Water level and stable Isotope based river aquifer interaction in different river styles of a semi-arid river

Abstract

Understanding the groundwater-surface water (GW-SW) interaction is critical for river management, especially in water-stressed regions such as semi-arid and arid areas. The pattern of GW-SW interaction may vary across variable valley settings, floodplain width and river planform. This study aims to analyse the pattern of GW-SW interaction in different River Styles reaches. The study is carried out in the Sabarmati River basin in using stable isotopes. Sabarmati River is an intermittent River of 419 km in length that drains a 21,085 km2 area in the semi-arid region of Western India. Representative sites of each River Styles were selected for water level measurement and stable isotope samples. The study was conducted in the post-monsoon period of 2021-22, which represents groundwater contribution to river system after major runoff seasons. Samples at each River Styles reach were collected along a cross-section. 1 river water and up to 4 groundwater samples (2 from each bank) were collected along the transect. A total of 48 samples were collected along 11 such transects. The depth of groundwater and stage of river water at each sample site was also measured.   The GW depth and river stage data indicate GW-SW connection for 9 sites (all except 1 are from the upstream region), while was inconclusive for 2 (all in the middle and lower reaches). Stable isotope-based analysis suggests a similar scenario. The upper reaches, which are gaining, have enriched δO18 composition and lower d-excess than the groundwater. The depleted isotopic composition of groundwater indicates faster groundwater recharge from the meteoric water. Such reaches are characterized by boulders and gravel beds. The reach-scale variability of the river from the losing-gaining stream also collaborates with the reach-scale variation of δO18 isotopic values. The losing reach has a depleted δO18 isotopic composition than the groundwater, thus indicating recharge of groundwater from the river water and the impact of evaporation. The integration of the River Styles map and GW interaction study suggests the following – (a) Generally, River Styles that were showing connected and gaining reaches were found to have low sinuosity  (b) River Styles with occasional and discontinuous flood-plains showed an inconclusive result about river aquifer connectivity by both methods (c) For all confined or partly unconfined reaches with bedrock margin-controlled settings, a connected river aquifer system was noted. The study highlights geomorphic control on the important process of GW-SW interaction in a semi-arid river channel.   Keywords: River style, stable isotope, river-aquifer interaction