Quantifying heterogeneities of vertical hydraulic gradients in a highly urbanized effluent river system in the Darjeeling Sub Himalayan foothills, India

The vertical hydraulic gradient (VHG) caused by differences in water surface elevation between the river and minipiezometer is regarded as a crucial factor in regulating groundwater-surface water interaction. However, very few studies have comprehensively analyzed VHG at both a large spatial and temporal scale, with an even more limited number of studies addressing this aspect specifically in the context of urban rivers. This study aims to examine the spatial and temporal variations of VHG in the two main tributaries of the Mahananda River Basin (MRB), which flows through the densely populated urban area in the Darjeeling sub-Himalayan foothills. This makes this study much more pertinent to the urban river system. To accomplish this, a total of 260-point measurements were taken across seventeen transects during the low-flow seasons of both 2020 and 2021. The result shows that the Balason River experienced a reversal of VHG from 2020 to 2021, while the Mahananda River showed a consistent upwelling pattern in both years. The overall upwelling gradient in the MRB indicates that groundwater is the main source of water in the Piedmont regions, especially during the low-flow seasons. Cross-valley and down-valley patterns revealed differences between the two rivers. High-resolution analysis of cross-valley VHG distribution reveals the potential for opposing flow directions within a few centimeters of spatial proximity. Among the various riverbed morphologies examined, the dune-ripple channel is found to be closely associated with upwelling. The spatial distribution of VHG is found to be more heterogeneous in areas where riverbed properties have been directly altered by excessive sediment extraction than in areas where wastewater is disposed of, indicating a stronger link between VHG and direct modification of riverbeds. These places are therefore susceptible to total removal of the hyporheic environment, including hyporheos.