Author: 1 Anshumali*: Assistant Professor 2 Parijat Tripathi: Senior Research Fellow Affiliation:

1. Author: 1Anshumali*: Assistant Professor 2Parijat Tripathi: Senior Research Fellow Affiliation: 1Department of Environmental Science and Engineering, Indian School of Mines, Dhanbad, Jharkhand, India-826004. E-mail: malijnu@gmail.com Telephone: +91-326-2235645 Fax: +91-326-2210028 2School of Environmental Sciences, Jawaharlal Nehru University, New Delhi- 110067, India

2. INDIAN SCHOOL OF MINES http://www.ismdhanbad.ac.in

3. Arsenic Contamination: History Groundwater contamination by Arsenic (As) has become a global concern in recent years. Most estimates of As pollution have focused on the predominance of As poisoning in the groundwaters of West Bengal (India) and Bangladesh (Ahmed et al., 2004; Ben et al., 2003). But recent studies in Middle Gangetic Plain revealed significant As contamination in the groundwaters (Tripathi et al., 2008).

4. Arsenic Concentrations The current drinking water quality guideline for As is 10 μg/l (WHO, 1993). The current standard for arsenic in drinking water in both Bangladesh and India is 50 µg/l. The average As content of the Earth’s crust is 1.8 ppm and is most abundant in shales (Mason, 1966). Based on the data compilation from West Bengal and Bangladesh, DPHE (1999) report that the average total arsenic contents of fluvio-deltaic sediments is 15.9 ppm for onshore samples and 10.3 ppm for offshore samples (Ravenscroft et al., 2005). Datta and Subramanian (1994) reported average As contents of riverbed samples to be 2.03 ppm in the Ganges, 2.79 ppm in the Brahmaputra, and 3.49 ppm in the Meghna.

5. Arsenic Enrichment in Groundwater Several authors suggested that the reductive dissolution of Fe (III)-oxyhydroxides in strongly reducing conditions of the young alluvial sediments is the cause for As mobilization (Ahmed et al., 2004, Bhattacharya et al., 1997; Harvey et al., 2002; McArthuret al., 2001; Nickson et al., 1998; Nickson et al., 2000). The reduction is driven by microbial degradation of sedimentary organic matter, which is a redox-dependent process consuming dissolved-O2 and NO3 (Stumm and Morgan, 1981; Nickson et al., 2000).

6. Study Area Map showing sampling locations in Ballia District, Eastern Uttar Pradesh, India

7. Hydrogeology Schematic model shows major As-contaminated zone in Newer Alluvium (T0-Surface) entrenched channels and floodplains and As-free zone in Older Alluvium upland terraces (T2-Surface) in Middle Ganga Plain (UP–Bihar). Newer Alluvium older floodplain (T1-Surface) are locally As-contaminated (Singh, 2004; Shah, 2008).

8. Geochemistry of Groundwater

9. Piper diagram of the Ballia groundwater All the groundwater samples lie within Ca-Na-HCO3, Ca-Mg-HCO3, Ca-Mg-Cl-HCO3, and Ca-Na-SO4-Clfacies indicating cyclic freshening/salinization phenomena operating in the groundwater systems of the study area.

10. Arsenic Enrichment The depth distribution of As shows a strong correlation between occurrence of As in groundwater and the depth of wells. In general, the highest As concentration and greatest spatial and temporal variability, occur 10-20 m below the ground surface and decreases rapidly below about 40 meters. Similar observations are reported by Karim et al. (1997) where a maximum As concentration was observed at a depth of 20 to 40 m with an As concentration declines below that.

11. As vs HCO3- Strong redox conditions are required to drive As mobility with depth (McArthuret al., 2001). Between 10 to 20 m well depths, the high concentration of As is associated with low concentration of HCO3- . The low concentration of HCO3- between 10 to 20 m depth represents the most important anion species, which competes with arsenic for adsorption sites at mineral surfaces (e.g., Fe/Mn oxyhydroxides, clay minerals, and weathered mica), consequently releasing As into the groundwater (Charlet et. al., 2007).

12. As vs Fe Moreover, the reduction of arseniferous iron oxyhydroxides may be coupled to precipitation of Fe2+ as FeCO3 (Sracek et al., 1998; Welch and Lico, 1998) when anoxic conditions develop during microbial oxidation of sedimentary organic matter (Bhattacharya et. al., 1997) and may be responsible for poor relationship between arsenic and iron.

13. As to NO3- This assessment is supported by poor correlation between As and NO3- (Fig. 12), which reveals that the apart from dissolved O2, NO3- is thermodynamically favored electron acceptor for microbial degradation of dissolved organic materials (Appelo and Postma, 1993; Drever, 1997) in the shallow aquifers (HP) of Ballia district. Biomediated reductive dissolution of hydrated iron oxide (HFO) by anaerobic heterotrophic Fe3+ reducing bacteria (IRB) play an important role to release sorbed As to groundwater (Nickson et al. 1998; Lovely and Chapelle 1995; Islam et al. 2004).

14. Geogenic vs Anthropogenic Pre-monsoon Post-monsoon The parallel behaviour of Cl-,As and SO4- - displaying decreasing content in reducing environment (increasing depth). but the most important observation is that what is the source of Cl- in the aquifers of Middle Gangetic Plain, which is almost equal to the concentrations present in the volcanic ground waters.

15. Factor analysis reveals an association of As with Cl- indicating the anthropogenic contributions, mainly through fertilizer and herbicide application in the agriculture fields. Moreover, Ravenscroft et al. (2001) studied the limiting depth of active groundwater circulation with anthropogenic influence on Cl- and As concentrations, which may be controlled by the subdued topography and occurrence of a silty clay layer in shallow wells in Gangetic basin.

16. Conclusion The cation and anion ratios reflect the seasonal variability in the weathering pattern of carbonate and silicate minerals, which is consistent with aquifer depth. Groundwater in the Ballia district is directly and/or indirectly influenced by the anthropogenic activities including extensive and intensive agriculture practices. The heterogeneous distribution and high arsenic in aquifers is governed by the depletion of anions like HCO3- and NO3- at shallow depth where redox reactions support the release of As from arseniferous iron oxyhydroxides in the groundwater.

17. Future Studies • Further following studies are needed to understand the mechanism of arsenic enrichment in the middle gangetic plain: • Arsenic mineral identification in soil profile • 2. Sources of nutrients • 3. Microbial assay • 4. Irrigation pattern • 5. Shift in geochemical properties of soil

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