Phosphate distribution and sources in the waters of Huangbai River, China: using oxygen isotope composition of phosphate as a tracer

We investigated the distributions of phosphate (PO₄-P) and used the oxygen isotope composition of phosphate (δ¹⁸O_P) to quantify PO₄-P sources in the waters of Huangbai River. According to the environmental characteristics of Huangbai River basin, the sampling stations in the Huangbai River were divided into three groups: sampling stations in the phosphate mining area, in the outcrop area of phosphate rock, and in the residential/agricultural area. The average PO₄-P concentration was highest (2.34 ± 1.00 μmol/L) in the outcrop area of phosphate ore, intermediate in the residential/agricultural area (1.06 ± 1.21 μmol/L), and lowest in the phosphate mining area (0.58 ± 0.31 μmol/L). The δ¹⁸O_P measured in the Huangbai River waters ranged from 6.0 to 20.9‰, with the highest average value in the outcrop area of phosphate rock (average: 14.6‰ ± 3.1‰). The majority of the measured δ¹⁸O_P values in the Huangbai River deviated greatly from the expected equilibrium values, indicating that δ¹⁸O_P in this area could be used to trace PO₄-P sources. We used two end-member mixing models to quantify the contribution of PO₄-P from different sources. In the phosphate mining area, the average fractions of PO₄-P from phosphate ore and sewage were 49.5% ± 23.8% and 50.5% ± 23.8%, respectively. In the outcrop area of phosphate rock, the average fractions of PO₄-P from phosphate ore and sewage were 60.1% ± 21.7% and 39.9% ± 21.7%, respectively. In the residential/agricultural area, the average fractions of PO4-P from fertilizer and sewage were 49.2% ± 23.2% and 50.8% ± 23.2%, respectively. These results indicate that phosphate mining activities was not an important source for PO₄-P in the waters of Huangbai River. The natural weathering of phosphate rock, fertilization, and domestic sewage contributed more to the high PO₄-P concentrations in the Huangbai River waters.