Nitrate (NO3 ?) contamination of surface- and groundwater is an environmental problem in many regions of the world with intensive agriculture and high population densities. Knowledge of the sources of NO3 ? contamination in water is important for better manage- mentof water quality. Stable nitrogen (d15N) andoxygen (d18O) isotope data ofNO3 ? have been frequently used to identify NO3 ? sources in water. This review summarizes typical d15N- and d18O-NO3 ? ranges of known NO3 ? sources, interprets constraints and future outlooks to quantify NO3 ? sources, and describes three analytical techniques (‘‘ion-exchange method’’, ‘‘bacterial denitrification method’’, and ‘‘cadmium reduction method’’) for d15N- and d18 ? O-NO3 ? determination. Isotopic data can provide evidence for the presence of dominantNO3 sources. However, quantification, including uncertainty assessment, is lacking when multiple NO3 ? sources are present. Moreover, fractionation processes are often ignored, but may largely constrain the accuracy of NO3 ? source identification. These problems can be ? overcome if (1) NO3 ? isotopic data are combined with co-migrating discriminators of NO3 sources (e.g. 11B), which are not affected by transformation processes, (2) contributions of different NO3 ? sources can be quantified via linear mixing models (e.g. SIAR), and (3) precise, accurate and high throughput isotope analytical techniques become available.
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