Rock–Sediment–Water Controls on Dissolved Inorganic Carbon, Δ¹³C and Water Quality in the Cauvery River, India

Abstract

Rivers draining silicate terrains carry carbon fixed by rock weathering to the ocean, yet rocks, sediments, water and dissolved carbon are rarely studied together, and the suitability of the same water for people is usually assessed separately. We combined whole-rock (n = 10), bed-load sediment (n = 15) and river-water chemistry (n = 15) with dissolved inorganic carbon (DIC) and its stable carbon isotope composition (δ¹³C-DIC; n = 11) along the lower Cauvery River, southern India. Gneiss-dominated rocks and immature sands (CIA 42–61) record plagioclase weathering, and the water plots at the silicate end-member (Ca/Na 0.14–0.31), yet bicarbonate balances only about one-third of the cation charge because of large chloride inputs from irrigation, effluents and evaporation. All river stations meet the Indian acceptable limits for chloride, calcium, magnesium and hardness, and TDS stays below the permissible limit, but magnesium hazard exceeds 50% at 12 of 14 stations, a risk for irrigated soils. DIC (8.3–40.9 mg C L⁻¹) follows the weathering solute load, while δ¹³C-DIC rises downstream from −9.5‰ to −1.3‰ (2.7‰ per 100 km; r = 0.84). As carbonate is absent, this enrichment records progressive exchange with the atmosphere and CO₂ evasion in the dam-regulated channel; an equilibration model gives an e-folding length of 190 ± 54 km. Treating all cations as weathering products would overestimate CO₂ consumption about threefold. We propose a Rock–Sediment–Water–Carbon (RSWC) framework, including water-quality screening, for use in other river basins.