『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