Response of groundwater quality to river-aquifer interactions during managed aquifer recharge: A reactive transport modeling analysis

Managed aquifer recharge (MAR) has been applied for groundwater restoration as an effective strategy in many overdraft regions. In Jinan, China, MAR through river infiltration was conducted regularly in recent years because of the most famous spring groups' drying up due to continuous decline of groundwater level caused by extensive pumping. A substantial rise of water table (similar to 20 m) has been observed near the river, however, the potential change in groundwater quality and geochemistry during MAR process remains unknown. In this study, a flow and multi-component reactive transport model was developed to evaluate the impact of MAR on chemical reaction processes and the response of groundwater quality to the interaction of river water and groundwater. The geochemical conditions of the river water and groundwater are significantly different, as river water con-tained more dissolved organic carbon (DOC) and less nitrate than groundwater did in the study area with intensive agricultural activity. This study revealed that nitrate dilution rather than denitrification was the main process responsible for observed groundwater quality change during MAR. Preferential flow occurring in the aquifer resulted in strong spatial variability of nitrate concentrations in the groundwater. River water DOC, aquifer hydraulic conductivity distribution, and river-aquifer hydrodynamic conditions can all affect the redox condition in the riparian zone by changing either reaction rate or residence time. The results from this study indicate that groundwater geochemistry can be significantly altered during MAR, suggesting the importance of regular monitoring plan to assess the changes in groundwater quality, especially in the area where river sedi-ments are rich in organic carbon. Such monitoring and reactive transport modeling analysis is critical to ensuring that the adverse impact on groundwater quality by potential MAR operations be kept to a minimum.