Weathering Characteristics in the Mahanadi River Basin, India

Abstract

The processes of weathering and erosion are the most important natural events that reshape the surface of the earth and regulate geochemical cycling of elements. The study of processes involved in rock weathering, sediment formation and subsequent transfer of weathered materials by the river to the ocean is essential for understanding different earth surface processes. The transport of sediments by rivers to the oceans represents an important link between the terrestrial and marine ecosystem. Chemical weathering of rocks releases soluble products and produce solid residues, which control the land-ocean-atmospheric fluxes and earth’s climate. The geochemistry of river sediments gives information about the provenance characteristics and characteristics of weathering and erosion in a basin. Therefore, this thesis aims to study the spatio-temporal variation of the sediment discharge and erosion rate, geochemistry of water and sediments of the Mahanadi river basin, one of the biggest rivers in India to understand the weathering characteristics of the basin.
The trend analysis study is conducted in the time series data (1981-2010) of water flux sediment discharge and rainfall (1990-2010) of the Mahanadi river to study the sediment load variation. The trend test result represents that the sediment load delivered from the Mahanadi river to the global ocean has decreased sharply at the rate of 0.42×106 tons/year between 1981 and 2010. Water discharge and rainfall in the basin show no significant decreasing trend except at only one tributary. The decline in sediment discharge from the basin to the Bay of Bengal is mainly due to the increase in the number of dams, which shows the increase from 70 to 253 during the period of 1980 to 2010. Over the past 30 years, the Mahanadi river discharges about 48.01±20 km3 of water and 14.52±12.7×106 tons of sediment annually to the Bay of Bengal whereas the mean erosional rate is 238±116 tons/km2/year. Based on the current data (2001–2010), sediment flux and water discharge to the ocean are 11.02±5×106 tons/year and 50.91±16 km3/year respectively; and ranking Mahanadi river second in terms of water discharge and sediment flux to the ocean among the peninsular rivers in India. The sediment load in the basin is mostly influenced by the variation in water discharge and relief.
The results of hydrogeochemical study of the Mahanadi river basin reveal that the dissolved loads in the basin are dominantly controlled by rock weathering particularly chemical weathering of silicates and carbonates. The TDS in the basin is higher than the global average. The estimated chemical weathering rates based on the forward model are 44.94 tons/km2/year in monsoon and 2.45 tons/km2/year in pre-monsoon with annual average chemical weathering rate of 23.69 tons/km2/year. The contributions of silicate weathering rates in the basin are 32.15 and 1.55 tons/km2/year during monsoon and pre-monsoon period respectively. The estimated CO2 consumption rate associated with chemical weathering in the basin is 13.3×105 mol/km/year during monsoon and 0.66×105 mol/km/year during pre-monsoon period with an average annual rate of 6.91×105 mol/km/year which is higher than the global average and most Indian rivers. The net rate of CO2 consumption by silicate weathering is estimated to be approximately 4.78×105 mol/km/year. It is observed that the runoff and lithology are the major factors influencing chemical weathering in the basin.
The geochemistry of sediment shows that they are mainly derived from felsic source rocks with a minor contribution from mafic and carbonate rocks. This is also evidenced from the ratio of Al2O3/TiO2, higher content of K2O and Rb along with the abundance of quartz, feldspar and variable quantity of dolomite and calcite in the sediments. Higher values of (La/Lu)N and LREE/HREE ratios suggest the presence of acidic source rock in the basin. The majority of sediments are chemically similar to arkose and litharenite sandstone. The values of the chemical index of alteration (CIA), index of compositional variability (ICV) and the ratio of Rb/Sr indicate most of the sediments are compositionally immature and undergone weak weathering. Positive Ce anomaly, as well as the ratios of Ni/Co and V/Cr, suggests oxidising environments of deposition. The sediments are deposited on a passive continental marginal setting with semi-arid to semi-humid climatic condition.