Event-Based Analysis of Recharge Dynamics in a Heterogeneous Hard-Rock Catchment using Hydrogeophysical Techniques

Understanding recharge processes in heterogeneous hard-rock terrains is essential for sustainable groundwater management. This study examines the spatiotemporal recharge dynamics of a 2 km² experimental micro-catchment in Rajasthan, India, using integrated hydrometeorological observations, Direct Current resistivity and Induced Polarization (DC–IP) imaging, Normalized Difference Vegetation Index (NDVI) time series, and aquifer tests. NDVI provided seasonal moisture variability, while ten DC–IP profiles delineated subsurface architecture and structural controls on flow.

Geophysical sections reveal a highly heterogeneous system dominated by massive high-resistivity sandstone blocks (>200 Ω·m) intersected by vertical to sub-vertical low-resistivity fracture zones (<50 Ω·m). Low to moderate chargeability (<10 mV/V) within these conductive features indicates groundwater-bearing fractures rather than shale layers. The fracture networks near Wells S1 and S2 are structurally isolated, despite being separated by only ~30 m. Slug tests indicate low hydraulic conductivities in both wells (Ks₁ = 2.42×10⁻⁸ m/s; Ks₂ = 1.06×10⁻⁸ m/s), with S1 being slightly more conductive.

Event-based analysis of 25 monsoon rainfall–recharge events shows contrasting well responses due to their structural positions. Well S1, located 5 m from an intermittent stream bank, exhibits a flashy early-season bank-storage response with high Specific Water Level Rise (SWLR), later transitioning to rapid recession and reduced efficiency as water levels exceed the streambed. In contrast, Well S2 (35 m from the stream bank) shows delayed but consistent responses, with stable SWLR and recession rates, characteristic of a structurally confined storage zone. Cross-correlation analysis confirms strong coupling in peak responses but moderate similarity in lag and recession behaviour.

These findings show that rapid water-level rises in hard-rock terrains often reflect transient drainage pathways rather than sustainable storage. Managed Aquifer Recharge strategies should therefore target structurally controlled, high-retention fractured zones (e.g., S2) instead of stream-connected fracture corridors (e.g., S1).