Contamination Of Groundwater By Arsenic In Ballia &Ghazipur Districts Of Eastern U.P. : A Review On Sources, Hydrochemistry And Temporal Variation Of Arsenic

Abstract

The escalating concern over the toxic nature and global ubiquity of arsenic has become a mounting apprehension for our nation and the international community at large. Recent analysis of groundwater samples collected from significant regions in the North-eastern States of India has unveiled elevated levels of arsenic, ranging from 50 to 986 g/l. This issue assumes a critical significance when the concentration of arsenic exceeds the established acceptable thresholds set by the World Health Organisation (WHO) and the Bureau of Indian Standards (BIS), which stand at 10 g/l and 50 g/l, respectively. In the forthcoming discourse, we shall delve into the origins, hydrochemical characteristics, and temporal fluctuations of arsenic within the Eastern U.P. districts of Ballia and Ghazipur. Groundwater primarily harbors two principal oxidation states of arsenic, namely arsenate (As5+) and arsenite (As3+). These states can undergo interconversion through the oxidation of As3+ into As5+ and the reduction of As5+ into As3+. Moreover, arsenic can manifest in an organic form resulting from biomethylation processes. Extensive investigations conducted in Ballia and Ghazipur districts of Eastern U.P. have elucidated a decline in arsenic concentrations in water samples during the post-monsoon period, indicating distinct temporal and seasonal variabilities across various water sources. This observation establishes a clear association between the behavior of arsenic and the amount of rainfall received. As the intensity of rainfall increases, the diffusion rate causes a reduction in arsenic concentration within groundwater. Notably, during the monsoon season, conspicuous decreases in arsenic content are observed. The elevation of groundwater levels in both surface and subsurface regions, coupled with the redox potential, influence the dynamic response of groundwater streams to seasonal rainfall patterns. The shallow basin and flood conditions during the monsoon season lead to rising groundwater levels, while the land remains moist for a significant portion of the year, resulting in low flushing rates of groundwater. Consequently, the average arsenic concentrations in surveyed cultivation areas contrast between the storm and pre-storm seasons. Arsenic ingress into groundwater occurs through the decomposition of iron(III) oxyhydroxide during the monsoon season, while it adsorbs reversibly onto iron(III) oxyhydroxide during the pre-monsoon season. These two processes play a pivotal role in effectively governing the arsenic concentration before and after the monsoon.