Multi-isotopic and statistical approaches to trace nitrate pollution sources and assess natural attenuation in groundwater: examples from nitrate vulnerable zones in Catalonia (NE Spain)

Abstract

In the last decades, anthropogenic inputs of nitrogen to groundwater have dramatically increased, and they nowadays represent one of the most important water resources concerns as NO3-N has become the most ubiquitous chemical contaminant in the world's aquifers. Agriculture, farming activities and wastewater seepage are the main anthropogenic sources of water contamination in rural areas. Another factor that is known to contribute to the decline of groundwater quality is excessive groundwater withdrawal in relation to the natural average recharge. Intensive groundwater exploitation regimes largely disturb hydrogeological systems modifying natural flow paths, altering relationships between groundwater recharge/discharge areas and modifying the flux among aquifer formations. All these human activities have affected the rates and quality of groundwater resources.

In order to address these issues, the origin, fate and transport of nitrate in groundwater have been extensively studied over the past decades. Stable isotope ratios of NO3 (d15N and d18ONO3), SO42-(d34S and d18OSO4), B (d11B) and C (d13CHCO3) have come to be successful tracers of pollution sources, and useful for assessing physico-chemical processes that affect pollutant fate. Aiming to fingerprint nitrate and sulfate sources and determine whether natural attenuation of pollution is occurring in groundwater, two nitrate vulnerable zones in Catalonia (NE Spain), the Selva and Baix Ter basins, have been studied applying multi-isotope and statistical approaches, in the frame of their hydrogeological settings. Both basins are characterized by regional and local, heterogeneous groundwater flow systems, disturbed by groundwater withdrawal from the different aquifer formations and at distinct rates and frequencies depending on the final water use (mainly for irrigation and for urban and farm supplies).

In accordance with potentiometric, hydrochemical and isotopic data, the hydrogeology of the Selva hydrogeological system has been described in order to characterize the alteration brought about in the system by intensive current groundwater withdrawal, and to define the resulting groundwater hydrodynamics. Hydraulic head data indicate the relationships between geological formations in the range areas and the sedimentary infill of the basin. Tectonic elements, such as fault zones and the basement fracture network, play a significant role in the flow behavior, since they have a direct effect on the recharge and allow upward vertical flow to the aquifers constituted by the sedimentary infilling. The use of fluoride and nitrate as tracers for the contribution of deep and shallow flow systems, respectively, provides a detailed portrait of the effects of pumping on the flow path distribution. Therefore, two distinct flow systems, with specific groundwater qualities, have been identified: a regional, large-scale, longer residence time nitrate-free system, recharged from the surrounding ranges, and a local flow system fed by rainfall infiltration in the lower areas of the basin, and affected by anthropogenic activities. The interaction between both flow systems produces a dilution effect that modifies nitrate concentration. Indeed, hydrochemical data, along with d15N, d18ONO3, d34S, d18OSO4 and d13CHCO3 information, confirmed mixing between regional and local flow systems. The d15N, d18ONO3 and d34S indicated that the predominant sources of contamination in the basin are pig manure and synthetic fertilizers. Apart from dilution processes that can contribute to the decrease of nitrate concentrations, the positive correlation between d15N and d18ONO3 agreed with the occurrence of denitrification processes. The d34S and d18OSO4 indicated that oxidation of pyrites is not linked to denitrification, suggesting organic matter to be an electron donor. However, d13CHCO3 did not point to the occurrence of organic matter oxidation. Thus, it is proposed that the mixing processes between deeper regional and local surface groundwater allow denitrification to occur due to the reducing conditions of the regional groundwater.

Results in the Baix Ter basin show a large range of groundwater nitrate concentrations, from no nitrate to up to 480 mg NO3 L-1. In the studied fluvio-deltaic formations d15N and d18ONO3 prove that natural denitrification is occurring, and in combination with d11B, confirm that pig manure application is the main vector of nitrate pollution, although sewage and mineral fertilizers are also isotopically fingerprinted. The natural reduction of nitrate happens in near­river environments and in areas hydrologically related to fault zones. d34S and d18OSO4 indicate that denitrification is not linked to the pyrite oxidation, but rather to the oxidation of organic matter.

A statistical treatment attending to the compositional nature of available data has been applied using samples from five nitrate vulnerable zones in Catalonia: Baix Ter, Selva, Lluçanès, Maresme and Osona. Three different sets of variables have been used: only geochemical data, only isotope data, or both together. The aims were twofold. First, to establish a graphical comparative tool to discriminate between the different zones affected by nitrate pollution, looking for combinations of logratios of variables that have significantly different average values between the sampled regions. Second: to put forward a statistical methodology that integrates isotope data together with geochemical data. According to these aims, a linear discriminant analysis entering compositional data has been performed and the corresponding discriminant biplot has been depicted. It is remarkable the notable discriminating power when using only the isotope data set, although the optimal separation of regions is achieved when using both geochemical and isotope data subsets, as predicted by the theory of discriminant analysis.

Obtaining all this information can help to understand the mechanisms that control groundwater nitrate contamination, and to evaluate the influence of anthropogenic activities and pressures over the aquifer system at a local as well as regional scale, as a basis for adopting an appropriate water management strategy.

Amb els objectius de traçar les fonts de nitrat i sulfat i de determinar si s'estan produint processos d'atenuació natural a les aigües subterrànies, s'han estudiat dues zones vulnerables a la contaminació per nitrats de Catalunya: les depressions de la Selva i del Baix Ter. Ambdues zones són hidrogeològicament complexes i estan caracteritzades per sistemes de flux regionals i locals afectats per una intensa extracció d'aigua subterrània i múltiples fonts de contaminació. La δ15N, δ18ONO3, δ34S i δ18OSO4 indiquen que la font de contaminació dominant de les aigües subterrànies és el purí aplicat als camps, tot i que els fertilitzants sintètics i les aigües residuals també s'han detectat isotòpicament. La mesura de la relació isotòpica del bor dissolt (δ11B), que no s’havia emprat abans en aquestes zones, confirma que la principal contribució de nitrogen és deguda a l’aplicació dels purins. La correlació positiva entre la δ15N i la δ18O del nitrat està d'acord amb l'existència de processos de desnitrificació natural. La δ34S i la δ18O del sulfat mostren que l'oxidació de pirites no està lligada a la desnitrificació, suggerint la matèria orgànica com el donador d'electrons que permet la reducció del nitrat. Les dades hidroquímiques i isotòpiques apunten que pot haver-hi una relació entre els processos de mescla d'aigua subterrània dels sistemes de flux local i regional i la desnitrificació natural del nitrat, sobretot en àrees associades a zones de falles i xarxes de fractures. Tenint en compte la naturalesa composicional de les dades obtingudes, s'ha aplicat un tractament estadístic apropiat a la hidroquímica i relacions isotòpiques de cinc zones vulnerables de Catalunya (Selva, Baix Ter, Lluçanès, Maresme i Osona) per tal d’establir-ne una comparativa.