Geochemical and isotopic characteristics of shallow groundwater within the Lake Qinghai catchment, NE Tibetan Plateau

Major ions, isotopic ratios of strontium (⁸⁷Sr/⁸⁶Sr), hydrogen (δD) and oxygen (δ¹⁸O) of groundwater samples were analyzed to decipher spatial variation, controlling factors, solute sources, and the rechargesource of shallow groundwaters within the Lake Qinghai catchment. Shallow groundwaters in this area are slightly alkaline, with 97% being fresh water of good quality, though there are high concentrations of nitrate and sulfide in Buha and lakeside groundwaters. Most of the shallow groundwatersare of the Ca²⁺-HCO₃^- type, whereas part of groundwaters surrounding the lake (LS) belongs to the Na⁺-Cl^- type as lake water (QHL). Groundwater geochemistry is controlled by regional lithological association, ion exchange, and mineral precipitation. The dissolved Sr concentrations and ⁸⁷Sr/⁸⁶Sr ratios vary from 1.0 to 15.6μmol/L, and from 0.709859 to 0.715779, respectively. The first quantitative calculation in groundwater using a forward model shows that 40% of dissolved Sr is from carbonate weathering, 33% from evaporite dissolution, 17% from silicate weathering, and the remainder from atmospheric input for the whole catchment. Carbonate weathering dominates groundwater geochemistry in Shaliu (SL), Hargai (HG) and Buha (BH) samples, while evaporite dissolution dominates LS and Daotang (DT) samples. δD and δ¹⁸O data show that rain water is the major recharge source of both river water and shallow groundwater within the Lake Qinghai catchment. Qinghai Lake water is characterized by Buha-type water, but its Sr geochemistry is different from the shallow groundwater due to carbonate precipitation. Although shallow groundwater contributes ~5% of the dissolved Sr to the QHL, groundwater must be taken into account when the chemistry and budget of lake water are characterized.