Isotope and microbiome data provide complementary information to identify natural nitrate attenuation processes in groundwater

Abstract

Natural attenuation processes alleviate the impact of fertilization practices on groundwater resources. Therefore, identifying the occurrence of denitrification has become a requirement for water quality management. Several approaches are useful for this purpose, such as isotopic and microbiological methods, each of them providing distinct but complementary information about denitrification reactions, attenuation rates and their occurrence in the aquifer. In this paper, we investigate the contribution of both approaches to describe denitrification in a consolidated rock aquifer (limestone and marls), with a porosity related to fracture networks located in the northeastern sector of the Osona basin (NE Spain). Isotopic methods indicated the origin of nitrate (fertilization using manure) and that denitrification occurred, reaching a reduction of near 25% of the nitrate mass in groundwater. The studied area could be divided in two zones with distinct agricultural pressures and, consequently, nitrate concentrations in groundwater. Denitrification occurred in both zones and at different levels, indicating that attenuation processes took place all along the whole hydrogeological unit, and that the observed levels could be attributed to a larger flow path or, in a minor extent, to mixing processes that mask the actual denitrification rates. Microbiological data showed a correlation between denitrifier genes and the isotopic composition. However, the groundwater microbiome and the distribution of denitrifying bacteria did not reveal a major influence on the denitrification level observed by isotopic methods. This focuses the interest of microbiological analysis to identify functional genes within the bacteria present in the aquifer. Results indicated that isotopic methods provide information of the overall denitrification ability of the hydrogeological unit, and that genomic data represent the processes actually acting nearby the well. A combination of both approaches is advised to support induced in situ attenuation actions in polluted sites.

An integrated assessment of nitrogen source, transformation and fate within an intensive dairy system to inform management change

“…In Sep 2014 NO 3 - -N < 5.65 was evident for all locations (Fig 5). In terms of N source groundwater NO 3 - -N isotopic composition clustered within the manure/sewage value range and along a 1:1–1:2 slope, suggesting a common organic source for these, and denitrification as the main biotransformation process (Fig 5) [41–43]. The denitrification lines for 2014 and 2017 samples share the same trend, which points to denitrification as the main process leading N attenuation.…”

Side Effects of Pesticides and Metabolites in Groundwater: Impact on Denitrification

“…As the first step (nitrate reduction into nitrite) of the denitrification pathway was shown to be mainly impacted, and in order to determine if the proportion of the denitrifying community changed among the microbial community, the relative abundance of narG and napA genes, both encoding nitrate reductase, was measured using qPCR approaches ( Figure 5 ). Data were normalized by taking into account molecular biomass (total DNA) ( Bru et al, 2007 ; Wakelin et al, 2011 ; Hernández-del Amo et al, 2018 ). The presence of pesticides or metabolites did not significantly modify the relative abundance of both genes even in conditions for which nitrate reduction rate was decreased (in particular ESA-metolachlor at 10 μg/L), suggesting an impact (inhibition) of pesticides and metabolites at the protein level rather than on the nitrate-reducing bacteria abundance within the bacterial community.…”

Strategies to Overcome Intermediate Accumulation During in situ Nitrate Remediation in Groundwater by Hydrogenotrophic Denitrification

“…The complementation with H 2 injection to stimulate hydrogenotrophic denitrifiers is therefore necessary and the effect of Fe(II)-containing minerals on the physiological characteristics of the underlying bacteria must also be examined. In this regard, Fe(II)-containing minerals may also be used as electron donors by numerous autotrophic denitrifiers (Otero et al, 2009;Jones et al, 2015;Hernández-del Amo et al, 2018;Margalef-Marti et al, 2020). As a result the abiotic reduction of NO 2 − with Fe(II) may occur alongside microbial denitrification, the two processes are even interconnected (Melton et al, 2014) and may catalyze NO 3 − remediation.…”