Heaton,T.H.E., Stuart,M.E., Sapiano,M. and Sultana,M.M.(2012): An isotope study of the sources of nitrate in Malta's groundwater. Journal of Hydrology, 414-415, 244-254.

『マルタの地下水中の硝酸塩源の同位体研究』


Abstract
 Levels of nitrate in Malta's groundwater are high. Median concentrations in the main sea-level aquifers of Malta and Gozo are 14 and 10 mg NO3-N L-1, respectively, and even higher in the younger groundwaters of the perched aquifers on Malta (37 mg NO3-N L-1). The wide variations in groundwater nitrate concentration are not due to denitrification, as 15N/14N and 18O/16O analyses of 47 samples from the three aquifer types found clear evidence for this process in only one sample. 90% of the groundwater nitrate samples had δ18O values in the range +3.1 to +6.1‰, which correspond exactly to those expected for nitrate formed by microbial processes in the presence of Maltese surficial waters (δ18O of H2O typically -5.3 to -4.3‰). The δ15N values of these groundwater nitrate samples, +7.7 to +11.7‰, were compared with those of a wide variety of potential nitrate sources in Malta (fertilizers, sewage, manure and soils). The closest correspondence was found for the organic N in cultivated soils (+6.0 to +11.2‰). These rerelativeky high δ15N values for soils may reflect greater fluxes of N from soils with a low C/N ratio and a long history of cultivation. While the isotope data support soil nitrification as the source for nitrate in the groundwaters, they do not rule out direct leaching of manure-derived nitrate as a source.

Keywords: Groundwater; Nitrate; Isotope; Soil; Sewage; Manure』

1. Introduction
2. Malta's groundwater
 2.1. Hydrogeology
  2.1.1. Perched aquifers
  2.1.2. Mean sea level (MSL) aquifers
 2.2. Groundwater residence time and chemistry
  2.2.1. Perched aquifers
  2.2.2. Mean sea level (MSL) aquifers
 2.3. Nitrate
3. Methods
 3.1. Groundwater collection and chemical analysis
 3.2. Sewage and animal wastes, fertilizers, and soils
 3.3. Preparation for isotope analysis
 3.4. Isotope analyses
4. Results
 4.1. Nitrate concentrations and historical trends
 4.2. Isotope compositions
5. Discussion
 5.1. Principles governing isotope composition of potential sources of nitrate
  5.1.1. δ15N values of potential sources
  5.1.2. δ18O values of potential sources
 5.2. Potential sources
  5.2.1. Fertilizer
  5.2.2. Sewage
  5.2.3. Manure
  5.2.4. Soils
 5.3. Sources of groundwater nitrate
6. Summary and conclusions
Acknowledgements
References


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