the sediments which creates reduced conditions in which iron‐bearing minerals (siderite/marcasite) has higher solubility causing enriching of the dissolved iron in the groundwater (Applin and Zhao 1989).
The dissolution of Fe in groundwater strongly depends on the concentration of dissolved oxygen and also on the pH of the water to a lesser extent. In groundwater, it generally occurs in two forms as Fe2+ and as Fe3+. Iron occurs as Fe3+ when the concentration of dissolved oxygen in groundwater is greater than 1–2 mg/L and as Fe2+ when dissolved oxygen is in low concentration. At normal pH of water, Fe2+ is soluble, and Fe3+ is insoluble. At low pH and dissolved oxygen, iron (also manganese) will dissolve readily in groundwater. Aquifers having higher depth and rich in organic matter typically contain less dissolved oxygen. Decomposition of the organic matter leads to depletion of the oxygen in the water, which results in the dissolution of the iron as Fe2+. In the oxygen‐deprived water, when pumped to the surface, a rust‐coloured iron mineral forms due to the reaction of dissolved iron and the oxygen (CGWB 2014).
In India, groundwater from a large number of areas is highly contaminated with iron. These areas include west Bengal, Rajasthan, Orissa, Goa, Haryana, Jammu and Kashmir, Andhra Pradesh, Karnataka, and Kerala (Garduño et al. 2011). Higher Fe constituent in the coastal aquifers may be due to the interface of oxidized Fe minerals and organic matter and subsequent dissolution of Fe2CO3 at a moderately lower pH.
The brown colour of groundwater after extraction from the aquifers is the primary sign of iron presence. The high concentration of Fe in water causes awful taste, staining, turbidity, and equipment problem in the water supply.
1.6.1.1 Health Impact
Permissible limit of Fe in drinking water prescribed by WHO (2011) and BIS (2012) is 300 μg/L. The use of contaminated groundwater having iron concentration above the permissible limit is highly objectionable as it causes health disorders. The reported health disorders due to Fe contaminated groundwater are disorders of the skin, digestive, respiratory and nervous system, kidney, spinal cord, and heart, as well as mental imbalance, miscarriage, and cancer (Achary 2014b). Deficiency of iron causes anaemia, whereas prolonged consumption of drinking water with a high concentration of iron may lead to a liver disease called hemosiderosis. The water with high iron concentration may seem brownish because of the precipitation of ferric hydroxide and taste astringent. The USEPA (US Environmental Protection Agency) maintains that though drinking water having iron may be consumed safely, iron‐bearing sediments may comprise trace impurities or harbour bacteria that might be damaging. Iron has nutritional value for human beings as it plays an important role in the formation of haemoglobin protein, enzymes, and also used in cellular metabolism. Lesser storage of iron in the body causes the iron deficit, anaemia, fatigue, and affects the immune system. Iron deficiency in children negatively disturbs mental growth, resulting in irritability and concentration ailment. Chronic consumption of surplus amounts of iron results in an ailment termed iron overload, which occurs due to gene mutation. Iron overload, if untreated, can lead to haemochromatosis, a severe illness that could harm the body's organs. Early symptoms of haemochromatosis are fatigue, weight loss, and joint pain; when not cured appropriately it can cause heart disease, liver complications, and diabetes (Garduño et al. 2011; CGWB 2014; Duggal et al. 2017).
1.6.2 Manganese
Manganese is the 12th most abundant element in the environment, and it is a crucial component of plants and animals. It has a strong positive correlation with iron, and its chemistry is also somewhat like iron as both metals take part in redox reactions in weathering conditions. Manganese substitutes the iron, magnesium, or calcium in silicate structures as it is not a crucial component of any of the common silicate rock minerals. Manganese exists in divalent form in several igneous and metamorphic minerals as a small component, but it is an essential component of basalt. It has occurred in minerals like olivine, pyroxene, and amphibole and a minor amount generally occurred in dolomite and limestone as a substitute for calcium (CGWB 2014).
In soil, manganese arises from mineral weathering and atmospheric deposition instigating from both natural and anthropogenic sources. In soil solution, only the divalent ion form is stable, whereas Mn (III) and Mn (IV) are steady in the solid phase of soil. Soil parameters like acidity, wetness, organic matter content, biological activity, etc., affect the mobility of manganese. The solubility of manganese in the soil is controlled by redox potential and soil pH. Its mobility rises at lower pH or lower redox potential because these favour the reduction of insoluble manganese oxides. Manganese forms bonds with organic matter, oxides, and silicates, which lead to a decrease in solubility at soil pH above 6. Hence high pH and higher organic matter content usually lower the availability and solubility of manganese, whereas in acidic soils with low organic matter, it is readily available. The anaerobic conditions (at pH above 6) and aerobic conditions pH below 5.5 both resulted in an increase of solubility of manganese (CGWB 2014).
The existence of different minerals at the aquifer influence the levels of Mn found in groundwater from natural leaching processes. It usually occurs in deeper wells where the water normally interacts with the rock for an extended time. In groundwater, manganese often occurs together with iron as it originated from ferromagnesian, but its concentration is usually lower than iron. Though it is crucial for humans and other living beings, at higher concentration, it is lethal. The higher concentration of manganese is stated primarily from West Bengal, Tamil Nadu, Orissa, UP, and Bihar (Garduño et al. 2011).
1.6.2.1 Health Impact
The permissible limit of Mn in drinking water given by WHO (2011) and BIS (2012) is 300 and 100 μg/L, respectively. Inhalation or contact with a higher concentration of Mn can cause damage to the central nervous system (Singh et al. 2011). It can readily accumulate in the brain, particularly in the basal ganglia, and can result in an irretrievable neurological syndrome like Parkinson's disease. Comparatively higher doses of manganese affect DNA replication and lead to mutations in the microorganism and mammalian cells. In mammalian cells, manganese causes DNA impairment and chromosome abnormalities. The higher concentration of it affect fertility in mammals and are deadly to the embryo and foetus (Singh et al. 2011).
1.6.3 Chromium
Chromium mostly occurs in trivalent and hexavalent forms depending on pH. In shallow aquifers, Cr (VI) is the dominant and toxic form, and its major species include chromate CrO4−2 and dichromate Cr2O7−2 (especially Ba2+, Pb+2, and Ag+) (Hashim et al. 2011). At low pH (<4), Cr (III) is the dominant form of Cr. In soil organic matter, S2− and Fe2+ ions under anaerobic conditions lead to the reduction of Cr (VI) to Cr (III) (Hashim et al. 2011). The Cr (VI) has higher mobility and toxicity than Cr (III), and it is more problematic to eliminate Cr (VI) in water. In water, chromium (III) is typically present as the free‐ion state, but it is also related to hydroxide ions depending on pH and forming chromium (OH)2+, chromium (OH)2+, chromium (OH) 3, and chromium (OH)4−. The major Cr species found in aqueous solution are SO42−, NH4+, and CN; these are formed by complexes of Cr3+ with organic and inorganic ligands. The equilibrium of Cr among the two oxidation forms has relied on oxidation kinetics which further depends on biochemical settings, such as pH, redox, and nutrient levels that direct microbial activity (CGWB 2014).
Chromium has been found in diverse parts of India. Its source has been generally attributed to anthropogenic inputs. However, chromite deposits in the Sukinda area of Orissa are identified as geogenic sources. In the Sukinda area, the lateralization progress including oxidation and change of the serpentines generates alkaline pore water which enables the
