Arsenic-safe alternate aquifers and their hydraulic characteristics in contaminated areas of Middle Ganga Plain, Eastern India.

Arsenic groundwater contamination exceeding 0.05 mg/l affecting the Newer Alluvial tracts of Patna and Bhojpur, the two worst affected districts located in the Middle Ganga Plain in the Bihar state, has been studied The area is underlain by two-tier Quaternary aquifer systems within a depth of 300 m below ground level, separated by a 15-32-m-thick clay/sandy clay aquitard. The upper part (<50 m depth) of the shallow aquifer system is arsenic-contaminated. The deeper aquifer system (lying below 120-130 m depth) exhibits low arsenic load (max 0.0035 mg/l), having hydraulic conductivity between 64.88 and 82.04 m/day. Groundwater in the deeper aquifer occurs under semi-confined to confined condition due to poor hydraulic conductivity of the middle clay (4.7×10(-2)-7.2×10(-3) m/day). Hydraulic head of the deeper aquifer remains close to the surface than the shallow aquifer. The two aquifer systems in the Newer Alluvium are replaced by a thick single aquifer system in the adjoining Older Alluvium, within a depth of 330 m below ground. In the arsenic-contaminated area, deeper aquifer is protected by a middle clay, which may be developed for community drinking water supply by deep tube wells having a yield capacity of 150 m3/h.