Page 291 - Hydrogeology Principles and Practice

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Table 1 Frequency distribution of arsenic concentrations in Even so, across much of southern Bangladesh, more than 50% of
groundwater from a regional survey of the 41 worst affected boreholes in the shallow aquifer have arsenic levels that comply
−1
districts of Bangladesh. After DPHE (1999). with the 50 mgL limit and so continued development of the allu-
vial aquifers may still be possible, at least in the medium term
Arsenic Frequency Percentage Percentage (Burgess et al. 2002; Ravenscroft et al. 2004).
concentration frequency in or above The problem of arsenic in groundwater is not only confined
−1
class*(mgL ) concentration to Bangladesh and West Bengal. According to Smedley and
class Kinniburgh (2002), the areas with large-scale problems of arsenic
in groundwater tend to be found in two types of environment:
<10 998 49 100 inland or closed basins in arid and semi-arid areas; and strongly
10–50 319 16 51 chemically reducing alluvial aquifers. The hydrogeological situation
50–100 209 10 35 in these areas is such that the aquifers are poorly flushed and
100–250 268 13 25 any arsenic released from the sediments following burial tends to
250–500 168 8 11 accumulate in the groundwater.
500–1000 57 3 3 Areas containing high-arsenic groundwaters are well known in
>1000 3 0.1 0.1 Argentina, Chile, Mexico, China and Hungary, but the problems
in Bangladesh, West Bengal and, additionally, Vietnam are more
−1
* WHO recommended limit = 10 mgL ; Bangladesh regulatory recent (Smedley & Kinniburgh 2002). In Vietnam, the capital Hanoi
−1
limit = 50 mgL . is situated at the upper end of the Red River Delta and analysis
of raw groundwater pumped from the lower Quaternary alluvial
aquifer gave arsenic concentrations of 240–320 mgL −1 in three of
the city’s eight treatment plants and 37–82 mgL −1 in another five
plants (Berg et al. 2001). In surrounding rural districts, high arsenic
are presently drinking water containing more than 50 mgL −1 of concentrations found in tubewells in the upper aquifers (48%
arsenic, while probably more than double this number are drinking above 50 mgL −1 and 20% above 150 mgL ) indicate that several
−1
water containing more than 10 mgL −1 of arsenic (DPHE 2000; million people consuming untreated groundwater might be at a
Nickson et al. 2000; Burgess et al. 2002). The number of persons high risk of chronic arsenic poisoning (Berg et al. 2001).
who must be considered ‘at risk’ of arsenic poisoning is even higher As in Bangladesh, the source of arsenic in the Red River Delta
because testing of the 5–10 million tubewells in Bangladesh will sediments is believed to be associated with iron oxy-hydroxides that
take years to complete. release arsenic to groundwater under chemically reducing condi-
The cause of the elevated arsenic concentrations in the
Ganges–Meghna–Brahmaputra deltaic plain is thought to relate tions. A characteristic feature of arsenic contamination of wells in
both Bangladesh and Vietnam is the large degree of spatial vari-
to the microbial reduction of iron oxy-hydroxides contained in the ability in arsenic concentrations at a local scale. As a result, it is
fine-grained Holocene sediments and the release of the adsorbed difficult to know when to take action to provide arsenic-free water
load of arsenic to groundwater. It has been proposed that the reduc- sources. For now, it appears safer to analyse each well until further
tion is driven by microbial metabolism of buried peat deposits research has been completed into the sources, controls and distribu-
(McArthur et al. 2001; Ravenscroft et al. 2001). The presence of tion of arsenic in susceptible areas.
abundant organic matter is expected in deltaic or fluvial areas that
supported peat formation during climatic optimums.
Severely polluted aquifers are all of Holocene age, although at
a local scale the distribution of arsenic pollution is very patchy.
There are cases of grossly polluted boreholes, pumping groundwa- Table 2 Distribution of arsenic concentrations in groundwater by
ter with arsenic at concentrations greater than 1000 mgL −1 being well depth from a regional survey of the 41 worst affected districts
separated spatially by only a few tens of metres from boreholes of Bangladesh. After DPHE (1999).
pumping groundwater with arsenic at concentrations less than 10
−1
mgL (Burgess et al. 2002). However, there are many cases where Well depth Number of Number of Percentage of
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almost all wells in a village contain more than 50 mgL . (m) samples samples above samples above
−1
−1
Data presented by DPHE (1999) show that the highest per- 50 mgL 50 mgL
centage of wells that contain arsenic concentrations above the
regulatory limits of 10 and 50 mgL −1 occur at depths between 28 <10 36 12 33
and 45 m (Table 2). Hand-dug wells are mostly less than 5 m deep 10–30 582 339 58
and are usually unpolluted by arsenic, but the risk of bacterio- 30–100 1032 334 32
logical contamination is high. Below 45 m there is a decrease in 100–200 92 18 20
the percentage of wells that are polluted, but the risk remains >200 280 2 0.7
significant until well depths exceed 150 m, the maximum depth of Total 2022 705 35
river channel incision during the Last Glacial Maximum at 18 ka.