Geochemistry and quality of groundwater of Gummanampadu sub-basin, Guntur District, Andhra Pradesh, India

Groundwater survey has been carried out in the area of Gummanampadu sub-basin located in Guntur District, Andhra Pradesh, India for assessing the factors that are responsible for changing of groundwater chemistry and consequent deterioration of groundwater quality, where the groundwater is a prime source for drinking and irrigation due to non-availability of surface water in time. The area is underlain by the Archaean Gneissic Complex, over which the Proterozoic Cumbhum rocks occur. The results of the plotting of Ca2+ + Mg2+ versus HCO3 (-) + CO3 (2-), Ca2+ + Mg2+ versus total cations, Na+ + K+ versus total cations, Cl- + SO4 (2-) versus Na+ + K+, Na+ versus Cl-, Na+ versus HCO3 (-) + CO3 (2-), Na+ versus Ca2+ and Na+: Cl- versus EC indicate that the rock-water interaction under alkaline condition is the main mechanism in activating mineral dissociation and dissolution, causing the release of Ca2+, Mg2+, Na+, K+, HCO3 (-), CO3 (2-), SO4 (2-) and F- ions into the groundwater. The ionic relations also suggest that the higher concentrations of Na+ and Cl- ions are the results of ion exchange and evaporation. The influences of anthropogenic sources are the other cause for increasing of Mg2+, Na+, Cl-, SO4 (2-) and NO3 (-) ions. Further, the excess alkaline condition in water accelerates more effective dissolution of F--bearing minerals. Moreover, the chemical data plotted in the Piper's, Gibbs's and Langelier-Ludwig's diagrams, computed for the chloro-alkaline and saturation indices, and analyzed in the principal component analysis, support the above hypothesis. The groundwater quality is, thus, characterized by Na+ > Ca2+ > Mg2+ > K+: HCO3 (-) + CO3 (2-) > Cl- > SO4 (2-) > NO3 (-) > F- facies. On the other hand, majority of groundwater samples are not suitable for drinking with reference to the concentrations of TDS, TH, Mg2+ and F-, while those are not good for irrigation with respect to USSL's and Wilcox's diagrams, residual sodium carbonate, and magnesium hazard, but they are safe for irrigation with respect to permeability index. Thus, the study recommends suitable management measures to improve health conditions as well as to increase agricultural output.