Geo-chemistry of Ground Water in Rajshahi City and Improving its Quality for Drinking Purpose

Abstract

The study was aimed at improving the fundamental understanding of the geochemistry of groundwater in Rajshahi City with a focus on constituents that pose a human health risks. The study was attempted to evaluate the major ionic chemistry, weathering and solute acquisition processes controlling water composition, and suitability of water quality for drinking purpose. A total of 240 ( 180 shallow and 60 deep) tubewells water samples were collected from 40 locations during pre, post-monsoon and monsoon season covering two years. Also a total of 30 aquifer soil samples were collected at depths (10-45 meter) from 30 locations during monsoon season in 2010 to know soil characteristics. A detailed laboratory analysis was carried to determine the characteristics of groundwater and aquifer soil samples of Rajshahi City. Also multivariate statistical analyses were conducted using various softwares (SPSS, AQUACHEM etc.) and the results were plotted to determine hydrochemistry, water quality and water type. The concentrations of Ca2+ and Cd2+ were the highest and lowest quantities among cations in the aquifer soil samples, respectively and their mean amounts were 166.08 and 0.133 mg/kg, respectively. Among anions, HCO3- and NO3- were the maximum and minimum concentrations and their concentrations were 580 and 3.87 mg/kg, respectively. The results of the analyzed aquifer soil samples showed that the mean concentrations of cationic parameters were followed the order: Ca2+> Mg2+> Na+> K+> Fe3+> Mn2+> Zn2+> Cu2+> Pb2+>As(total)> Cd2+and that of the anions were: HCO3-> Cr> SO/>NO3-. The analyzed results of the hydro-chemical composition of the shallow and deep tubewells water in the Rajshahi City showed that the groundwater of the study area was slightly acidic to neutral pH, hard to very hard and fresh category. Ca2+ was the dominant ionic species among the cations in both the shallow and deep tubewells water and their mean concentrations were 78.354 mg/L for the shallow and 78.541 mg/L for deep tubewells. HCO3- was the dominant species with an average of 245.30 mg/L for shallow and 188.362 mg/L for deep tubewells water. The analyzed results show that the mean concentrations of cations and anions in both shallow and deep tubewells water samples were followed the order: Ca2+> Mg2+> Na+> K+> Fe3+> Mn2+> Pb2+> Zn2+> Cu2+> As(total)> Cd2+ and HCO3-> Cl> SO/> NO3-, respectively. Most of the groundwater samples contained concentration of TH, HCO3-, ca2+, Mn2+, Fe3+ and Pb2+ that exceeded the permissible limit for World Health Organization (WHO) and Bangladesh Drinking Water Standard (BD-DWS). About 40% groundwater samples exceeded As(total) concentration for the WHO standard limit. The study observes that most of the physico­chemical parameters were found to be higher in the pre and post-monsoon seasons compared to the monsoon season may be the cause of decreasing the volume of water in aquifer. ----------------