Evaluation of the influence of river bank infiltration on groundwater in an inland alluvial fan using spectral analysis and environmental tracers

Understanding the influence of river-water infiltration on groundwater is essential for effective management of groundwater resources adjacent to a losing river. This study evaluates the influence of river-water infiltration in the alluvial fan of the Nalenggele River basin, northwest China, using (1) spectral analysis to analyze river discharges and groundwater levels, and (2) environmental tracers. The fluctuation period of groundwater levels was similar to that of river discharge. However, the time lag was about 50–150 days (increasing with distance from the channel), and the fluctuation period also increased with distance from the channel, from 297 to 990 days; this indicated a weakening influence towards the flanks of the alluvial fan. The total dissolved solids and δ 18 O values of the groundwater were close to those of the river water when the influence of river bank infiltration was strong. The spatial distribution of groundwater 14 C activity reflected the influence of river bank infiltration and the groundwater residence time; 14 C activities larger than 50 pMC in groundwater indicated a strong mixing with river water and fast groundwater flow, and reflected the strong influence of river bank infiltration. Data confirmed that the zone of strongest influence was near the river channel (horizontal distance ~5 km, maximum depth 150 m), whereas the zone of weakest influence was in areas far away from both sides of the river channel and near where groundwater seeps to the surface. River bank infiltration should be considered when developing groundwater resources to ensure the security of water supply.