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Fecal contamination of shallow tubewells in Bangladesh inversely related to arsenic.

van Geen A, Ahmed KM, Akita Y, Alam MJ, Culligan PJ, Emch M, Escamilla V, Feighery J, Ferguson AS, Knappett P, Layton AC, Mailloux BJ, McKay LD, Mey JL, Serre ML, Streatfield PK, Wu J, Yunus M - Environ. Sci. Technol. (2011)

Bottom Line: In this study, 125 tubewells 6-36 m deep were sampled in duplicate for 18 months to quantify the presence of the fecal indicator Escherichia coli.On any given month, E. coli was detected at levels exceeding 1 most probable number per 100 mL in 19-64% of all shallow tubewells, with a higher proportion typically following periods of heavy rainfall.The frequency of E. coli detection averaged over a year was found to increase with population surrounding a well and decrease with the As content of a well, most likely because of downward transport of E. coli associated with local recharge.

View Article: PubMed Central - PubMed

Affiliation: Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA. avangeen@ldeo.columbia.edu

ABSTRACT
The health risks of As exposure due to the installation of millions of shallow tubewells in the Bengal Basin are known, but fecal contamination of shallow aquifers has not systematically been examined. This could be a source of concern in densely populated areas with poor sanitation because the hydraulic travel time from surface water bodies to shallow wells that are low in As was previously shown to be considerably shorter than for shallow wells that are high in As. In this study, 125 tubewells 6-36 m deep were sampled in duplicate for 18 months to quantify the presence of the fecal indicator Escherichia coli. On any given month, E. coli was detected at levels exceeding 1 most probable number per 100 mL in 19-64% of all shallow tubewells, with a higher proportion typically following periods of heavy rainfall. The frequency of E. coli detection averaged over a year was found to increase with population surrounding a well and decrease with the As content of a well, most likely because of downward transport of E. coli associated with local recharge. The health implications of higher fecal contamination of shallow tubewells, to which millions of households in Bangladesh have switched in order to reduce their exposure to As, need to be evaluated.

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Related in: MedlinePlus

Time series of data from the two study areas. (a) Variations in water levels measured in Char Para (one 10 m deep well and a pond, both referenced to the land elevation near the well) and inside the embankment in Matlab (one pond inside the embankment and two 10 m deep wells inside and outside the embankment, respectively; all referenced relative to the land elevation at the well monitored inside the embankment). Also shown is weekly precipitation measured in Matlab, with data gaps filled using adjusted rainfall data from Chittagong. (b) Monthly variations in the fraction of shallow (6−36 m) wells with detectable E. coli in the two study areas separated in three categories according to their As content. Also shown are 95% CIs for the lowest and highest category of As concentrations; the confidence intervals for the intermediate category are comparable but not shown for clarity. The time of sampling was shifted by a few days between different categories, also for clarity.
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fig2: Time series of data from the two study areas. (a) Variations in water levels measured in Char Para (one 10 m deep well and a pond, both referenced to the land elevation near the well) and inside the embankment in Matlab (one pond inside the embankment and two 10 m deep wells inside and outside the embankment, respectively; all referenced relative to the land elevation at the well monitored inside the embankment). Also shown is weekly precipitation measured in Matlab, with data gaps filled using adjusted rainfall data from Chittagong. (b) Monthly variations in the fraction of shallow (6−36 m) wells with detectable E. coli in the two study areas separated in three categories according to their As content. Also shown are 95% CIs for the lowest and highest category of As concentrations; the confidence intervals for the intermediate category are comparable but not shown for clarity. The time of sampling was shifted by a few days between different categories, also for clarity.

Mentions: The precipitation pattern in 2008 and 2009 was typical of the monsoonal climate of the region, with heavy rainfall during the months of April through October and little if any rain during the remaining months (Figure 2a). In response, water levels in ponds monitored inside the embankment in Matlab and Char Para varied by ∼1 m between the dry and wet season. Relatively deep ponds were selected for monitoring; many ponds dry out entirely during winter. The seasonal range in groundwater levels measured within the embankment in Matlab is somewhat wider (∼1.5 m), more so outside the embankment (2.5 m) and even more so in Char Para (4 m). The difference between surface and groundwater levels is a roughly constant 0.5 m downward within the embankment in Matlab over much of the year (downward head gradient of 0.05 given the 10 m depth of the monitoring well), with the exception of March and April when groundwater and pond levels were essentially the same (Figure 2a). The limited pond data available from Char Para indicate a more pronounced difference of 2 m between the pond and groundwater (downward gradient of 0.2) in June 2009 and no significant gradient later in August through October.


Fecal contamination of shallow tubewells in Bangladesh inversely related to arsenic.

van Geen A, Ahmed KM, Akita Y, Alam MJ, Culligan PJ, Emch M, Escamilla V, Feighery J, Ferguson AS, Knappett P, Layton AC, Mailloux BJ, McKay LD, Mey JL, Serre ML, Streatfield PK, Wu J, Yunus M - Environ. Sci. Technol. (2011)

Time series of data from the two study areas. (a) Variations in water levels measured in Char Para (one 10 m deep well and a pond, both referenced to the land elevation near the well) and inside the embankment in Matlab (one pond inside the embankment and two 10 m deep wells inside and outside the embankment, respectively; all referenced relative to the land elevation at the well monitored inside the embankment). Also shown is weekly precipitation measured in Matlab, with data gaps filled using adjusted rainfall data from Chittagong. (b) Monthly variations in the fraction of shallow (6−36 m) wells with detectable E. coli in the two study areas separated in three categories according to their As content. Also shown are 95% CIs for the lowest and highest category of As concentrations; the confidence intervals for the intermediate category are comparable but not shown for clarity. The time of sampling was shifted by a few days between different categories, also for clarity.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3037737&req=5

fig2: Time series of data from the two study areas. (a) Variations in water levels measured in Char Para (one 10 m deep well and a pond, both referenced to the land elevation near the well) and inside the embankment in Matlab (one pond inside the embankment and two 10 m deep wells inside and outside the embankment, respectively; all referenced relative to the land elevation at the well monitored inside the embankment). Also shown is weekly precipitation measured in Matlab, with data gaps filled using adjusted rainfall data from Chittagong. (b) Monthly variations in the fraction of shallow (6−36 m) wells with detectable E. coli in the two study areas separated in three categories according to their As content. Also shown are 95% CIs for the lowest and highest category of As concentrations; the confidence intervals for the intermediate category are comparable but not shown for clarity. The time of sampling was shifted by a few days between different categories, also for clarity.
Mentions: The precipitation pattern in 2008 and 2009 was typical of the monsoonal climate of the region, with heavy rainfall during the months of April through October and little if any rain during the remaining months (Figure 2a). In response, water levels in ponds monitored inside the embankment in Matlab and Char Para varied by ∼1 m between the dry and wet season. Relatively deep ponds were selected for monitoring; many ponds dry out entirely during winter. The seasonal range in groundwater levels measured within the embankment in Matlab is somewhat wider (∼1.5 m), more so outside the embankment (2.5 m) and even more so in Char Para (4 m). The difference between surface and groundwater levels is a roughly constant 0.5 m downward within the embankment in Matlab over much of the year (downward head gradient of 0.05 given the 10 m depth of the monitoring well), with the exception of March and April when groundwater and pond levels were essentially the same (Figure 2a). The limited pond data available from Char Para indicate a more pronounced difference of 2 m between the pond and groundwater (downward gradient of 0.2) in June 2009 and no significant gradient later in August through October.

Bottom Line: In this study, 125 tubewells 6-36 m deep were sampled in duplicate for 18 months to quantify the presence of the fecal indicator Escherichia coli.On any given month, E. coli was detected at levels exceeding 1 most probable number per 100 mL in 19-64% of all shallow tubewells, with a higher proportion typically following periods of heavy rainfall.The frequency of E. coli detection averaged over a year was found to increase with population surrounding a well and decrease with the As content of a well, most likely because of downward transport of E. coli associated with local recharge.

View Article: PubMed Central - PubMed

Affiliation: Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA. avangeen@ldeo.columbia.edu

ABSTRACT
The health risks of As exposure due to the installation of millions of shallow tubewells in the Bengal Basin are known, but fecal contamination of shallow aquifers has not systematically been examined. This could be a source of concern in densely populated areas with poor sanitation because the hydraulic travel time from surface water bodies to shallow wells that are low in As was previously shown to be considerably shorter than for shallow wells that are high in As. In this study, 125 tubewells 6-36 m deep were sampled in duplicate for 18 months to quantify the presence of the fecal indicator Escherichia coli. On any given month, E. coli was detected at levels exceeding 1 most probable number per 100 mL in 19-64% of all shallow tubewells, with a higher proportion typically following periods of heavy rainfall. The frequency of E. coli detection averaged over a year was found to increase with population surrounding a well and decrease with the As content of a well, most likely because of downward transport of E. coli associated with local recharge. The health implications of higher fecal contamination of shallow tubewells, to which millions of households in Bangladesh have switched in order to reduce their exposure to As, need to be evaluated.

Show MeSH
Related in: MedlinePlus