Elsevier

Journal of Hydrology

Volumes 414–415, 11 January 2012, Pages 244-254
Journal of Hydrology

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

https://doi.org/10.1016/j.jhydrol.2011.10.037Get rights and content

Summary

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.

Highlights

► Examines sources of nitrate in some of Europe’s highest nitrate groundwaters. ► Measures, rather than assumes isotope composition of potential sources. ► Indicates soil nitrification as the most likely source. ► Examines implications for a country with a long history of cultivation.

Introduction

The Republic of Malta is largely comprised of two islands, Malta (246 km2) and Gozo (67 km2), and with a population of 412,000 is one of the most densely populated countries in the world (DOIM, 2009). 23% of the land is built upon, and 60% of the remaining area is used for agriculture (NSO, 2006). The combined domestic, tourist, industrial and farming activities place strong demands on water supplies, with Malta having one of the world’s highest Water Competition Indexes (population per unit of available water; Mangion et al., 2005). The demand is met predominantly by groundwater abstraction (56%) and seawater desalinization (34%), with more limited use of harvested rainwater runoff and treated sewage effluent (Sapiano et al., 2006). Over half of the water put into public supply is desalinated seawater. But whilst this provides a reliable supply of good quality water, the high cost of desalinization, and its dependence on imported oil, means that groundwater continues to be a vital resource.

The utility of this groundwater resource, however, is being compromised by poor chemical quality: especially seawater intrusion (some parts of aquifers having chloride levels above WHO limits) and high levels of nitrate (MEPA, 2006). Average nitrate concentrations in the main aquifers sampled as part of the present study exceed the 11.3 mg NO3–N L−1 limit imposed by the European Union’s (EU) Nitrate Directive and Water Framework Directive, with nitrate levels in some of the minor groundwater bodies reaching levels more than six times the EU quality standard (EC, 1991, EU, 2000, EEA, 2008). As part of Malta’s programme to address this problem, the British Geological Survey and the Malta Resources Authority conducted a preliminary study on the identification of the sources of nitrate contamination in groundwater in Malta. Geochemical aspects of groundwater movement are reported elsewhere (Stuart et al., 2010); the present work reports on the results of the isotope study of nitrate.

Section snippets

Hydrogeology

A schematic section of the main island of Malta is shown in Fig. 1, and illustrates features of the geohydrology relevant to both Malta and Gozo. The Tertiary geology is essentially made up of two limestone sequences separated by an impermeable clay-marl: the Blue Clay (Pedley et al., 1976; Fig. 1).

Groundwater collection and chemical analysis

In January and March, 2008, 50 groundwater samples were collected from the mean sea level (MSL) aquifers on Malta and Gozo, and from some of the perched aquifers on Malta (Fig. 3, Table 1). Sample locations were selected to represent the different landuse types (agricultural and urban areas) and proximity to potential point sources of pollution (sewer galleries, livestock enclosures). The landuse classifications (Table 1) were based on specific activities known to occur within a 100 m radius of

Nitrate concentrations and historical trends

The nitrate concentrations are listed in Table 1 and summarised in Fig. 2. All three aquifers display similar values for their lowest nitrate concentrations: about 6 to 10 mg NO3–N L−1. These levels are well above those typically thought to represent background concentrations in Europe (less than 2 mg NO3–N L−1, EEA (1999)), and suggest that there may be no part of Malta where the groundwater is unaffected by human activity. The perched aquifers display the largest range of concentrations, to

Principles governing isotope composition of potential sources of nitrate

With the exception of some of the fertilizers, the potential sources of nitrate analysed in this study are in the form of ammonium or organic nitrogen. We therefore consider the probable δ15N and δ18O values of nitrate which might be expected to form from mineralisation and nitrification of these sources on theoretical grounds. The values are compared with those of the groundwater in Fig. 5.

Summary and conclusions

Fertilizers, manure, sewage systems, and soil cultivation are regarded as the main potential sources of high nitrate concentrations in Maltese groundwater. With water chemistry influenced by saline intrusions, however, and a very mixed pattern of human activity scattered over islands of limited land area, it has proved difficult to identify possible nitrate sources by using chemical tracers or examining spatial relationships to specific activities (Stuart et al., 2010). Hence the focus on an

Acknowledgements

We gratefully acknowledge the samples, information, and assistance provided by staff of Malta’s Water Services Corporation, the Ministry for Rural Affairs and the Environment, the Agricultural Services and Rural Development Division, and the Agricultural Cooperative of Malta. We also thank BGS colleagues Carol Arrowsmith, John Chilton, Louise Maurice and Peter Williams for their important contributions to project management, sampling and analytical work. The project was funded by a Technical

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