Constraining the groundwater flow system and aquifer properties using major ions, environmental traces and a simple physical model in China’s Jilantai Basin

Abstract

This study uses major ions and environment isotopes (87Sr/86Sr, δ13C and 14C) together with a proposed physical model to constrain hydrogeochemical processes, groundwater ages, recharge rates and physical parameters in China’s Jilantai Basin. The major processes are identified using major ions, which include calcite dissolution and the exchange of Na for Ca in upstream areas and gypsum dissolution, carbonate-incongruent dissolution and silicate weathering in downstream areas. Sr and major ions indicate the dissolution of carbonate minerals or gypsum and silicate weathering. 87Sr/86Sr variations indicate that shallow groundwater in discharge areas consists of more than 90 % groundwater from rainfall in the Ulan Buh Desert (UBD), and middle groundwater comes entirely from the Helan Mountains. The major ions, 87Sr/86Sr ratio and physical model reflect that inter-aquifer mixing has occurred between shallow unconfined and upper confined aquifers, but not in middle confined formations. The 14C ages range from the present to 15 ka, and the distribution of 14C residence times indicate that inter-aquifer flow occurs on a regional scale and over the long term between shallow unconfined and upper confined aquifers. The travel times using the physical model proposed in this paper are comparable to the 14C ages, indicating that this model is suitable for calculating travel times for the middle confined aquifers that have effective aquitards. The vertical leakage rates are 0.01–0.03 m/day, and the long-term recharge rates are 15 mm/year (approximately 5 % of annual rainfall). The groundwater from this region is a locally valuable resource, and the correct understanding of groundwater flow systems, inter-aquifer mixing and recharge rates is critical to managing these groundwater resources.