Multi‑temporal investigation of brine hydrogeochemical evolution under sustained potash mining in the Qarhan Salt Lake, Qaidam Basin, China

Abstract The Qarhan Salt Lake, located in the eastern Qaidam Basin of Qinghai Province, China, is the largest potash-producing base in the country. Decades of sustained brine extraction for potassium chloride (KCl) production have profoundly altered the subsurface hydrogeological and hydrogeochemical regime. However, systematic multi-temporal assessments of these changes remain scarce. This study presents a comprehensive investigation of brine dynamics across approximately 177 monitoring sites in the Qarhan mining district, comparing initial (pre-mining baseline), 2020 (fourth quarter), and 2024 (fourth quarter) datasets. Results indicate a progressive decline in brine water levels, with median depth to water table (DWT) increasing from − 0.44 m (baseline) to − 2.98 m (2020) and − 3.95 m (2024), reflecting cumulative drawdown induced by long-term extraction. Concurrently, the median KCl content decreased from 1.22% (baseline) to 0.90% (2020) and 0.57% (2024), indicating a gradual lowering of brine grade over the observation period. Between 2020 and 2024, additional hydrogeochemical shifts were observed: NaCl increased slightly from 6.39% to 7.08%, LiCl rose from 293 mg/L to 397 mg/L, while MgCl 2 declined from 17.4% to 16.6%, B 2 O 3 from 457 mg/L to 341 mg/L, total dissolved solids (TDS) from 335 g/L to 324 g/L, and specific gravity from 1.24 g/cm 3 to 1.23 g/cm 3 . Spatial contour mapping revealed pronounced heterogeneity in the distribution and temporal evolution of these parameters, potentially linked to the spatial configuration of extraction well fields and recharge pathways. These findings provide critical data support for sustainable resource management and offer new insights into the long-term hydrogeochemical evolution of continental evaporite aquifer systems under anthropogenic stress.