Factors characterizing phosphate oxygen isotope ratios in river water: an inter-watershed comparison approach

Abstract

We compared the oxygen isotope ratio of dissolved phosphate $$\left( {\delta^{{{18}}} {\text{O}}_{{{\text{PO}}_{{4}} }} } \right)$$ in two rivers with different land-cover and geological features (Ado River and Yasu River) within Lake Biwa basin, central Japan, to explore what factor primarily characterizes the $$\delta^{{{18}}} {\text{O}}_{{{\text{PO}}_{{4}} }}$$ . Mean values of $$\delta^{{{18}}} {\text{O}}_{{{\text{PO}}_{{4}} }}$$ in river water were 19.0 ± 2.4‰ (n = 7) in Ado River and 13.1 ± 2.3‰ (n = 15) in Yasu River, which were significantly different. Comparisons of $$\delta^{{{18}}} {\text{O}}_{{{\text{PO}}_{{4}} }}$$ between river water and potential sources of phosphate revealed that in the Ado River, the $$\delta^{{{18}}} {\text{O}}_{{{\text{PO}}_{{4}} }}$$ was similar to that in rocks from the accretionary complex and decreased with increasing sedimentary rock coverage. In the Yasu River, the $$\delta^{{{18}}} {\text{O}}_{{{\text{PO}}_{{4}} }}$$ was low in the upper forested areas, but increased with paddy field coverage. These results demonstrate that river $$\delta^{{{18}}} {\text{O}}_{{{\text{PO}}_{{4}} }}$$ strongly reflects inputs from geological substances, but is also impacted by land-use activities and varies with anthropogenic land coverage in the watershed. Thus, river $$\delta^{{{18}}} {\text{O}}_{{{\text{PO}}_{{4}} }}$$ relates to land or bedrock coverage differentially in each river. Regression analysis showed that residuals of the $$\delta^{{{18}}} {\text{O}}_{{{\text{PO}}_{{4}} }}$$ tended to converge to zero with increasing drainage area, suggesting that river $$\delta^{{{18}}} {\text{O}}_{{{\text{PO}}_{{4}} }}$$ more explicitly reflects land-cover and geological features on a larger watershed scale.