Evaluation of surface runoff and road dust as sources of nitrogen using nitrate isotopic composition.

Stable nitrogen and oxygen isotope ratios of nitrate (δ(15)N-NO(3) and δ(18)O-NO(3)) have recently been used to identify nitrogen sources in water environments. However, there have been no investigations designed to determine nitrate isotopes in non-point sources in urban areas for evaluating the impact of surface deposits on nitrogen in surface runoff. In this study, we collected rainwater, surface runoff and surface deposits (road dust, roof dust and soil) to evaluate the nitrogen sources in surface runoff using nitrate isotopes. There were no large differences in δ(15)N-NO(3) among rainwater (-0.3‰ to 1.5‰), surface runoff (-2.7‰ to 0.4‰), leachates from road dust (-5.8‰ to 6.2‰) and soil (-11.5‰ to 0.6‰). In contrast, the δ(18)O-NO(3) in surface runoff (28.5-47.9‰) was lower than that in rainwater (62.7-78.6‰), and higher than that in leachates from road dust (6.1-27.6‰) and soil (-1.1‰ to 6.6‰). δ(18)O-NO(3) is a useful indicator for evaluating the NO(3)-N sources in surface runoff. Using this indicator, NO(3)-N from road dust was estimated to account for more than half of the NO(3)-N in surface runoff. This is consistent with a result based on a comparison of their loads per unit surface between rainwater and surface runoff, which also showed that most of the nitrogen in surface runoff was derived from surface deposits.