April, 2003
Selective Analysis of Tubewell Arsenic Screening Data from 36
Upazillas (BAMWSP Phase-II)
( Draft Prepared by M. Khaliquzzaman, Environment Team, WBOD)
Executive Summary
Data archived at the National Arsenic Mitigation Information Centre (NAMIC) and collected
during the screening of tubewells in 36 Upazillas for Arsenic concentration have been
selectively analyzed to look for trends. It is seen that the nature of Arsenic contamination looks
quite different when viewed at different depths. The availability of Arsenic safe aquifers at
different geographical areas also look quite interesting. Using proxy variables for analysis, it
appears that water abstraction from the aquifers has not affected the aquifer arsenic
concentration up to now. Attempts have been made to relate prevalence of arsenicosis to
arsenic ingestion using a proxy parameter like average arsenic concentration but significant
trends have not been found. It appears that the observations from such analysis may be useful
for cost effective mitigation work and for better understanding of some of the aspect of the
Arsenic problem in the groundwater. It is necessary to maintain a quality assured national
archiving and data management system to this end.
1.
Introduction
The approach in this report is purely empirical. Data obtained during the Arsenic screening
program for 36 Upazillas contain many more parameters than Arsenic concentrations in
tubewells. These information have been analyzed to look for systematic trends. Consistency
of such trends with current understanding of the Arsenic problem has not been specifically
considered. However, certain definitions and issues have been discussed here for making the
report self standing for the readers who are not so technically inclined and some discussions
have been done on the different topics in terms of these terminologies.
Below the zone of aeration extending from surface to only a few meters below the ground,
the soil in Bangladesh is saturated with water. Saturation means that all pore spaces in the
soil is filled with water and this zone is defined by a groundwater table. Bangladesh soils
consist of unconsolidated sediments and the pore spaces are simply the openings between the grains.
Lithologic drill logs show layers of gravel, sands of different grades, silt and clay down to
great depths in succession.
Hydrogeologists classify soil layers as to their ability to yield water to wells or springs. A
layer which is permeable enough to supply water to wells or springs is referred to as an
"Aquifer", while an “Aquiclude” is impermeable and an “Aquitard” tends to be very poorly
permeable. The aquifers are sand or gravel layers that may be a few centimeters to many
meters’ thick. Like other similar areas in the world, the sand intervals which constitute the
aquifers are probably lens-shaped with varying degree of lateral and vertical
interconnectedness. This interconnectedness usually decreases with depth. The extent of the
of aquifers may vary from a few km2 to many thousands of km2 . So, a large aquifers may
easily contain a trillion liters or even more water. Even a small aquifer would probably
contain a couple of billion liters.
1
An aquifer is referred to as confined when it is bounded by aquicludes or aquitards that
impede flow into it. The primary source of recharge to the aquifers is assumed to be historic
runoff from the rainfalls. Groundwater is in principle renewable but in certain cases the
period needed for replenishment (100s to 1000s of years) is very long in relation to the
normal time-frame of human activity. For this reason, it is valid in such cases to talk of the
utilization of non-renewable groundwater or the ‘mining of aquifer reserves’. Water in a
confined aquifers can be literally thousands of years old. This is the reason for concern about
the aquifer systems and their specific susceptibilities to negative impacts under abstraction
stress. For water balance studies three and four aquifer models have been shown to be
adequate. However, in reality in many regions of Bangladesh more aquifers can be found
stacked on top of one another where from water can be extracted. The hydrogeology of
Bangladesh area has been studied for more than fifty years and the details are available
elsewhere[1-4].
Groundwater is a vital natural resource for the reliable and economic provision of potable
water supply in both the urban and rural environment. It thus plays a fundamental role in
human well-being. Until the emergence of the Arsenic problem, groundwater was hailed for
providing the access to clean drinking water in Bangladesh. In the screening process
tubewells have been found all depths up to 1000 ft or even more in a few cases. The relevant
questions here to ask are: (i) at what depths good Arsenic safe aquifers are available at
different geographic locations; (ii) if the tubewells currently safe will remain so in the future;
(iii) do the concentration of Arsenic vary with time in the aquifers or depend on extraction;
(iv) how Arsenic concentrations are related to prevalence of the arsenicosis symptoms etc.
These and similar issues have been looked into in this report on the basis data obtained from
the 36 Upzillas’ screening.
2.
Geographical depth distribution of Arsenic concentrations
Maps showing geographic distribution of Arsenic concentrations from different surveys are
now available. However, most maps do not show if there are any variations depending on the
depth of the aquifers. In the absence of complete understanding of Arsenic mobilization in
the aquifer systems, we have to rely on the empirical knowledge on the availability of arsenic
safe water in the aquifers. In fact such availability can be considered as proxy for our lack of
knowledge of the given aquifer. During the survey, the depth information for all the
tubewells were collected from the owners. As written records were not available in many
cases, the depth values have come from people’s memory. The reliability of the memory
decreases with lapse of time, so for older tubewells the depth data may be less reliable than
the recent ones. Again depths of 1000ft or more have been reported for tubewells dating back
to sixties and seventies. It is not clear why people would spend money to sink tubewells to
such depths when water could be found at shallower depths. So, there may be either
recording, data entry or plain reporting errors in some such numbers. The 36 Upazilla
screening were done using Merck kit which probably can be relied upon at 100ppb level. So,
in preparing depth segregated maps, tubewells have been considered safe at this level. The
depth segregated maps of 36
2
Fig. 1 – Depth segregated Map showing relative number of arsenic-safe (green) and
contaminated aquifers (red) for 36 Upazillas. Red colour indicates contaminated fraction.
3
Upazillas for Arsenic safe and contaminated wells are shown in figure 1. It can be seen that
arsenic-safe water can be found at all depths depending on the area. Aquifers at depth
between 50-250 ft are most frequently contaminated and aquifers are least contaminated
between 500-1000ft. It is interesting to note that contamination frequency appear to change
for worse beyond 1000ft. This could be related to the inaccuracy of depth data as pointed out
earlier. It is clear that empirical knowledge obtained in the screening program can be used in
a cost effective manner to provide mitigation measure in contaminated areas by sinking
tubewells at least depths where safe water is available.
3.
Statistics of Safe and Contaminated tubewells
In the previous section, we have looked at the depth distribution of Arsenic concentrations at
different locations in a qualitative and visual way. We want look at the issue in a more
quantitative way in this section. Selected cases are considered here. The graphical data for all
the Upazillas are given in annex-1 (to be included).
3.1 Low contamination areas
% of tubewells
Three typical cases of statistical distribution of safe and contaminated tubewells in 3 low
contamination Upazillas (~20% or less) are shown the figure-2 below.
120
100
80
60
40
20
0
1
2
3
4
5
6
7
8
(a) Alamdanga (20.3%)
120
100
80
60
40
20
0
1
2
3
4
5
6
7
8
(b) Rajhat(3.2%)
4
120
100
80
60
40
20
0
1
2
3
4
5
6
7
8
(c) Kulaura(3.9%)
Fig-2. Percentage of safe and contaminated tubewells in three low contamination areas as
function of depths. (--- Safe, --- Contaminated) Depth(ft) Groups: 1. <=15, 2. >15 to <50, 3.
>=50 to <150, 4. >=150 to <250, 5. >=250 to <500, 6. >=500 to <750, 7. >=750 to <1000, 8.
>=1000
It can be seen from these cases that the percentage of safe and contaminated wells do not
appear to depend on depth. It is known that misclassification of tubewells can arise due to
limitation of kits used in the analysis [5]. In the case of Rajhat and Kulaura, the percentages
of contaminated wells at all depths are less than 10%. Such level of contamination in a
survey especially using Merck kit could arise from misclassification. Even in the case of
Alamdanga where percentage reaches around 20%, the depth distribution appears random.
Such situation could arise from analytical skill related problems. So, in areas where pattern of
contaminations as in fig-2 are observed, careful laboratory based quality assurance checks
should be carried out. It is likely the areas showing this type of pattern may be entirely
uncontaminated.
It very surprising to note that of the about 38,000 total tubewells in Rajhat more than 15,000
have been sunk to depth of 1000ft or more although screening shows it to be low
contamination area of average 3.2% contamination level and the probability of getting
Arsenic safe water at lower depth is very high. In the absence of other water quality problem,
this shows that considerable saving in cost could be done by using shallower aquifers if they
are present.
3.2 High contamination areas
Four cases of statistical distribution of safe and contaminated tubewells in highly
contaminated Upazillas (70% or more) are shown the figure-3 below.
100
90
80
70
60
50
40
30
20
10
0
1
2
3
4
5
6
7
8
(a) Faridganj (93.6)
5
120
100
80
60
40
20
0
1
2
3
4
5
6
7
6
7
8
(b) Raipur (79%)
120
100
80
60
40
20
0
1
2
3
4
5
8
(c) Tungipara (70.3%)
100
90
80
70
60
50
40
30
20
10
0
1
2
3
4
5
6
7
8
(d) Daudkandi (77.3%)
Fig-3. Percentage of safe and contaminated tubewells in some high contamination areas as
function of depths. (--- Safe, --- Contaminated) Depth(ft) Groups: 1. <=15, 2. >15 to <50, 3.
>=50 to <150, 4. >=150 to <250, 5. >=250 to <500, 6. >=500 to <750, 7. >=750 to <1000, 8.
>=1000
Although the curves look similar in three out of 4 cases shown here, the depths where safe
water is available are quite different. Safe water is available at depth greater than 750ft at
Faridganj. This depth is 500ft for Raipur and 250ft for Tungipara. It appears that for
Daudkandi safe water with good probability is available only at the 250-500ft depth
window. So, here again are the examples were screening information can be used for cost
effective mitigation. Rather that wholesale sinking of tubewells at 1000ft or more depths
6
(which is being reportedly done now in many areas ), if data obtained in the screening
process is used in an opportunistic and rational manner considerable cost saving can be done.
4. Arsenic concentration variation with time
One of the questions which is of concern to many is whether the tubewells currently safe will
remain so in the future or the concentration of Arsenic will change with time in the aquifers
for worse due to stress imposed by abstractions. In order to obtain answer to such questions
data have to be collected over extended period of time. Delay for such observation are not
possible as answers are needed immediately. We have to find suitable proxy parameters
which can reasonably replace the time parameter. One such proxy could be the age of the
tubewells. As tubewells were sunk without specifically taking into consideration the aquifer
properties, the average concentration of Arsenic in tubewells sunk at any given period should
be statistically the same as any other period. So, any change in the concentration of Arsenic
with tubewell age should represent its abstraction induced effect.
Average concentrations for Arsenic in tubewells grouped by age were calculated for all the
tubewells and also for the depth segregated groups. The results are shown in figure-4.
Concentration Vs Year of Installation at Different Depth
Mean Concentration (Mg/L)
0.2
0.15
0.1
0.05
1996 to
2001
1991 to
1995
1981 to
1990
1961 to
1980
1941 to
1960
1921 to
1940
1900 to
1920
0
Year of Installation
All Depth
<50
50-150
251-500
501-750
751-1000
151-250
Figure 4- Variation of average concentrations of Arsenic for all tubewells and depth
segregated groups according to age.
7
It can be seen that for all the groups except >1000ft, the concentrations do not appreciably
change with tubewell age. There appears to a slight decrease with time but this may not be
very significant as these mostly relate to tubewells dating back to sixties. As pointed out
earlier data for these tubewells may not be all that accurate. In the case of tubewells of depth
greater than 1000ft, there may be even greater problem with data as discussed earlier and this
is probably reflected in the increase in the Arsenic concentration for the tubewells between
the period 1940-60.
5.
Prevalence of Arsenicosis Symptoms
Prevalence of arsenicosis data were collected during screening. The normalized prevalence
rate per 100,000 population has been plotted as function of average Arsenic concentration
for all the Upazillas in figure-5.
Patients vs Mean Concentration
Patients/100,000
350.0
300.0
250.0
200.0
150.0
100.0
50.0
0.0
0.000
0.100
0.200
0.300
0.400
0.500
0.600
Concentration (mg/l)
Figure-5 Prevalence rate for patients per 100,000 population as function of average
concentration of Arsenic in tubewell water for 36Upazillas.
The linear relationship between cumulative dose and prevalence rate is expected. If average
concentration can be taken as proxy for cumulative dose, this linear relationship should be
expected for the graph in fig-6 also. Although there appears to be a general increase, the
scatter is too great to show any definitive correlation. For average concentration to represent
the cumulative dose, the underlying assumption is that age distribution of tubewells is the
8
same in all the areas. In order to assess the impact of length of exposure the prevalence rates
for phase-I and II screening have been compared this is shown in table 1.
Table 1: Arsenicosis prevalence data from screening -Phase I & II
Phase
Upazi
llas
I
6
II
36
Total
42
Tube wells
Total
Contaminated(%)
Population Patients
80,390
48.19
1,698,410
1,139
544,651
46.49
8,674,202
9,188
625,041
46.71
10,372,61
10,327
Patients
Per
100,000
67
106
Comments
Merck Kits
used
Hach Kits
used
100
As the degree of contamination in both the phases are rather similar and the measurements are
shifted in time (by ~ 2 years), the rate of increase in the observed prevalence of patients may be
statistically significant. It appears that the prevalence rate for patients has increased
approximately at the rate of 20 per hundred thousand per year.
Attempt was therefore made to look at the age distribution tube wells in different regions. Two
selected age distributions are shown here (fig-6). The distributions do not looks any different. It
is therefore not understandable why there is such poor correlation between concentration and the
prevalence rate.
Singair:TW Count
3000
2500
2000
1500
1000
500
19
50
19
53
19
56
19
60
19
63
19
66
19
69
19
72
19
75
19
78
19
81
19
84
19
87
19
90
19
93
19
96
19
99
0
9
Kalaroa: TW Count
2000
1800
1600
1400
1200
1000
800
600
400
200
98
01
20
19
92
95
19
19
89
19
83
86
19
19
77
80
19
19
71
74
19
19
65
68
19
19
61
19
55
19
19
50
0
Figure-6: Age distribution of tubewells in two randomly selected Upazillas
6.
Discussions
Selective analyses of the data from NAMIC show that a considerable insight into the Arsenic
problem can be obtained from such data which is basically a byproduct of the BAMWSP
screening program and obtained at minimal incremental cost. Such data and their analyses can
supplement and in some cases may even substitute data that can be obtained through very
expensive research and over a long period of time. Present analysis has shown that the nature
of geographic distribution of Arsenic contamination looks quite different when viewed at
different depths. The availability of Arsenic safe aquifers at different geographical areas also
look quite interesting. Using proxy variables it appears that water abstraction from the aquifers
has not affected the aquifer arsenic concentration up to now. A general raising trend for the
prevalence of arsenicosis has been observed with higher arsenic ingestion represented by the
proxy parameter of average arsenic concentration but significant correlations have not been
found. It appears that observations of such analysis may be useful for cost effective mitigation
work and for better understanding of some of the aspect of the Arsenic problem in the ground
water.
It is to be noted that all the analyses in this report were performed by the staff members of
(NAMIC). The data base available at NAMIC and the further data from the on going larger
screening program (147Upazillas) will be invaluable for improving the cost effectiveness of
the future mitigation programs. It is, therefore, necessary to ensure that the archived data and
management system of NAMIC be maintained beyond the tenure of BAMWSP.
10
7. References
1. D. G. Kinniburgh and P. N. Smedley, Arsenic contamination of groundwater in
Bangladesh, BGS Technical Report WC/00/19, Vol. 1-4 (2001).
2. UNDP., Groundwater Survey: The Hydrogeological Conditions of Bangladesh, UNDP
Technical Report DP/UN/BGD-74-009/1(1982)
3.
J.A. Barker and R. Herbert, The pilot study into optimum well design: IDA 4000
Deep Tubewell II Project, Vol-4: Well and Aquifer modeling: Part 2, BGS Technical Report
WD/89/11(1989).
4.
P. K. Aggarwal, P.K. Basu, R.J. Poreda, K.M. Kulkarni, K. Froehlich, S.A. Tarafdar,
M. Ali, N. Ahmed, A. Hussain, M. Rahman, S. R. Ahmed, Isotope Hydrology of
Groundwater in Bangladesh: Implications for Characterization and Mitigation of Arsenic in
Groundwater, Report IAEA – TC Project: BGD/8/016 (2000)
5. M. Khaliquzzaman , Quality Assurance Sampling Plan for Tubewell Monitoring with
Hach Kit, Unpublished Report, Environment Team, WBOD (October, 2002)
11