Understanding transport and transformation of dissolved inorganic carbon (DIC) in the reservoir system using δ13CDIC and water chemistry

Abstract

In order to advance our understanding of the driving factors controlling carbon dynamics and evolution of water quality in river-reservoir systems for the generation of hydropower, we conducted a study in a karst deep-water reservoir (Wujiangdu Reservoir), southwest China. Water samples were collected from the inflow/outflow of the Wujiangdu Reservoir, four vertical columns along the reservoir, and from three tributaries to the reservoir in January, April, July, and October 2017. The dissolved inorganic carbon (DIC) concentrations and carbon isotope composition (δ13CDIC) varied greatly (1.99–3.45 mmol/L and −10.7‰ to −6.0‰, respectively) and were controlled by multiple processes including CO2 outgassing, primary production, and organic matter degradation. In the four vertical profiles, the difference between the values of samples with those at 15 m of Δ[DIC] and Δ[δ13CDIC], Δ[DIC] and ΔCa2+ in the same water column had positive correlations, and the variation in dissolved O2, partial pressure of CO2, and flux of CO2 suggested that primary production dominated above the epilimnion and degradation of OM dominated below the epilimnion during the warm season. Continuous CO2 outgassing was found from riverine water to surface water of the reservoir before the dam based on carbon isotopic compositions and water chemistry. These processes would cause isotopic fractionation between the residual DIC and CO2 (aqueous), the degree of which varied among different seasons (July > April > October > January). Carbon transport and biogeochemical processes were highly controlled by the hydraulic retention time (HRT) in the river-reservoir system. It was estimated that approximately 71.5% of the annual DIC flux was produced in the water column below 15 m depth during the study year, suggesting that the processes of DIC generation and consumption occurred at the same time. The results highlight that carbon behavior in the impounded rivers is influenced by multiple processes. The carbon transformation processes should be taken into account for improving the estimation accuracy of the carbon budget calculations and the management of water quality for river-reservoir systems.