arsenic pollution groundwater geology.pdf

arsenic pollution

1. Groundwater Arsenic Pollution
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2. Geochemistry of Groundwater Arsenic
Basic information
of Arsenic:
Geologicalenvironments: Igneous, Metamorphic and Sedimentary
rocks have
arsenic. Arsenic is a semi-metal,odorless and tasteless.
Crustal abundance: 1.8
ppm, ranging
from 0.1 to several hundred ppm.
Major source of anthropogenic
arsenic mobilization: Gold mine waste deposits often
associated with arsenopyrite.
Natural waters, arsenic is mostly found in inorganic form as oxyanions of trivalent
arsenite [As(1)] or pentavalent arsenate [As(V)]. several oxidation states (-3, 0, +3,
+5). As(l) is more toxic (40-60 times) than As(V).
As(111) exists in most natural water as As(OH); and is more mobile than As(V
because it is less strongly absorbed on most mineral surfaces than the negatively
chargedAs(V) oxyanions. Arsenateis strongly absorbed by Fe0OH and precipitated.
Arsenopyrite
Orpiment
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3. Concentrations in solids: mass per mass
1 part per
million (ppm) is 1 milligram
in 1 kilogram i.e. 1 mg/kg
or 1 microgram
in 1
gram i.e. 1 4g/g
Scientist sometimes write this as 1
ug g'
A part per billion
(ppb)
is 1
ug kg'
Concentrations in liquids: mass per volume
1 part per million (ppm) is 1 milligram in 1 liter i.e. 1 mg/L
or 1 microgram
in 1 mL i.e. 1 ug/mL
Scientist sometimes write this as 1 ug mL1
A part per billion (ppb) is 1 ug L'
1000 ppb=1 ppm
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4. What makes Arsenic Unique as a Contaminant?
Most toxic trace metals occur in solution as cations (e.g. Pb*, Ni**,
Cd2) which become insoluble as the pH increases.
As a result, arsenic anions can persist in solution at relatively
high concentrations even at near-neutral pH values.
Arsenic is problematic in the environment due to its relative
mobility over a wide range of natural processes,such as:
Weathering reactions
Biological activity
Volcanic emissions
Range of anthropogenic activities
Arsenic
is also
distinct by becoming relatively mobile under
reduced conditions.
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5. Causes of Arsenic Contamination
Pyrite Oxidation Hypothesis
Arsenic is assumed to
be
present in
certain sulphide minerals (pyrites) that are
deposited within
the
aquifer sediments. Due
to the lowering
of water table
below deposits, arseno-pyrite oxidized in the vadose zone releases arsenic as
arsenic adsorbed on iron hydroxide. During the subsequence recharge period,
iron hydroxide releases arsenic into groundwater.
Oxy-hydroxide Reduction Hypothesis
Arsenic
is assumed to be present in alluvial sediments with high concentrations
in sand grains as a coating of iron hydroxide. The sediments were deposited in
valleys
eroded in the delta when the stream base 1evel was lowered due to the
drop in sea level during the last glacial advance. The organic matter deposited
with the sediments reduces the arsenic bearing iron hydroxide and releases
arsenic into groundwater. According to this hypothesis, the origin of arsenic rich
groundwater is due to a natural process, and it seems that the arsenic in
groundwater
has been present
for thousands of
years
without being flushed from
the delta.
Competitive exchange with phosphorus
Phosphate (PO,*)
Dissolved As and P exchange for one another
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6. Oxidation of Arsenopyrite:
4 FeAsS +13 0,
+6
H,OI 4FeSO,
+
4H,AsO,
Add Oxygen and water -Lower the water table in an
Arsenopyrite-rich aquifer
-Arsenic will be mobilized
As (III)
Reduction
pH
Oxidation
As (V)
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7. Overview of As Release
Rain Water
Rain
0
0
000
00 a 0 0
.Vadose Zone (unsaturated)
_Vadose Zone
Phreatic Zone(saturated)
Aerobic organisms consume
O
************
Phreatic Zone
Anaerobic microbes reduce
FeOOH Microbial
Activity Releases Fe and As
Dissolved As levels
FeOOH
As
Biological activity
Adsorption reactions
o
Adsorption
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8. Groundwater Arsenic Pollution in Holocene Sediments in Fluvial
Floodplain and Deltaic Environments in India, Bangladesh and Nepal
Terai Belt of
Nepal is
Arsenic
INDEX
QUATERNARY SEDIMENTS |1-Ganga-
Brahmaputra
MEGA FANS OF
AHIMALAYAN RIVERS
CALCUTTA/YPaa
INDIABANGLA2-Bengal
DESH Basin
DELHI
affected in
ARSENIC AFFECTED
AREASI SPOT LOCATIONS groundwater
BAY OF
HIMALAYAN
BENGAL A-Allahabad
V-Varanasi
BX-Buxar
B-Balia
C-Chhapra
P-Patna
BG-Bhagalpur
500 km
NEPALB
RANGE
GHAGR
GANGA
UNA
DELH
BRAHMAPUTR
cHAM BAL
BxGANGA BG
DAMODAR 2
CALCUTTA
SON
NAR MADA
BENGAL BASIN
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Arsenic in drinking water WHO=10 ug/l;
BIS
=
10 ug/l; PHED supplying
< 50 ug/l

9. Sources of Arsenic
As-rich sulphide deposits
in
the Himalayas erode and
As-rich
Sediment is continuously deposited in the Ganges Delta.
Mobilization in the well water.
Oxidation of As rich pyrite with lowering of water table and
Oxygenation of ground water. Fe2*
precipitates as Fe-oxyhydroxides scavenging As.
oxidizes to Fe3* and
Dissolution
of
Fes" oxyhydroxides with the
release of As from
the surface in reduced conditions in the aquifer.
Phosphate, from
seawater flooding area, can replace
As
in
absorption sites
on Fe-oxyhydroxides and in sulphides. As
released in aquifer.
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10. Bengal Basin Arsenic
The sedimentation in the Ganga- Bramputra- Meghna delta was
strongly influenced by sea-level changes during Late
Pleistocene-Holocene. Arsenic contamination is confined to
parts of the Younger Delta Plain (Holocene), whereas, the Older
Delta Plains are unaffected in Bengal Basin.
Bengal Basin & other alluvial aquifer possibly triggered by
excessive extraction of GW & enhancing movement of GW that
contain dissolved organic product which promote reduction of
hydrated iron oxide(HFO) by anaerobic heterotropicFe(II)-
reducing bacteria with release of As.
Biotite-chlorite, other ferro-mag minerals, magnetite supplied As
in solution or sorbed in HFO that occur dispersed as coating in
hah sediment grain.

11. Health effects of (chronic) arsenic exposure
Cardiovascular disease
Skin lesions (few years of exposure)
Cancers of the skin, lung, liver, and bladder
decades of exposure)
(several
Arsenic poisoning is different from
acute poisoning, and results from a
chronic and long term exposure to
arsenic which produces
over time.
symptomns
The main symptoms of arsenic
exposure include stomach pain,
diarrhea, vomiting, nausea, black spot
in hands and feet,
discoloration of the skin,
thickening and
partial
hah paralysis,and blindness.

12. Arsenic removal methods
Alum Method
-
This cheap
method allows 70% removal of arsenic. 300-500
grams of alum wrapped in a clean cloth is sinked in a bucket
full of
arsenic contaminated water
for 12 hours. Upper
2/3rd
of the water is separated using decantationor two layered
clean cloth. Lower
third of water contains arsenic and not
suitable for drinking.
Ferric oxide-manganese dioxide clay based filter columns
-
This earthen column packed with the above substances is
attached to the tube well outlet. The method is claimed to
remove significant amount of arsenic at the water flow rate of
90-110 mL/minute and
the system
can filter up
to 5000 L of
water betore disposal.
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