“…Trace element concentrations (e.g., As, Cr, Mo, Pb, Sb, Se, Sr, Tl, and U)combined with 87 Sr/ 86 Sr ratios provide powerful discriminatory leverage for distinguishing among the variety of natural and anthropogenic solute sources to a river, including inputs from weathering, soil erosion, groundwater, mining, agriculture, and urban runoff (Barats et al, 2014;Christofaro and Leao, 2009;Fitzpatrick et al, 2007;Le Pape et al, 2012Luo et al, 2014;Ollivier et al, 2011;Potot et al, 2012;Xu and Han, 2009).For example, Sr is an important trace element for evaluating natural weathering processes in a watershed, and 87 Sr/ 86 Sr ratios can be used to distinguish between silicate and carbonate weathering (Bickle et al, 2005;Brennan et al, 2014;Chetelat et al, 2008;Jacobson et al, 2002;Voss et al, 2014;Wei et al, 2013). Furthermore, weathering of specific types of bedrock such as black shales may contribute other elements, including Se and U (Bates, 1957;Galindo et al, 2007;Lavergren et al, 2009a;Rawat et al, 2010;Stillings and Amacher, 2010;Tuttle et al, 2014aTuttle et al, , 2014bWen and Carignan, 2011).…”