The state of toxicity and cause of elevated Iron and Manganese concentrations in surface water and groundwater around Naga Thrust of Assam-Arakan basin, Northeastern India

Abstract

An alarming situation of exceedingly high Fe and Mn concentrations in both surface water and groundwater has been identified in a tectonically active region located in Assam-Arakan basin of northeast India. Cross plots, Pearson correlation, principal component analysis and flow net methods were applied to elucidate the present state and possible sources of Fe and Mn contamination. Out of 32 water samples collected for the study, all of them found exceeding 28 times more for Fe and 136 times more for Mn than EPA limit. Correlation and cross plots of pH, TDS, EC, Fe, Mn, Cu, Zn suggest the reduction of both Fe-hydroxides and Mn-oxides in redox condition into dissolve states leading to Fe and Mn elevations in groundwater. Principal component analysis exhibits three PC factors in which PC1 and PC3 reflected oxidized condition and PC2 reflected reducing environment that controlled Fe and Mn concentrations. The study also revealed that source of high Fe and Mn contaminations was controlled by groundwater flows. Water table contour flow net suggests that the Chathe river acts as influent stream and supplies elevated Fe and Mn water into the shallow aquifers. Baseflow from the discharge areas into low lying areas causes accretion of Fe and Mn in pond, oxbow lake, backswamp and shallow aquifers.