Variations in the dissolved carbon concentrations of the shallow groundwater in a desert inland river basin

Due to its potentially important role in the global carbon cycle, the dissolved carbon of groundwater systems is severely affected by climate change and oasisization in desert inland river basins. However, the spatial distribution and driving mechanisms of groundwater dissolved carbon are still poorly understood. Here, we use groundwater data from 72 monitoring wells to reveal the spatial variations in the dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) concentrations of the shallow groundwater in the Tarim River basin, an arid region in northwest China. Due to the unsaturated characteristics of the groundwater recharge, the DOC concentration of groundwater increased significantly with greater lateral distance from the downstream river, rather than the DIC concentration of groundwater. There was a significant positive linear correlation between DOC and DIC. Redundancy analysis showed that groundwater depth (GD) and evapotranspiration influenced the DOC concentration, while the DIC concentration was greatly affected by the total salinity of groundwater and farmland. Likewise, both DOC and DIC concentrations were most affected by Ca2+. Further analysis showed that a decrease in the GD will lead to a decrease in the DOC concentration (dilution effect), but it has no obvious influence on the DIC concentration. High evapotranspiration in the desert region drives an increase in the DOC concentration (concentration effect) and a decrease in the DIC concentration. Our study highlights that average annual evapotranspiration and GD jointly drive the differences in the DOC and DIC concentration changes in the shallow groundwater of the Tarim River basin.