Sulfur, Nitrogen and Heavy Metals in Headwater Streams of Upper Qinghai Lake, Northeast Qinghai–Tibet Plateau: Possible Pollution by Atmospheric Deposition

To understand the effects of atmospheric sulfur (S), nitrogen (N), and heavy metal (HM) deposition on streamwater chemistry in Buha River (BHR), the upstream of Qinghai Lake (QHL) on Qinghai–Tibet Plateau (QTP), water, mosses, and soil were collected from three representative small watersheds. Mosses had low S contents (0.09 ± 0.05%), slightly high N, zinc (Zn), and lead (Pb) contents (1.36 ± 0.46%, 78.7 ± 19.4 mg·kg–1, and 10.5 ± 0.83 mg·kg–1), and quite high copper (Cu) contents (27.3 ± 4.65 mg·kg–1), compared with the global background values. The δ15N in mosses and soil Pb isotopic composition revealed the contribution of coal combustion to soil pollution via atmospheric deposition. The average SO42– and NO3– concentrations of streamwater were 21.9 and 1.74 mg·L–1. Elevated concentrations and increasing ratios of SO42–/Cl– and NO3–/Cl– compared with historical levels indicated a significant impact of atmospheric deposition. However, the average Zn, Pb, and Cu concentrations in streamwater were 6.99 ± 2.36, 0.52 ± 0.87, and 2.22 ± 1.50 μg·L–1, respectively, with very limited effects of HM deposition. The high pH of streamwater provided by the chemical weathering of carbonate rocks prevented water acidification and transport of HMs from soil to surface waters. Given QTP's ecological fragility and potential climate changes, the significance of long-distance atmospheric transportation to surface water still mattered.