Spatial Variation in the Sources of Dissolved Inorganic Carbon in a Large Arid River Basin, Northwestern China

Abstract:

The Heihe River basin (HRB) is the second largest endorheic river basin of China located in arid northwestern China, characterized by runoff-generating mountainous terrains in the upstream, well developed irrigated agriculture in the middle basin, and Gobi deserts in the downstream. We sampled the Heihe river water along the middle section during base flow and high discharge (irrigation periods), as well as groundwater scattered in the middle HRB. δ¹³C-DIC, nitrate, and cation were measured to constrain water and carbon sources. Considering positive correlations of δ¹³C-DIC with Ca²⁺ and Mg²⁺ concentrations and the distribution of maize in the midstream areas, the weathering of silicates with CO₂ from C4 plants was the main source of DIC in the Heihe river. During base flow and high discharge, river water δ¹³C-DIC had similar values (-2.26‰ for average) at the mountain exit and sharply fell to averages of -4.36‰ at the baseflow and -5.80‰ at high discharge, as the river enters the midstream plain where larger fluxes of groundwater discharging to the river water were observed. δ¹³C-DIC values during high discharge than during baseflow conditions can be attributed to lower inputs of groundwater and stronger soil respiration in the growing season. A negative correlation between δ¹³C-DIC and nitrate at high discharge indicated stronger influences of soil respiration on river water DIC through irrigation return flows. The binary mixing model using groundwater and mountain exit as end-members was used to evaluate longitudinal variation in the fraction of groundwater contributing to river water. The proportion of groundwater increased from the boundary of the diluvial fan (about 20% at high discharge and 45% at base flow) to the exit edge of the middle basin (about 45% at high discharge and 60% at base flow).