Natural Sources and Anthropogenic Influences on the River Water and Groundwater Chemistry of the Lower Mahanadi Basin: Insights From Radiogenic Sr Isotopes and Major Ion Chemistry

Abstract

The Mahanadi River ranks second among the rivers in the peninsular India in terms of water potential and flows through a geologically heterogeneous terrain. The present study uses a multiproxy approach, involving concentrations of major ions, and 87Sr/86Sr of the dissolved phases in seasonally collected river water and groundwater samples from the Lower Mahanadi Basin to investigate the sources of ions, the Cl-enrichment in the river water, the influences of the man-made structures like the Naraj Barrage, and the role of fertilizers on the chemistry of the river water. This study also provides the first estimate of the radiogenic Sr-flux of the Mahanadi River to the Bay of Bengal. Both inverse and forward models were used to evaluate the contributions of different sources to the dissolved ions of the Mahanadi River over different seasons. The results suggest that even in the predominantly silicate watershed of the Lower Mahanadi River, the riverine chemistry is modulated primarily by carbonate dissolution farther upstream followed by silicate weathering although, the contributions vary seasonally. The Naraj Barrage, which divides the main channel of the Lower Mahanadi River into several distributaries marks a divide between the less polluted upstream and the more polluted downstream. The radiogenic 87Sr/86Sr of the Mahanadi River water can be explained by mixing of four endmembers: Proterozoic carbonates, Archean silicates, rainwater, and fertilizers. The dominance of agricultural input during monsoon is consistent with high loadings of nitrates and phosphates used as fertilizers in the Mahanadi River basin along with a strong positive relationship between phosphate concentrations and 87Sr/86Sr of the river water samples. The flux-weighted averages of Sr concentration and 87Sr/86Sr to the Bay of Bengal were found to be 1.03 μmol/l and 0.72154, respectively. The groundwater composition of the Lower Mahanadi Basin is modulated by the mixing of four endmembers, viz. weathering of silicate and carbonate rocks, Bay of Bengal seawater, and fertilizer inputs. The groundwater samples of the Mahanadi basin show an average Sr concentration of 5.45 μmol/l and an average 87Sr/86Sr of 0.71772.