Lithological controls on chemical weathering signatures in the semi-arid climatic regime: Study based on tropical small scale catchments

Lithological controls on chemical weathering can be better constrained by studying catchments having different lithologic settings in similar climatic conditions. This study compares the weathering fluxes in two small catchments having different lithology in semi-arid region (500–800 mm year^-1 rainfall) in western India: The West Banas River (2100 km² area, 372 m relief) draining granitic/gneissic rocks and the Berach River (610 km² area; 542 m relief) flowing through shales. Sample collected in the year 2016 and 2017, were analysed for chemical composition to evaluate chemical weathering fluxes. Inverse model analysis and Soil and Water Assessment tool (SWAT) simulated runoff were combined to estimate the annual weathering fluxes. Total dissolved solids (TDS) in the West Banas ranged from 71 to 428 mgL⁻¹(avg. 227 mgL⁻¹), while the Berach River showed TDS of 190–712 mgL⁻¹(avg. 370 mgL⁻¹), with a few higher values due to anthropogenic sources. Silicate weathering rate (SilWR) is estimated as 7.7 ± 1.7 tons km⁻²y⁻¹ for the granitic West Banas catchment whereas 5.7 ± 1.2 tons km⁻²y⁻¹ for the shale lithology in the Berach River. The weathering susceptibility ratio of shale to granite is derived as 2:3. The weathering intensity is highly controlled by the susceptible minerals present in these rocks. The posteriori results of elemental ratios of the silicate endmember in shale lithology basin (Berach River) shows strong indication of incongruent weathering in the basin as a result of their constituting minerals has already gone through at least one cycle of chemical weathering during their formation. Saline-alkaline soils (SAS) contribute significantly to dissolved loads, especially in the Berach (38 ± 12 %), compared to the West Banas (26 ± 7 %). The cations derived from different lithologic sources have a dependency on the drainage basin area. However, other topographical factors showed minor control on chemical weathering.