Arsenic contamination of Groundwater

water contamination, affects of arsenic on human health, reactivity of arsenic, sources of arsenic, natural and human induced sources of arsenic, arsenic bearing minerals, rocks containing arsenic, health affects of arsenic, redox and oxidation of arsenic

1. ARSENIC CONTAMINATION OF GROUNDWATER


HYDROGEOLOGY

2. ABSTRACT
Arsenic water contamination is a big problem in today’s time. Arsenic is present on earth naturally
and is added into ground water and stream both naturally and due to human activities also. Major
three types of rocks Igneous, Metamorphic and sedimentary all contain arsenic in different
proportion. Many individual minerals also contain arsenic. Upon certain conditions (redox and
oxidation) naturally occurring arsenic is freed and is dissolved into water. Many human activities
including mining, agricultural and industrial waste also adds arsenic into water. This water
contamination causes serious health issue and has affected millions of people all across the globe.

3. 1. ARSENIC CONTAMINATION
Arsenic contamination is groundwater pollution which is due to presence of naturally occurring
high concentrations of arsenic in groundwater. It has become a big issue due to the use of deep tube
wells for water supply for drinking, food etc., causing serious arsenic poisoning to large numbers
of people. In 2007, a study found that over 137 million people are affected by water contamination
in more than 70 countries. Mass poisoning of water in Bangladesh made everyone aware of the
issue of arsenic contamination. Arsenic contamination of ground water has affected countries all
across the globe, including Pakistan. 20 major incidents of groundwater arsenic contamination
have been reported uptil now, including 4 major in Thailand, Taiwan, Asia, and Mainland China.
1.1. Introduction
Arsenic is a natural component of the earth’s crust and it is widely distributed throughout the
environment in the air, water and land. It is highly toxic when in its inorganic form. People are
exposed to high levels of inorganic arsenic through drinking contaminated water, using
contaminated water in food preparation and irrigation of food crops, industrial processes, eating
contaminated food and smoking tobacco. Exposure to inorganic arsenic for too long, mainly
through drinking water and food, can lead to chronic arsenic poisoning. Skin lesions and skin
cancer are the most characteristic effects.
1.2. Exposure to Arsenic
The exposure of public to arsenic originates from using contaminated groundwater. Inorganic
arsenic is naturally present at high levels in the groundwater of a number of countries, including
Argentina, Bangladesh, Chile, China, India, Mexico, and the United States of America. Arsenic
contamination is higher in areas where there is geothermal activity. Drinking contaminated
water, crops irrigated with contaminated water and food prepared with contaminated water are
the sources of exposure. Fish, shellfish, meat, poultry, dairy products and cereals can also be
dietary sources of arsenic, although exposure from these foods is generally much lower
compared to exposure through contaminated groundwater. In seafood, arsenic is mainly found
in its less toxic organic form. Main and direct exposure of humans to arsenic is through drinking
contaminated water.
1.3. Effects of Arsenic

4. Arsenic appears to be essential for some plant and animal species. A possible safe dose for humans
as a dietary mineral is 15-25 μg. This amount could be absorbed from food without any trouble.
The total amount of arsenic in a human body is about 0.5-15 mg. Many arsenic compounds are
absorbed 60-90%, but they are also easily excreted. Humans can develop resistance to certain
arsenic concentrations. Shortly after absorption arsenic can be found in liver, spleen, lungs and
digestive tract. Most arsenic is excreted, and residues may be found in skin, hair, nails, legs and
teeth. Under conditions of prolonged exposure, many organs may be damaged, skin pigmentation
may occur, hair may fall out and nail growth may stop. Arsenic poisoning is a medical condition
that occurs due to elevated levels of arsenic in the body.
If arsenic exposure occurs over a brief period of time symptoms may include vomiting, abdominal
pain, encephalopathy, and watery diarrhea that contains blood, these are also called acute effects
of arsenic.
Long-term exposure can result in thickening of the skin, darker skin, abdominal pain,
diarrhea, heart disease, numbness, and cancer. The most common reason for long-term exposure
is contaminated drinking water.
Recommended levels in water are less than 10–50 µg/l (10–50 parts per billion). Other routes of
exposure include toxic waste sites and traditional medicines. Most cases of poisoning are
accidental. Arsenic acts by changing the functioning of around 200 enzymes. Diagnosis is by
testing the urine, blood, or hair. Prevention is by using water that does not contain high levels of
arsenic. This may be achieved by the use of special filters or using rainwater. There is not good
evidence to support specific treatments for long-term poisoning
2. ARSENIC BEARING MINERALS
Pure arsenic is found in lesser amounts, but most of it is found combined with the atoms of other
elements. Most arsenic is obtained as a byproduct when rock containing other metals is processed.
There are several common arsenic minerals.
2.1. Arsenopyrite
The main arsenic containing mineral is arsenopyrite (also called mispickel), in which arsenic is
combined with iron and sulfur. Arsenopyrite is found in high temperature hydrothermal veins, in
pegmatites, and in areas of contact metamorphism or metasomatism. Much of the arsenopyrite that

5. has been mined formed as a high-temperature mineral in hydrothermal veins. Arsenopyrite has
also been mined from sulfide deposits formed by contact metamorphism.
2.2. Orpiment
Orpiment is another arsenic mineral, in which arsenic is combined with sulfur. The name orpiment
comes from Latin meaning “gold pigment”. Orpiment is very poisonous, and it also discolored the
other pigments that artists used. Orpiment is found in volcanic fumaroles, low temperature
hydrothermal veins, and hot springs.
2.3. Realgar
Realgar, also known as "ruby Sulphur" or "ruby of arsenic". Realgar most commonly occurs as a
low-temperature hydrothermal vein mineral associated with other arsenic and antimony minerals.
It also occurs as volcanic sublimations and in hot spring deposits.
2.4. Cobaltite
Cobaltite is a sulfide mineral composed of cobalt, arsenic, and sulfur, Co As S. It contains up to
10% iron and variable amounts of nickel. It occurs in high-temperature hydrothermal deposits and
contact metamorphic rocks.
2.5. Proustite
Proustite is a sulfosalt mineral consisting of; silver sulfarsenide, Ag3AsS3, known also as light red
silver or ruby silver ore, and an important source of the metal. Proustite occurs in hydrothermal
deposits as a phase in the oxidized and supergene zone.
2.6. Tennantite
Tennantite is a copper arsenic sulfosalt mineral with an ideal formula Cu12As4S13. It is found in
hydrothermal veins and contact metamorphic deposits in association with other Cu–Pb–Zn–Ag
sulfides and sulfo-salts, pyrite, calcite, dolomite, siderite, barite, fluorite and quartz.
3. ARSENIC BEARING ROCKS
3.1.Earth’s crust
Few estimates exist for the concentration of arsenic in the earth’s crust. However, the concentration
is generally taken to be low with average arsenic concentration in the lithosphere as about 2 mg/kg.

6. 3.2. Igneous rocks
Arsenic concentrations in igneous rocks are generally similar to those found in the crust, with
average value of 1.5 mg/kg. Volcanic glasses are only slightly higher with an average of around
5.9 mg kg–1. Despite not having exceptional concentrations of Arsenic, volcanic rocks (especially
ashes) are often implicated in the generation of high Arsenic waters. This may relate to the reactive
nature of recent acidic volcanic material, especially fine-grained ash and its tendency to give rise
to sodium rich high-pH groundwater.
3.3.Metamorphic rocks
Arsenic concentrations in metamorphic rocks tend to reflect the concentrations in their igneous
and sedimentary precursors. Most contain around 5 mg/kg or less. Pelitic rocks (slates, phyllites)
typically have the highest concentrations with on average 18 mg/kg.
4. ARSENIC BEARING MINERALS IN SED. ROCKS
The concentration of Arsenic in sedimentary rocks is typically in the range 5–10 mg/kg, slightly
above average terrestrial abundance. Average sediments are enriched in Arsenic relative to igneous
rocks.
4.1. Sandstone
Sands and sandstones tend to have the lowest concentrations, reflecting the low Arsenic
concentrations of their dominant minerals, quartz and feldspars. Average sandstone Arsenic
concentrations are around 4 mg/kg.
4.2. Argillaceous
Argillaceous deposits have a broader range and higher average Arsenic concentrations than
sandstones, typically an average of around 13 mg/kg. The higher values reflect the larger
proportion of sulphide minerals, oxides, organic matter and clays. Black shales have Arsenic
concentrations typically at the high end of the range principally because of their enhanced pyrite
content. Marine argillaceous deposits have higher concentrations than non-marine deposits. This
may also be a reflection of the grain-size distributions, with potential for a higher proportion of
fine material in offshore pelagic sediments as well as systematic differences in sulphur and pyrite
contents. Marine shales tend to contain higher sulphur concentrations. Sediment provenance is also

7. a likely important factor. Particularly high Arsenic concentrations have been determined for shales
from mid-ocean settings. Atlantic Ridge gases may in this case be a high Arsenic source.
4.3. Organic Coals
Concentrations in coals and bituminous deposits are variable but often high. Samples collected of
organic-rich shale from Germany have Arsenic concentrations of 100–900 mg/kg. Some coal
samples have been found with extremely high concentrations up to 35,000 mg/kg but generally
low concentrations of 2.5–17 mg/kg are also reported.
4.4. Carbonates
Carbonate rocks typically have low concentrations, reflecting the low concentrations of the
constituent minerals. Some of the highest observed Arsenic concentrations, often several thousand
mg/kg, are found in ironstones and Fe-rich rocks. Phosphorites are also relatively enriched in
Arsenic (values up to ca. 400 mg/kg having been measured).
4.5. Unconsolidated Sediments
Concentrations of Arsenic in unconsolidated sediments are not notably different from those in their
indurated equivalents, muds and clays having typically higher concentrations than sands and
carbonates. Values are typically 3–10 mg/kg, depending on texture and mineralogy. Elevated
concentrations tend to reflect the amounts of pyrite or Fe-oxides present. Increases are also
typically found in mineralized areas. Placer deposits in streams can have very high concentrations
as a result of the abundance of sulphide minerals.
Average Arsenic concentrations for stream sediments are in the range 5– 8 mg/kg. Similar
concentrations have also been found in river sediments where groundwater-arsenic concentrations
are high: Datta and Subramanian (1997) found concentrations in sediments from the
 River Ganges averaging 2.0 mg kg–1 (range 1.2–2.6 mg/kg),
 Brahmaputra River averaging 2.8 mg kg–1 (range 1.4–5.9 mg/kg)
 Meghna River averaging 3.5 mg kg–1 (range 1.3–5.6 mg/kg).
Arsenic concentrations in lake sediments ranges between 0.9–44 mg/kg (median 5.5 mg/kg) with
highest concentrations present down-slope of mineralized areas. The upper baseline concentration
for these sediments is likely to be around 13 mg/kg.

8. Arsenic concentrations in glacial till ranges from 1.9–170 mg/kg (median 9.2 mg/kg) and highest
concentrations down-ice of mineralized areas. Relative arsenic enrichments have been observed
in reducing sediments in both near shore and continental-shelf deposits noted concentrations
increasing with depth (up to 30 cm) in continental shelf sediments as a result of the generation of
increasingly reducing conditions.
4.6. Soils
Baseline concentrations of Arsenic in soils are generally of the order of 5–10 mg/kg. Peats and
bog soils can have higher concentrations with average 13 mg/kg, principally because of increased
prevalence of sulphide mineral phases under the reduced conditions.
Acid sulphate soils which are generated by the oxidation of pyrite in sulphide-rich terrains such as
pyrite-rich shales, mineral veins and dewatered mangrove swamps can also be relatively enriched
in Arsenic. Arsenic concentrations up to 45 mg/kg has been noticed in the acid sulphate soils
derived from the weathering of pyrite-rich shales in Canada.
Although the dominant source of Arsenic in soils is geological, and hence dependent to some
extent on the concentration in the parent rock material, additional inputs may be derived locally
from industrial sources such as smelting and fossil-fuel combustion products and agricultural
sources such as pesticides and phosphate fertilizers.
4.7. Contaminated Surficial Deposits
Arsenic concentrations much higher than baseline values have been found in sediments and soils
contaminated by the products of mining activity, including mine tailings and effluent.
Concentrations in tailings piles and tailings-contaminated soils can reach up to several thousand
mg/kg. The high concentrations reflect not only increased abundance of primary arsenic rich
sulphide minerals, but also secondary iron arsenates and iron oxides formed as reaction products
of the original ore minerals. The primary sulphide minerals are susceptible to oxidation in the
tailings pile and the secondary minerals have varying solubility in oxidizing conditions in ground
waters and surface waters. Scorodite (FeAsO4.2H2O) is a common sulphide oxidation product
and its solubility is considered to control arsenic concentrations in such oxidizing sulphide
environments. Scorodite is metastable under most groundwater conditions and tends to dissolve
incongruently, forming iron oxides and releasing arsenic into solution. There is some confusion in

9. the analysis of these solubility relationships between congruent dissolution, incongruent
dissolution and sorption/desorption reactions. Secondary arsenolite (As2O3) is also relatively
soluble. Arsenic bound to iron oxides is relatively immobile, particularly under oxidizing
conditions.
5. SOURCES
The contamination of groundwater by naturally occurring Arsenic is much more serious, it is
urgently necessary to understand and investigate the major geochemical pathways involved in
the transformation and mobilization of Arsenic in aquifer sediments. The geochemistry of
Arsenic is a complex phenomenon found in the environment, and it generally is a function of
multiple oxidation states, speciation, and redox transformation. The contamination of a drinking
water source by arsenic can result from either natural or human activities. Arsenic is an element
that occurs naturally in rocks and soil, water, air, plants, animals and is used for a variety of
purposes within industry and agriculture. It is also a byproduct of copper smelting, mining, and
coal burning. Volcanic activity, the erosion of rocks and minerals, and forest fires are natural
sources that can release arsenic into the environment. Arsenic is used for wood preservative
purposes, paints, drugs, dyes, soaps, metals, semi-conductors and agricultural applications.
Arsenic can enter the water supply from natural deposits in the earth or from industrial and
agricultural pollution. It is widely believed that naturally occurring arsenic dissolves out of certain
rock formations when ground water levels drop significantly. Once released, arsenic remains in
the environment for a long time. Arsenic is removed from the air by rain, snow, and gradual
settling. Once on the ground or in surface water, arsenic can slowly enter ground water. High
arsenic levels in water wells may come from certain arsenic containing fertilizers used in the past
or industrial waste. It may also indicate improper well construction or overuse of chemical
fertilizers or herbicides in the past.
5.1. How Arsenic enters water
In the absence of oxygen, some bacteria living in deposited sediments can use arsenic and iron
oxide particles as an alternative means of respiration. The microbes separate the arsenic and iron
oxides and transfer the toxin into underlying groundwater.

10. Arsenic is present in many minerals, which further forms rocks and different formation. In
sedimentary rocks, matrix contain arsenic bearing minerals (e.g. Iron oxide and Pyrite) in them.
Arsenic can be released from iron oxides if groundwater has these characteristics:
 neutral to slightly alkaline pH (a pH of about 7 or slightly greater),
 “Reducing” redox conditions (indicated by low oxygen and high iron concentrations).
5.2. Speciation of Arsenic
Arsenic is unique among trace metalloids and oxyanion forming metals. It is very sensitive to
change in pH of water (6.5-8.5). It mostly exist as inorganic form in water as trivalent arsenite [As
(III)] or pentavalent arsenate [As (V)], although it can occur in (-3, 0, +3 and +5) forms. Organic
forms are formed by biological activity but are not quantitatively important, they can be significant
in industrial polluted area.
Arsenic may be solubilized when pH is high, arsenic can be released from surface binding sites
that lose their positive charge. When water level drops and sulfide minerals are exposed to air,
trapped arsenic in sulfide minerals can be released into water. In organic carbon presence in water,
bacteria are fed by directly reducing As (V) to As (III) or by reducing the element at the binding
site, releasing inorganic arsenic.
5.3. Mobilization and Redox Transformation of Arsenic
Natural geochemical and biologic processes play a vital role in controlling the mobilization and
transformation of Arsenic in the surface and subsurface environment. In groundwater,
mobilization of Arsenic takes place in the range of pH 6.5 to 8.5 under both oxidizing and
reducing conditions. Both abiotic and biotic processes favor the natural redox transformation
and mobilization of Arsenic in humid as well as arid environments.
5.4. Redox transformation of Arsenic in water
Under reducing conditions in natural environments, deltaic and alluvial sediments usually are
associated with the mobilization of Arsenic. Aquifer sediments, along with a layer of clay or silt,
may act as a cap, which effectively restricts the penetration of atmospheric oxygen to the aquifers,
thereby creating an anaerobic environment. Highly reducing conditions (anaerobic) also may be
developed in the presence of natural organic matter deposited with sediments. These highly

11. reducing conditions facilitate the release of Arsenic adsorbed on amorphous Fe oxides commonly
occurring in the aquifer sediments. Reductive dissolution of As-bearing amorphous Fe (III) oxides
plays a key role in the mobilization of Arsenic from aquifer sediments to ground waters. Dissolved
organic carbon (DOC) deposited in sediments and anaerobic metal-reducing bacteria also play a
vital role in Arsenic mobilization. The Arsenic concentration in ground waters is more in reducing
environment depends on several factors, including areal and vertical distributions of peat deposits,
the degradation of which is the major redox controller, the redox driver in the groundwater system,
groundwater movement, pH, HCO3
−
, Fe, Mn, and Al oxides, and DOC concentrations of
sediments.
5.4.1. Reductive Dissolution of Iron Oxide
Iron hydroxides Fe (OH)3 are one of the most common phases associated with aquifer sediments.
The desorption mechanisms of As from Fe (OH)3 are directly responsible for the existing high
concentrations of mobile As(III) in ground waters
5.5. Oxidation of Arsenite
The form of As (III) is thermodynamically unstable in aerobic conditions; hence, it oxidizes easily
to the less mobile form of As (V). However, this reaction is a slow process, when oxygen becomes
the only oxidant. The presence of some other redox-sensitive species, such as ferric iron [Fe (III)],
manganese oxides (MnO2), clay minerals, and some microorganisms, can intensively increase the
rate of As (III) oxidation converting into the less toxic As (V) form. These oxidation reactions are
highly favorable in arid and semiarid areas as a result of extensive mineral reactions and
evaporation. In the presence of Fe (III) in aqueous solutions, the rate of As (III) oxidation is
accelerated below pH 7. Manganese oxides commonly associated with aquifer solids have been
recognized as an important oxidant to oxidize As (III) to As (V), and this reaction is
thermodynamically feasible over a wide range of pH values.
5.5.1. Oxidation of Sulfide Minerals
The primary source of Arsenic in the environment is the oxidation of Arsenic sulfides, such as
FeAsS and FeS2 minerals. Oxidation of As-bearing sulfides is recognized as an important cause
of Arsenic contamination of groundwater due to production of acid drainage containing toxic

12. inorganic pollutants at higher levels. In oxygen-rich waters, Arsenic is released predominantly
through pyrite and FeAsS minerals
5.5.2. Arsenic Release from Pyrite
FeS2 oxidation takes place via several reaction pathways; the first step involves the chemical
oxidation of FeS2 in the presence of dissolved oxygen (DO), resulting in a certain amount of
Arsenic in ground waters.
5.5.3. Arsenic Release from Arsenopyrite
Dissolution of FeAsS also is of environmental concern and therefore has received much attention
in recent years. FeAsS can be oxidized by Fe (III), a process more than 10 times faster than
oxidation of FeS2
6. NATURAL BARRIERS FOR ARSENIC MOBILIZATION IN DISSOLVED
PHASE
6.1. Geochemical Processes in Controlling Arsenic Mobility and Transformation
Adsorption is the most prominent geochemical process that controls the mobility and transport of
Arsenic in ground waters. Adsorption reactions between Arsenic and mineral surfaces generally
are the most critical phenomena in controlling the dissolved concentrations of Arsenic in ground
waters. Adsorption of Arsenic depends on several external factors, such as solid surface, pH, Eh,
concentration of Arsenic and competing ions, and Arsenic speciation. Redox-active oxide surfaces
of iron, aluminum, and manganese minerals are potentially the most important sources of Arsenic
in aquifer sediments
6.2. Arsenic Adsorption on Iron Oxides
The solubility of Arsenic in ground waters is strongly influenced by adsorption at the Fe (OH)3
surfaces, which exist as discrete particles or as coatings on other mineral surfaces. Both As (V)
and As (III) may be adsorbed and co-precipitated on Fe (OH)3, depending on several important
factors, including pH, the amount of Fe (OH)3 deposited, and the concentration of other competing
ions found in the medium. The adsorption of Arsenic onto Fe (OH)3 occurs mainly at the
oxic/anoxic boundary, referred to as ferrihydrite (Fe2O3·2H2O), which has a large specific area
resulting in an increased adsorption capacity.

13. 6.3. Arsenic Adsorption on Aluminum Oxides
Oxides and hydroxides of Al also have significant adsorption capacity for Arsenic. Because the Al
(III) atom has the same charge and a nearly identical radius as the Fe (III) atom, the common
hydrous Al oxide phases are structurally similar to hydrous ferric oxide. As(V) species such as
CH3AsO(OH)2 and (CH3)2AsOOH can be absorbed by amorphous Al(OH)3, crystalline
Al(OH)3 (gibbsite), α-Al2O3, and β-Al2O3 up to 7.0 pH, and adsorption decreases significantly
at higher pH values, whereas As(III) adsorption increases with increasing pH.
6.4. Arsenic Adsorption on Manganese Oxide
The oxidation of As (III) to As (V) by manganese oxide is an important process in the natural
cycling of Arsenic, as well as in reducing the concentration of dissolved As (III) in groundwater
systems. As (III) is oxidized by MnO2 followed by the adsorption of the As (V) reaction product
on the MnO2 solid phase. The As (V)–MnO2 complex likely is a bidentate binuclear corner-
sharing complex occurring at MnO2 crystallite edges and interlayer domains [100, 125].
Manganese oxides may act as an electron acceptor in the oxidation process and therefore can
adsorb significant amounts of As.
6.5. Arsenic Adsorption on Clay Minerals
Adsorption and oxidation/reduction reactions of Arsenic at clay mineral surfaces also play a crucial
role in the natural attenuation and transformation of Arsenic in groundwater systems.
Aluminosilicate clay minerals are composed of alternating layers of silica oxide and aluminum
oxide, providing several types of binding sites to adsorb a variety of metal ions. The OH groups
associated with Al ions bound on the surface of clay particles act as proton acceptors, forming
anionic species of Arsenic.

14. 7. SUMMARY
Arsenic contamination is a serious problem in this modern world. World population is increasing
and providing them clean water for drinking is becoming more difficult. We have to better
understand how ground water is contaminated by the arsenic naturally and how humans add up to
the contamination. With better understanding we can reduce, if not completely prevent, the water
contamination.

15. 8. References
Source and behavior of arsenic in natural waters Pauline L Smedley and David G
Kinniburgh British Geological Survey, Wallingford, Oxon OX10 8BB, U.K.
Mobilization of arsenic by dissolved organic matter from iron oxides, soils and sediments
by Markus Bauer T, Christian Blodau
https://link.springer.com/article/10.1007/s40726-016-0028-2
https://en.wikipedia.org/wiki/Arsenic_contamination_of_groundwater
https://www.lenntech.com/periodic/water/arsenic/arsenic-and-water.htm
https://www.cdc.gov/healthywater/drinking/private/wells/disease/arsenic.html