1. Importance of Evaluating
Phosphate Levels in Tubewells
in High Arsenic Areas of Asia
Tom Mahin1,2, Tommy Ngai2, Susan Murcott1, Mohon Mondal3
1 - Massachusetts Institute of Technology (MIT)
2 - Centre for Affordable Water & Sanitation Technology (CAWST)
3 - LEDARS – Bangladesh
Presented at 33rd WEDC International Conference Accra, Ghana

2. Arsenic in Asia
From: World Bank “Study: Arsenic Contamination of
Groundwater in South and East Asian Countries” 2005

3. Impact of Phosphates on
Arsenic Removal by Adsorption Systems
Numerous studies have shown that
phosphates (PO4) have a significant impact
on arsenic removals by some of the most
common and cost effective arsenic treatment
(iron-based) systems.
Because PO4 has a similar chemical structure
to arsenate, they compete with arsenic for
adsorption sites on iron oxides.
PO4 are relatively common in tubewells in
high arsenic areas of Asia.

4. Origin of Phosphates in Ground Water
Phosphates are believed to result from the
decomposition of organic matter. Also similar
to the release of arsenic, phosphates can be
released from sediments in aquifers by:
(1) dissolution of iron oxides under anoxic
conditions (absence of dissolved oxygen),
(2) desorption of phosphates from iron
oxides iron (such as high pH conditions).
Fertilizers & latrines can also contribute
phosphates to shallow groundwaters.

5. Phosphates Often are Elevated When
Arsenic is at High Levels (red line is 50 ug/L)
Pakistan
India (West Bengal)
Above from Nickson et al., Applied Geochemistry Above from McArthur et al., Applied Geochemistry
China - Inner Mongolia
Bangladesh
From KTH Sweden by Fei Shi (2004) From KTH – Sweden by Jonsson & Lundell

6. Bangladesh Phosphate
Results (DPHE/BGS)
• 0.3 mg/L P (median)
for 3,530 samples
• But when As > 10 ppb
P averaged 1.3 mg/L
(median - 1 mg/L)

7. West Bengal
(north of Barasat in southern West Bengal)
PO4 ranged from
(1–6.5 mg/L).
“
From: McArthur et al., Natural organic matter in sedimentary basins and its
relation to arsenic in anoxic ground water: the example of West Bengal and its
worldwide implications” Applied Geochemistry 19 (2004) 1255–1293

8. China - Inner Mongolia (Huhhot Basin)
> 50 ppb As Wells Have Elevated Phosphates
50 ug/L - As
Wells > 100 m ( ) have higher arsenic, phosphate & pH levels
From : “Mobilisation of arsenic and other trace elements in fluviolacustrine aquifers of the
Huhhot Basin, Inner Mongolia” Smedley et al, Appied Geochemistry (18) 2003

9. Example of How Phosphate, Iron & Arsenic Levels
Vary by Individual Wells (Bangladesh - Chandpur)
For 2 different wells and iron levels are very similar
but phosphates differ by a factor of 10
From: Targeting safe aquifers in regions with arsenic-rich groundwater in Bangladesh
Case study in Matlab Upazila - Jonsson and Lundell (2004) Swedish University of

10. Iron to Phosphate Ratios
The ratio of iron to phosphates (Fe/PO4-P)
is important because higher iron levels
increase and higher phosphates reduce
arsenic % removals. So higher Fe/PO4-P
ratios are associated with > arsenic %
removals.
Phosphate results are reported as either
PO4-P or P, both are the same. However
results reported as PO4 are about 3 times
higher than as P.
.

11. Red River Delta Vietnam
Parameter Average Median
(n=74)
Arsenic* 115 ug/L 98 ug/L
(range 9-382 ug/L)
PO4-P* 0.75 mg/L 0.56 mg/L
Fe* 13. 7 mg/L 13 mg/L
Fe/PO4-P ratio 18
(avg./avg.)
Conclusion: PO4-P levels high but Fe also high, sand
filters can remove arsenic for some wells in Vietnam
* Data from: Berg et al. “Arsenic Removal from Groundwater by Household Sand Filters –
Comparative Field Study, Model Calculations, and Health Benefits”– E, S & T

12. High Arsenic Area of Cambodia (Kandal Province)
PO4-P (mg/L)* As (ug/L)* Fe (mg/L)*
Average - 0.66 Average – 233 Average – 2.8
Range:<0.2–3.14 Range:1 -1340 Range<0.05-16
Calculated Average Fe/PO4-P Ratio 4
Conclusion – Generally high
PO4-P levels & moderate Fe
levels, supplemental iron
needs to be used not just filters
* - Data from Swiss Institute of Aquatic Science and
Technology, Eawag, as published in Environ. Sci. Technol.

13. The Impact of Phosphates on % Removal
of Arsenic by Sand Filters
Pink circle = very high phosphates (P > 2.5 mg/L)
.
* Note as iron levels increase, the
% As removals increase even with
very high phosphates
Adapted from graph by Berg et al., “Household Sand filters for
Arsenic Removal – Technical Report” – EAWAG 2004

14. Fe/PO4-P Ratios and % As Removal
Arsenic As - % As Iron PO4-P Fe/PO4-P
(As) well filtered removed (well) mg/L ratio
ug/L water mg/L
Vietnam
223 21 91% 11 0.05 220
137 49 64% 11 2.8 4
117 45 62% 10 2.6 4
70 9 87% 7 0.2 34
55 44 20% 6 3.7 2
Bangladesh
8 + iron 3 without nails
171 25 85% with nails from nails
2.3
17 with nails
Raw Data from: Berg et al. “Arsenic Removal from Groundwater by Household Sand Filters – Comparative Field
Study, Model Calculations, and Health Benefits”– E S & T, Ratios by Mahin

15. Conclusions
Even where PO4 levels are generally
lower, individual wells may have high PO4.
Fe/PO4 ratios should be considered when
evaluating treatment performance results &
when preparing sampling plans.
MIT/CAWST/LEDARS pilot tested Kanchan
Arsenic Filter (KAF) in areas of Bangladesh
with high arsenic/phosphate levels. The KAF
uses nails that rust adding iron & resulting in
high Fe/PO4 & higher As removals.