33. One field site as example
Residence time
10’s of y
1000’s of y
Site A, Zheng et al., 2005

34. Processes controlling He & Ne concentrations
He (ccSTP/g)
Ne (ccSTP/g) A B C E F G
ASW+air
Degassing during recharge
CE model
No degassing correction for 3H/3He
CE degassing and excess air correction for CFCs and SF6
excess air
Solubility
equilibrium
radiogenic He
degassing

35. Noble gas & water temperature4 6 8 10 12 14 16 18 20 21 22 23 24 25 26 27 28
Twater
TNG
depth (m)
Temperature (C)
shallow water temp very consistent: 26oC
lower NGT due to
O2 consumption?
increased pressure in unsaturated zone as a result of flooding?

36. Groundwater 3H+3He & 3H in precipitation
Stute et al., 2007
3H+3He%time2

37. 3H/3He age and [As]
Fairly uniform increase of As concentrations with 3H/3He age
Hydrology important factor controlling variability of As concentrations
Stute et al., 2007
As%age3

38. Two potential mechanisms:
As mobilization kinetics slow: groundwater accumulates As at a ~uniform rate
As mobilization kinetics fast: dissolved As equilibrates quickly with solid phase As, shorter groundwater residence time
higher degree of As removal
Lower dissolved As concentration

39. Laboratory studies Release rate (unamended): 90-110 mgL-1yr-1
Radloff et al, 2007

40. ‘dating’ with CFCs, SF6 and 3H/3He
Plummer et al., 2000

41. CFCs in groundwater CFCs are almost completely degraded expected
3H/3He recharge year
Horneman et al., in press

42. CFCs in soil air
Groundwater is likely sink for CFCs in unsaturated zone
Horneman et al., in press

43. CFC degradation rate in groundwater
0.1 1 10
100 4 8 12
Time (yr)
% CFC of expected
MW-A
MW-C
MW-F
CFC-12
Site
CFC-11 yr -1
CFC-12 A
~0.4 to ~0.5
~0.25 to ~0.3 C
~5.1 to ~6
~4.8 to ~5 F
~1.3 to ~1.5
~1.6
Degradation rate
=> Degradation: CFC-11 > CFC-12

44. SF6 in soil air => SF6 excess in local atmosphere
Horneman et al., in prep.

45. SF6 in groundwater Detectable SF6 in 3H- free groundwater
Horneman et al., in prep.

46. 3H/3He and SF6 ages
Horneman et al., in prep.

47. Potential explanations?
Partitioning into organic phases?
Exchange with
(old) bubbles?
Figure 7 Comparison of the noble gas corrected SF6 concentrations and 3H/3He calculated groundwater ages. Lines show expected SF6 for water in contact with atmosphere at 26°C as well as the effects of gas exchange with trapped (initially SF6 free) old air at different air-to-water ratios, retardation by a peat layer, and the cumulative effect of 0.27% and 2.83% organic carbon in sands and silts.
Horneman et al., in prep.

48. Options? Remediation options Existing wells Alternative sources
New wells Surface water
As removal Well switching Shallow wells Deep wells Pond water Rain collection
Safi
3-kolshi
Tube well sand filter
Maintenance
Monitoring
Bacterial growth
Spatial variability
Social resistance
Dug wells
Seasonality
Pathogens
$50 for 150 ft
Installation
Distribution
Pond sand filter
50 ea.
Bacteria 1/100
Aquaculture
Boiling
Rainwater harverster
1 $160/$40 ea.
Storage-seasonality