**1. Introduction**

Groundwater is by far the largest fresh water resource on the globe. It is often a safe drinking water source from bacteriological point of view, and the development of safe wells in many coun‐ tries has saved millions of children in particular. However, the development of deeper sources of groundwater has implied redox reactions that may mobilise elements that are toxic, such as arsenic and manganese. Other elements of health concern whose mobility is redox‐dependant

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

may be introduced from natural and/or anthropogenic sources such as chromium, uranium and nitrogen. This contribution will present the processes behind the mobility with examples from hydrogeology and hydrochemistry on a global scale.

To get a simple overview of the redox reactions in nature a so‐called aerobic/anaerobic stair case including only major redox‐sensitive species in groundwater could be used (**Figure 1**). In reality, the 'steps' are numerous if all elements, including trace elements are taken into account.

While pH is related to the activity of protons, Eh or redox potential is related to the activity of electrons (**Figure 2**). To define a specific redox level, Eh measurements with a platinum electrode and a reference electrode, commonly an Ag/AgCl electrode, are helpful but often difficult to interpret [1]. However, it is essentially the Fe(II)/Fe(III) couple that can be relied upon provided the concentrations of both species are not too small. Most redox measure‐ ments mirror a mixed potential [2]. Many species are not electrically active at the platinum electrode. ZoBell's solution is commonly used to check the electrodes [3]. The redox potential should preferably be measured in a so-called flow-through cell to avoid any air to come in contact with the groundwater. Speciation of the form of the elements is another tool that can be applied for instance for separating As(III) from As(VI). Numerous methods of speciation are published (e.g. [1, 4]).

In general, the species that are more mobile are anions as the cations in the form of most heavy metals are quite strongly adsorbed to clay minerals and organic matter at around neutral and alkaline pH while anions, adsorbed onto ferric oxyhydroxides and aluminium compounds, are less adsorbed at moderately alkaline conditions and not at all above zero points of charge (ZPC) (**Figure 3**) which is at a pH of 8.2 for ferric oxyhydroxides. Reducing conditions with the reduc‐ tion of ferric iron to soluble ferrous iron is another case of the failure of adsorption of anionic species.

**Figure 1.** The aerobic/anaerobic 'staircase' for major elements in groundwater.

**Figure 2.** The Eh‐pH diagram for arsenic with an overlay of iron.

**Figure 3.** Adsorption of cationic and anionic species.

#### **2. Arsenic**

may be introduced from natural and/or anthropogenic sources such as chromium, uranium and nitrogen. This contribution will present the processes behind the mobility with examples from

To get a simple overview of the redox reactions in nature a so‐called aerobic/anaerobic stair case including only major redox‐sensitive species in groundwater could be used (**Figure 1**). In reality, the 'steps' are numerous if all elements, including trace elements are taken into

While pH is related to the activity of protons, Eh or redox potential is related to the activity of electrons (**Figure 2**). To define a specific redox level, Eh measurements with a platinum electrode and a reference electrode, commonly an Ag/AgCl electrode, are helpful but often difficult to interpret [1]. However, it is essentially the Fe(II)/Fe(III) couple that can be relied upon provided the concentrations of both species are not too small. Most redox measure‐ ments mirror a mixed potential [2]. Many species are not electrically active at the platinum electrode. ZoBell's solution is commonly used to check the electrodes [3]. The redox potential should preferably be measured in a so-called flow-through cell to avoid any air to come in contact with the groundwater. Speciation of the form of the elements is another tool that can be applied for instance for separating As(III) from As(VI). Numerous methods of speciation

In general, the species that are more mobile are anions as the cations in the form of most heavy metals are quite strongly adsorbed to clay minerals and organic matter at around neutral and alkaline pH while anions, adsorbed onto ferric oxyhydroxides and aluminium compounds, are less adsorbed at moderately alkaline conditions and not at all above zero points of charge (ZPC) (**Figure 3**) which is at a pH of 8.2 for ferric oxyhydroxides. Reducing conditions with the reduc‐ tion of ferric iron to soluble ferrous iron is another case of the failure of adsorption of anionic

hydrogeology and hydrochemistry on a global scale.

226 Redox - Principles and Advanced Applications

**Figure 1.** The aerobic/anaerobic 'staircase' for major elements in groundwater.

account.

species.

are published (e.g. [1, 4]).

Arsenic is an element that has been known and used by mankind since the Bronze Age to make bronze stiffer. It has been introduced in society for different purposes such as for the removal of air bubbles in glass in medieval times. In the eighteenth century, it was used to cure different ailments. Its toxicity has been known for long time. A step forward in detection of low concentrations of arsenic was done by Berzelius, a Swedish chemist, who invented a qualitative but sensitive analysis [6]. Arsenic in groundwater has been known for quite some time; however, an extensive epidemiological investigation warranted the lowering of the safe level from 50 to 10 μg/l by WHO and that was adopted in most countries.

Arsenic is mobile in groundwater under two conditions, in a reducing environment as arse‐ nite (As(III)) and in an oxidising environment at elevated pH as arsenate As(V). The mobility is closely related to the chemistry of iron oxyhydroxides. Under ferric‐reducing conditions, the arsenite adsorbed onto the ferric compounds is released when the adsorbent is mobilised as soluble ferrous iron. Under oxidising conditions, the arsenate is mobilised at pH above 8.2, the ZPC for ferric oxyhydroxides.

In the 1990s, it was discovered that groundwater in many aquifers in south and southeast Asia had levels of arsenic that threatened the health of millions of people. In the Bengal delta in Bangladesh, 35–75 million people are exposed to excess arsenic depending on whether the 50 or the 10 μg/l level limit is used. Symptoms of arsenicosis were seen by a doctor in West Bengal in India and groundwater analysis showed high contents of arsenic [7]. In Bangladesh, the child mortality was high before the 1960s due to the use of bacteriologically polluted surface water. The switching over to cheap wells down to a depth of ~30–50 m meant a radi‐ cal decrease in mortality but after 10–15 years, the slow poisoning with arsenic from those wells became evident. The discovery evoked a discussion about the mechanisms behind the elevated levels of arsenic amounting even up to mg/l. An initial hypothesis was that the intro‐ duction of wells had lowered the groundwater level and allowed oxygen to diffuse into the sub‐ground level causing oxidation of arsenopyrite (**Figure 4**). However, a common feature of the polluted groundwater was high contents of dissolved iron and it turned out to be a completely internal process in the sediments where organic matter degraded under anaerobic conditions by bacteria using ferric iron as an oxidant dissolving ferric oxyhydroxides, releasing arsenic in the form of arsenite into the groundwater [8] (**Figure 4**). The arsenic content in the sedi‐ ments is moderately higher than elsewhere due to sources like rocks in the Himalayas [9]. This is, however, not the cause for the mobilisation of arsenic in the groundwater in the Bengal delta and the Gangetic plain; the redox level in the sediments is the reason.

Further work has shown that deeper wells are safe with a higher redox level than the shallower. This can be traced back to glacial times. The sediments deposited during Pleistocene, before the last glacial maximum (LGM) when the sea level was lowered at a geologically rather fast rate,

**Figure 4.** A hypothesis regarding the mobilisation of arsenic in the Bengal delta.

were deposited under rather oxidising conditions, and they were due to the lowering of the sea level subject erosion and resedimentation. Contrary, after the LGM when the sea level rose, there was more formation of wetlands allowing the introduction of more organic matter in the postglacial sediments.

of low concentrations of arsenic was done by Berzelius, a Swedish chemist, who invented a qualitative but sensitive analysis [6]. Arsenic in groundwater has been known for quite some time; however, an extensive epidemiological investigation warranted the lowering of the safe

Arsenic is mobile in groundwater under two conditions, in a reducing environment as arse‐ nite (As(III)) and in an oxidising environment at elevated pH as arsenate As(V). The mobility is closely related to the chemistry of iron oxyhydroxides. Under ferric‐reducing conditions, the arsenite adsorbed onto the ferric compounds is released when the adsorbent is mobilised as soluble ferrous iron. Under oxidising conditions, the arsenate is mobilised at pH above 8.2,

In the 1990s, it was discovered that groundwater in many aquifers in south and southeast Asia had levels of arsenic that threatened the health of millions of people. In the Bengal delta in Bangladesh, 35–75 million people are exposed to excess arsenic depending on whether the 50 or the 10 μg/l level limit is used. Symptoms of arsenicosis were seen by a doctor in West Bengal in India and groundwater analysis showed high contents of arsenic [7]. In Bangladesh, the child mortality was high before the 1960s due to the use of bacteriologically polluted surface water. The switching over to cheap wells down to a depth of ~30–50 m meant a radi‐ cal decrease in mortality but after 10–15 years, the slow poisoning with arsenic from those wells became evident. The discovery evoked a discussion about the mechanisms behind the elevated levels of arsenic amounting even up to mg/l. An initial hypothesis was that the intro‐ duction of wells had lowered the groundwater level and allowed oxygen to diffuse into the sub‐ground level causing oxidation of arsenopyrite (**Figure 4**). However, a common feature of the polluted groundwater was high contents of dissolved iron and it turned out to be a completely internal process in the sediments where organic matter degraded under anaerobic conditions by bacteria using ferric iron as an oxidant dissolving ferric oxyhydroxides, releasing arsenic in the form of arsenite into the groundwater [8] (**Figure 4**). The arsenic content in the sedi‐ ments is moderately higher than elsewhere due to sources like rocks in the Himalayas [9]. This is, however, not the cause for the mobilisation of arsenic in the groundwater in the Bengal delta

Further work has shown that deeper wells are safe with a higher redox level than the shallower. This can be traced back to glacial times. The sediments deposited during Pleistocene, before the last glacial maximum (LGM) when the sea level was lowered at a geologically rather fast rate,

level from 50 to 10 μg/l by WHO and that was adopted in most countries.

and the Gangetic plain; the redox level in the sediments is the reason.

**Figure 4.** A hypothesis regarding the mobilisation of arsenic in the Bengal delta.

the ZPC for ferric oxyhydroxides.

228 Redox - Principles and Advanced Applications

While the removal of arsenic from the groundwater functions technically, it does not work socially as women who are expected to handle the filters are too burdened by daily tasks [10, 11]. However, it has been found that safe sediments can be identified by their colour. This is a practice that was found out by local drillers in search for low‐iron groundwater. What they did not know was that low‐iron groundwater is also low in arsenic (**Figure 5**) [12]. The colour code tool has been developed from a large number of sediment samples paired with many groundwater analyses. The colour scheme that has been used is the Munsell Colour code [13]. As indicated above, the Pleistocene sediments at around 100 m depth were likely to be a good target. The colour code was tested on 243 wells drilled to around that depth [14]. The predic‐ tion of safe groundwater below 10 μg/l is 91%, while low manganese, below WHOs technical guideline at 400 μg/l, can be achieved in 89% of the cases [14].

Another colour tool mirroring the redox conditions in the groundwater is the colour of the platforms at hand pumps in the Bengal delta [15]. Red precipitates of ferric hydroxide indi‐ cate an iron‐reducing groundwater which has often, in the Bengal delta, elevated arsenic con‐ centrations. A black platform with precipitates of manganese oxides mirrors a higher redox level where the arsenic concentrations are lower.

Another mechanism of mobilisation of arsenic may occur under oxidising conditions at an elevated pH above the ZPC of the major adsorbents, ferric oxyhydroxides of different types.

**Figure 5.** A simplified colour code to identify low arsenic groundwater in the Bengal delta [12].

This is common in the Andes in South America [16]. The arsenic is then present as arsenate, and the high pH of groundwater tends to be of the Na‐HCO3 type [17].
