An isotope study of the sources of nitrate in Malta’s groundwater

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 relatively 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.