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It has been accepted that variations in stable N isotope ratios (15N/14N) can potentially provide useful, sometimes unique, information on the sources of nitrate in groundwater, because N isotope compositions are generally different among various N pools such as atmospheric N2, soil, chemical fertilizer, and manure. However, this review strongly suggests that interpretation of the nitrate source based on a single measurement of δ15N may not be accurate because the isotopic composition of groundwater nitrate is a function of its source and any isotope fractionation that may occur during its generation or transport to groundwater. This review, therefore, shows that to evaluate the nitrate contamination source using δ15N technique, it is more reasonable to use correlation between the concentrations and δ15N of NO3 −. For example, a positive correlation indicates15N-enriched source such as manure, while15N-depleted source results in a negative correlation. The correlation analysis can also be used in evaluation of denitrification and nitrification. Denitrification is a key process affecting δ15N of NO3 −, i.e. if significant amount of nitrate is removed through denitrification, it is not easy to discriminate fertilizer-derived nitrate from manure-or compost-derived one because denitrification increase the δ15N of the remaining NO3 −. Therefore, it is important to assess whether denitrification may be occurring or not when evaluating isotope data on nitrate. By using dual (δ15N air and δ18O vsmow ) isotopic indicators, denitrification can be evaluated more accurately. [ABSTRACT FROM AUTHOR]

Abstract: An understanding of the long-term changes in the nitrate contamination pattern of unconfined groundwater is critical to conservation of drinking water in rural areas supporting mixed land-use activities such as cropping, livestock farming, and residence. To examine the effect of different land-use activities on nitrate contamination, groundwater samples were collected monthly for 3 years (1997–1999) from 12 wells in rural areas with different land-use activities and analyzed for the concentrations and N isotopic ratios (δ15N) of nitrate. The characteristics of nitrate contamination clearly differed with land-use activities. The percentages of samples that had a nitrate concentration exceeding the national standard for drinking water (10mgNL−1) were 0, 23, 43, and 67% for the uncontaminated natural area, cropping area, cropping-livestock farming complex area, and residential area, respectively. The range of δ15N values was between +1.4 and +4.5‰ for groundwater nitrate from the uncontaminated natural area. In the cropping area, the δ15N values were slightly different with the type of fertilizer applied to fields in the vicinity of the well, and the values ranged between +8.7 and +14.4‰ for the compost-applied area and between +4.5 and +8.5‰ for the area where urea was applied with compost. The δ15N values of the cropping-livestock farming complex area ranged from +1.0 to +17.7‰, probably resulting from mixed contamination sources (inorganic fertilizer and livestock manure). The well located closest to the livestock feedlot had relatively higher δ15N values, with a range between +8.7 and +17.6‰. In the residential area, a higher δ15N (most frequently above +10‰) of nitrate suggested that the major source of contamination was effluent from leaky septic tanks. Our data showed that unconfined groundwater is susceptible to land-use activities above the aquifers, and the impacts of the activities could be more precisely inferred from long-term data on the concentration and δ15N of nitrate. By determining the impacts, more effective (specific to contamination sources) measures for preventing groundwater quality could be implemented. [Copyright &y& Elsevier]