Simulation of Groundwater Dissolved Organic Carbon in Yufu River Basin during Artificial Recharge: Improving the SWAT-MODFLOW-RT3D Reaction Module

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Abstract
To maintain the spring water supply and groundwater level, the Chinese government has adopted a series of water regulation measures in Jinan. However, as a result of water exchange, considerable volumes of dissolved organic carbon (DOC) can reach groundwater. Changes in DOC concentrations in groundwater can affect the physical and chemical stability of groundwater, thereby altering the subsurface microecological environment. In addition, the Chinese government's targets for carbon peaking and carbon neutrality have posed new challenges to the Yufu River watershed's ability to maintain and enhance its carbon sinks and storage capacity. This study aimed to develop a SWAT-MODFLOW-RT3D model specifically designed for the Yufu River watershed, with the purpose of simulating variations in DOC concentration in groundwater under diverse artificial recharge scenarios. The ultimate objective was to offer valuable insights into the effective management of water resources in the designated study region. The modified SWAT-MODFLOW-RT3D model simulated the variations of DOC concentration in groundwater under three artificial recharges scenarios, which are (a) Recharged by Yellow River water; (b) Recharged by Yangtze River water; (c) Recharged by Yangtze River and Yellow River water. The study revealed that the distribution of DOC concentration in groundwater within the watershed exhibited notable spatial variations as a result of the impact of surface water infiltration recharge. The concentration of DOC in groundwater exhibits an increase in proximity to the river, whereas it remains relatively constant in other regions within the watershed. Additionally, as artificial recharge periods increase, the impact of rivers on groundwater DOC concentrations grows steadily stronger. The primary cause of the ongoing rise in groundwater DOC concentration in the watershed is the long-term storage of water resources. By 2030, the maximum level of DOC in the basin will exceed 6.20 mg/l. The Yellow River water recharge scenario offers greater groundwater recharge and the lowest DOC input unit volume in the research area when compared to the Yangtze River water and combined water resources recharge scenarios. Lithology is also a key factor that determines how the model performs in the end. The primary source of DOC recharge in groundwater is lateral recharge from rivers, not precipitation infiltration. The findings of this study indicate that, particularly when recharge water supplies are enhanced with organic carbon, DOC concentrations in groundwater may alter dramatically during artificial storage. In order to lessen the detrimental effects of water backfilling on groundwater quality, this coupled modeling analysis is crucial.

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