GROUNDWATER HYDROGEOCHEMISTRY AND ITS IMPLICATIONS FOR ARSENIC MOBILIZATION IN SHALLOW AND DEEP AQUIFERS IN ARAIHAZAR, BANGLADESH

A hydrogeochemical study was conducted in Araihazar, Bangladesh to compare and contrast groundwater compositions in the shallow, As containing Holocene aquifer (5-30 m) and deep, low-As aquifers (30 – 91 m) that are of Holocene and Pleistocene strata. Groundwaters from 37 monitoring wells from 6 nests, as well as 21 existing private wells were sampled in January, 2003 and January 2001 respectively. In addition to parameters measured in the field such as ORP, pH, alkalinity, dissolved oxygen, ammonia, As(III) was separated from total As by anion exchange resin on site. Other parameters quantified are major cations, anions, trace elements, and nutrients (NO₂, NO₃, NH₄, ortho-Phosphate, total organic carbon and total nitrogen).

Groundwater is HCO₃ type, although the shallow aquifer water is Ca-Mg rich while the deep Pleistocene water is Na rich. Shallow wells with elevated As concentrations (> 50 mg/L) are all reducing with negative ORP values between -50 mV and -200 mV, with predominantly As(III). PHREEQC calculations show that Fe and P are super saturated with respect to siderite, vivianite and hydroxyapatite in shallow groundwaters from 5 sites except one that is influenced by irrigation pumping (Site F). In general, shallow aquifer waters contain much higher nutrients (TOC, TN, NH₄ and P) than the deep aquifer waters. This suggests that the role of nutrients on the microbial activity and the mobility of As in both aquifers warrant further investigation.