Fate of Urban Groundwater in Shallow Confined Aquifers. Case Study of Baldia Town, Karachi, Pakistan

Abstract

A study was carried out to assess the groundwater quality for drinking purpose in Baldia Town, Karachi. For this purpose, groundwater samples (n = 18) were randomly collected from various depths (> 100 feet) through boring wells after monsoon season. Data revealed that except two (BT-5, 8) which were turbid and smoky, rest of the samples were colorless, non-turbid and sweet in taste. Groundwater temperature fluctuates between 19-26 ˚C. The pH varies between slightly acidic to slightly basic (range: 6.8-7.3) where two third of total samples have pH < 7. All the samples have very high TDS content (range: 1240-16910 mg/L; mean: 6832 mg/L) which exceeded the national drinking water quality standard (1000 mg/L) set by PCRWR. Hardness values varied in the extreme range (1000-9500 mg/L; mean: 2366 mg/L). Relative abundance of major cations follows the order of Mg > Ca > Na> K while anions varied in the order of HCO3> SO4> NO3> Cl. Dissolved Fe+3 (mean: 0.01 mg/L) varies within WHO permissible limit (0.3 mg/L) while Mn showed concentration < 0.01 mg/L. Concentration of trace elements declined in the order of Ni > Zn > Cr > Co. It is concluded that groundwater of study area not fit for drinking purpose. It is strongly influenced by semi-arid climate and water rock interaction which is manifested by geochemical signatures of limestone (Ca, Zn) and clays (Ni, Co, Cr). Due to confined aquifer system the anthropogenic contamination is not significant.