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104 Clemente Recio and high N in drinking water has been linked to diseases such as metheglobinemia and cancer (Vigil et al., 1965). The issue is widely acknowledged by international organizations. In Europe, the “Nitrates Directive”, 91/676/EEC, establishes mandatory maximum limits of 50 mg/l NO - 3 (11,3 mg/l N, although it recommends not to exceed 25 mg/l NO - 3 or its equivalent 5,6 mg/l N, as established in the “Drinking Water Directive”, 80/778/EEC). These are very similar values to those recommended by the WHO 1 , and the ones currently adopted by the US's EPA. Process Reaction , fractionation factor N 2 fixation N 2 NH 4 + organic N 0.991 to 1.0041 N 2 O reduction N 2 O NH 4 + organic N 1.00343 Denitrification NO 3 - N 2 O 1.028; 1.033 NO 3 - assimilation NH 4 + assimilation NO 2 - assimilation Nitrification NH 4 + NO 2 - Diffusion NO 3 - NH 4 + organic N 1.0027 to 1.03 NH 4 + organic N 1.0091 to 1.02 NO 2 - NH 4 + organic N 1.007 NH 4 + , NH 3 , or NO 3 - in solution 1.025 to 1.035 ~ 1.00 NH 3 diffusion in gas phase 1.018 Table 4.- Nitrogen isotope fractionations during biologically-related reactions.From Handley and Raven (1992). Fractionation factors are composites from a variety of organisms cultured in laboratory or field experiments. Transcription of the European Directive to Spanish national legislation (R.D. 261/1996, 16 February; B.O.E. # 61, 11/3/1996), keeps the concentration of nitrate mentioned, and includes the publication of the so called “Código de Buenas Prácticas Agrarias” (Good Agricultural Practice Code), that, among other things, regulates fertilizer application (quantity and season) and density of livestock per surface area, such that the nitrogen load is always lower than 170 KgHa -1 yr -1 . This represents between 367 1 The WHO set an upper limit of 10 mg/l of N derived from NO 3 - , and 1 mg/l N derived from NO 2 - (equivalent to 44,3 and 3,3 mg/l of the respective ion in drinking water; World Health Organization, 1984, Guidelines for Drinking-Water Quality; WHO, Geneva)
Alike as two water drops: distinguishing one source of the same substance from another 105 and 2125 Kg/Ha/year of chemical fertilizer (depending on its nature; Orús et al., 2000), or a livestock density that varies between 580 hens and 2 cows per Ha and year (Regulation CEE 2092/91). It is therefore evident from the above that control instruments must be sought in the search for possible links between source and sink. But cause-effect relationships are not enough; it is necessary to search for parameters that allow a firm association between source and pollutant, as well as quantifying relative contributions in the case of multiple sources. Classical chemical methods can easily determine nitrate in water; however, they are not suited to identify the source of pollution. Whatever its origin, the different forms of soil N are subject to transformations that mask its origin. The stable isotopic ratios D/H, 13 C/ 12 C, 15 N/ 14 N, 18 O/ 16 O and 34 S/ 32 S, however, can fulfil the requirements. Different substances have characteristic isotopic ratios, that allow not only identification, but also quantification of individual pollutants (Kendall and McDonnell, 1998). Natural soil N derives ultimately from the atmosphere via biological fixation. Except in very particular locations, the geological materials are not a significant source of N. Anthropogenic nitrate or ammonium, however, can locally or regionally represent a noticeable contribution. Atmospheric N 2 is also used to produce NH 3 by the Haber method, and this is the base to synthesise chemical fertilizers. It is estimated that by the year 2000, the amount of fertilizer-N already equalled the amount of N fixed in organic matter by natural processes. Once incorporated into the soil, organic matter is employed as substrate by multiple microbial reactions. Nitrates are highly soluble; they are not as reactive with organic complexes as ammonia, and can be easily transported by water percolating through the soil. Each of the steps described has the potential for fractionating N isotopes. Except fixation and assimilation as a nutrient, other processes can result in fractionations between 10 to 30‰ (Table 4), or even larger (the product depleted in 15 N with respect to the reagent), as observed after fertilizer application. Nitrate 15 N is useful in tracing its provenance (for example, Aravena et al., 1993, Durka et al., 1994; Wassernaar, 1995; Ging et al., 1996; Kendall et al., 1997). However,
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SEMINARIOS DE LA SOCIEDAD ESPAÑOLA
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Page 82 and 83: 84 Clemente Recio Isotope Terminolo
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Page 88 and 89: 90 Clemente Recio beers. Some brand
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Page 92 and 93: 94 Clemente Recio By using GC-C-IRM
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