Natural and Induced Nitrate Attenuation Processes in Pétrola Basin (Albacete, Spain)

Abstract

One of the main problems of diffused groundwater contamination is produced by nitrate (NO3‾). NO3‾ pollution is linked with fertilizer application, manure management and wastewater. Many areas of Spain have NO3‾ concentration over the human consumption (0.8 mM NO3‾) established by the Directive 98/83/CE. Endorheic basins located in semiarid or arid regions constitute one of the most vulnerable environments to NO3‾ pollution. The Petrola basin (Central Spain) is an outstanding example of these endorheic systems. Nitrate concentration in surface and groundwater from Petrola basin ranged from <1.6x10-3 mM to 2.5 mM. The observed variation in NO3‾ concentration could be linked with naturalattenuation. In this basin two different sediments are able to produce denitrification: 1) the organic and sulphide-rich sediment from the Utrillas Facies (Lower Cretaceous) and 2) the bottom lake sediment. Laboratory experiments coupled with field data can be applied to improve the understanding of the nitrogen cycle in Petrola basin.

The study of Cretaceous Utrillas Facies and bottom lake sediments in laboratory experiments combining multi-isotopic techniques and chemical data showed that natural attenuation produced by both materials was mainly linked with the oxidation of the organic matter. In the case of Utrillas Facies, isotopic values of δ34SSO4 and δ18OSO4 ruled out pyrite as electron donor. Bottom lake sediments showed higher potential to remove NO3‾ than the Utrillas Facies. Important differences between both sediments were observed in the amount of reactive organic carbon, and in the nitrogen reduction rate. These differences were linked to the variation in the reactivity of the different organic carbon sources. Despite the remarkable differences, similar isotopic fractionation of N and O during denitrification for the Utrillas Facies and the bottom lake sediment were observed. The Utrillas Facies displayed values of ‾11.6‰ and ‾15.7‰ for εN and ‾12.1‰ and ‾13.8‰ for εO whereas denitrification produced by the bottom lake sediments showed εN of ‾14.7‰ and εO of ‾14.5‰.

The hydrogeological study showed that groundwater flow in the Petrola basin can be considered as the result of two main flow components: a regional groundwater flow, from the recharge areas (Zone 1) to the discharge ones located near the lake (Zone 2) and the density driven flow from surface water from the lake towards the underlying aquifer (Zone 3). Isotopic results from the field showed that denitrification was taking place in the basin although it was restricted to the surrounding area (Zone 2) and under the lake (Zone 3). In addition, the using the isotopic fractionations obtained in the laboratory experiments, the percentage of NO3 ‾ attenuation was calculated. NO3‾ removed ranged from 0% to 60% with an average percentage of 20%. The degree of denitrification observed at field scale showed an important spatial variability related with the amount, availability and reactivity of labile organic matter as well as spatial variation of NO3‾ supply. Attenuation processes by heterotrophic denitrification reactions were more likely to occur in zones of the saline lake-aquifer system. Density downflow from surface water effectively transported the DOC to aquifer zones under reducing conditions; however, its influence decreased with depth becoming negligible in the deepest part of the aquifer.

As consequence of the incomplete NO3‾ attenuation observed in groundwater of Petrola basin a biostimulation treatment was proposed. Periodically injecting an organic carbon source might be a suitable method to remove NO3‾ from groundwater during long-term treatments. The induced attenuation experiment showed that the injection of glucose enhanced denitrification. Remarkable presence of NO2‾ was produced during the first days due to enzymatic repression of nitrite reductase as well as the pH conditions of the system. Changes in C/N ratio to stoichiometric or lower ratio did not further affect the achieved complete NO3‾ attenuation because secondary carbon sources were available in the system (DOC in the input water and/or biofilms). The isotopic fractionation values of N and O during induced denitrification were ‾8.8‰ and ‾8.0‰ respectively.

Esta tesis se centra en la descripción y cuantificación de los procesos de atenuación natural del nitrato en la cuenca endorreica de Pétrola. Para ello se realizan diversos experimentos en columna y batch con sedimentos de la cuenca para determinar el potencial de atenuación de todos ellos caracterizándolos, química e isotópicamente. Los resultados obtenidos permitieron evaluar ambos sedimentos como una fuente de electrones capaz de producir procesos de atenuación mediante desnitrificación heterótrofa. A continuación los procesos y conocer la extensión de los mismos. La atenuación producida en la cuenca llega hasta el 60% pero la media es tan solo de un 20%. Mediante el estudio hidrogeológico se identifican las zonas de la cuenca donde se está produciendo la atenuación así como el papel de los sedimentos estudiados. Finalmente, como la atenuación natural no consigue la completa eliminación del nitrato, se propone un tratamiento de remediación, realizando un primer ensayo de bioestimulación periódica en columna utilizando glucosa. Los resultados obtenidos mostraron como un tratamiento viable utilizando una cantidad de glucosa por debajo de la estequiométrica debido a los fuentes secundarias de electrones presentes en el sistema.