Combination of nitrate (N, O) and boron isotopic ratios with microbiological indicators for the determination of nitrate sources in karstic groundwater

International audienceA new approach based on measurements of nitrate and boron isotopic composition associated with microbiological indicators for the determination of nitrate origin in karstic groundwater (SW, France) is presented. Nitrate and boron isotopic data indicate an animal source of nitrate (delta N-15-NO3- > 5 parts per thousand, delta O-18-NO3- < 10 parts per thousand and delta B-11 similar to 25 parts per thousand). Microorganism detection (bacteriophages) confirmed contamination from animal sources and proved fast water transfer (2-3 days) from surface to groundwater

Model selection to improve multiple imputation for handling high rate missingness in a water quality dataset

In the current era of “information everywhere”, extracting knowledge from a great amount of data is increasingly acknowledged as a promising channel for providing relevant insights to decision makers. One key issue encountered may be the poor quality of the raw data, particularly due to the high missingness, that may affect the quality and the relevance of the results’ interpretation. Automating the exploration of the underlying data with powerful methods, allowing to handle missingness and then perform a learning process to discover relevant knowledge, can then be considered as a successful strategy for systems’ monitoring. Within the context of water quality analysis, the aim of the present study is to propose a robust method for selecting the best algorithm to combine with MICE (Multivariate Imputations by Chained Equations) in order to handle multiple relationships between a high amount of features of interest (more than 200) concerned with a high rate of missingness (more than 80%). The main contribution is to improve MICE, taking advantage of the ability of Machine Learning algorithms to address complex relation- ships among a large number of parameters. The competing methods that are implemented are Random Forest (RF), Boosted Regression Trees (BRT), K-Nearest Neighbors (KNN) and Support Vector Regression (SVR). The obtained results show that the hybridization of MICE with SVR, KNN, RF and BRT performs better than the original MICE taken alone. Furthermore, MICE-SVR gives a good trade-off in terms of performance and computing time

Upscaling nitrogen removal capacity from local hotspots to low stream orders’ drainage basins

International audienceDenitrification is the main process removing nitrate in river drainage basins and buffer input from agricultural land and limits aquatic ecosystem pollution. However, the identification of denitrification hotspots (for example, riparian zones), their role in a landscape context and the evolution oftheir overall removal capacity at the drainage basin scale are still challenging. The main approaches used (that is, mass balance method, denitrification proxies, and potential wetted areas) suffer from methodological drawbacks. We review these approaches and the key frameworks that have been proposed to date to formalize the understanding of the mechanisms driving denitrification: (i) Diffusion versus advection pathways of nitrate transfer, (ii) the biogeochemical hotspot, and (iii) the Damköhler ratio. Based on these frameworks, we propose to use high-resolution mapping of catchment topography and landscape pattern to define both potential denitrification sites and the dynamic hydrologic modeling at a similar spatial scale (<10 km2). It would allow the quantification of cumulative denitrification activity at the small catchment scale, using spatially distributed Damköhler and Peclet numbers and biogeochemical proxies. Integration of existing frameworks with new tools and methods offers the potential for significant breakthroughs in the quantification and modeling of denitrification in small drainage basins. This can provide a basis for improved protection and restoration of surface water and groundwater quality

Geochemical identity of pre-Dogon and Dogon populations at Bandiagara (Mali, 11th–20th cent. AD)

Bone geochemistry of pre-Dogon (11th–16th cent. AD) and Dogon (17th–20th cent. AD) populations buried in two caves of the Bandiagara Cliff (Mali) was examined for the purpose of exploring their diet and mobility. While the Dogon were the subject of extensive ethnographic studies, the lifestyle of the pre-Dogon, so-called “Tellem” is not known. We therefore compared the geochemical composition of Dogon bones with the results obtained from modern dietary surveys in Mali, to establish the parameters of a dietary model that was further applied to the pre-Dogon in order to expand our knowledge concerning their way of life. The exceptional preservation of the bones of both populations was confirmed not only at the macroscopic scale, but also at the mineralogical, histological and geochemical levels, which resemble those of fresh bones, and therefore offered ideal conditions for testing this approach. 15The application of the Bayesian mixing model FRUITS, based on bone δ13C (apatite and collagen) and bone δ N values, suggested a dietary continuity through time, from the 11th century to today. Bone barium (Ba) content revealed very restricted mobility within the Cliff while bone δ18O values indicated that Pre-Dogon and Dogon most likely occupied the Bandiagara Plateau and the Cliff, respectively

Utilisation du traçage isotopique naturel pour caractériser et quantifier les processus de nitrification et de dénitrification à l'échelle du réseau hydrografique de la Seine

PARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Sci.Terre recherche (751052114) / SudocSudocFranceF

Denitrification in a hyporheic riparian zone controlled by river regulation in the Seine river basin (France)

The purpose of this paper is to study denitrification and the conditions for its development in a hyporheic zone. The study site is the riparian zone of a former branch of the Seine River, where the river stage is kept almost constant during the year by hydraulic regulation. Hydrological and geochemical surveys were performed by monitoring four wells, ten shorter piezometers and the river over a 15-month period. The water fluxes originating from the chalky hillsides and the river converge in a zone parallel to the river that acts as a drainage flow path through the floodplain. The riparian zone between this flow path and the river shows an important depletion of nitrate during the summer and autumn period, which cannot be explained by a simple mixing of waters coming from the river and the chalky hillsides. It can be attributed to denitrification as it occurs when oxygen concentration is below 2 mg l1, and goes along with a consumption of dissolved organic carbon and a decrease of redox potential. The river completely controls these hydro-geochemical conditions. It also keeps the wetness of the riparian zone almost constant, which allowed us to isolate the high temperatures in summer and autumn as an important triggering factor for denitrification through its influence on the reaction rate and oxygen deficits. We also found a small isotopic enrichment of nitrate, suggesting that denitrification occurs after diffusion of nitrate through the sediment and riparian zone matrix, which is consistent with the hyporheic functioning of the study site

Transformations of nutrients (N, P, Si) in the turbidity maximum zone of the Seine estuary and export to the sea

International audienceNutrients (N, P, Si) were studied in the turbidity maximum zone (TMZ) of the Seine estuary at various moments of tidal cycles from April to October in 2001, 2002 and 2003, covering a salinity gradient from 0 to 27. Contrasted hydrological conditions were observed (extremely wet in 2001, unusually dry in 2003). Nutrient fluxes were analysed in terms of an indicator of coastal eutrophication potential (ICEP) using nutrient stoichiometry. Besides these three years, nutrient fluxes and corresponding ICEP values were analysed for a 17-year period, from 1990 to 2006. N, P and Si forms did not show ample transformations in the salinity gradient, revealing that the estuary plays a limited buffer role at the interface between river and coastal zone, however not inconsiderable during dry hydrology when longer water and particle residence times are compatible with the biological growth rates. The simplified LIFT model of the TMZ, already used for analysing organic matter transformation, was implemented here to simulate the nutrients' behaviour in the salinity gradient and to quantify nutrient fluxes after their transit in the TMZ. Abatements ranging from 15% to 40% of incoming N and P were calculated, without significant changes for silica. Although P-ICEP values did not show a severe risk for coastal eutrophication for the past few years, contrary to the situation from 1970 to 2000, the high N-ICEP, due to a steady use of fertilisers in agriculture, is a real threat for eutrophication and harmful algal blooms

Approches isotopiques intégrées du processus de dénitrification

La dénitrification, processus de réduction du nitrate (NO3-) en diazote (N2), est considérée comme un processus d épuration des milieux. Néanmoins, certains des intermédiaires réactionnels, tels que le nitrite (NO2-) et le protoxyde d azote (N2O), lorsqu ils s accumulent, entraînent des effets néfastes sur l environnement. Afin de limiter leur présence, résultant très largement des activités anthropiques, il est nécessaire de mieux comprendre les processus qui les produisent et les réduisent. Ce travail a consisté à tester, avec une approche couplée, les mesures des isotopomères du N2O, des d15N et d18O du N2O et du d15N des NO2- afin de tracer le processus de dénitrification dans son entièreté. L originalité de ce travail repose sur la confrontation d expérimentations menées en laboratoire avec des mesures in-situ. Les mesures in-situ réalisées sur des sites connus pour dénitrifier ont mis en évidence que la mesure seule des abondances isotopiques naturelles ne permet pas d aboutir à la caractérisation de l origine du N2O, si celle-ci n est pas couplée à la détermination des valeurs de SP-N2O. Des expérimentations réalisées sur des communautés bactériennes en milieu contrôlé, ont permis d expliquer la présence de valeurs élevées de SP-N2O dans l environnement. Ainsi, ces valeurs de SP-N2O peuvent résulter (1) d une part de N2O produit par la nitrification ou (2) d autre part, de N2O en cours de réduction en N2 lors de la dénitrification. Enfin, l'étude de l impact des valeurs de d15N du NO2- sur celles du e15N associé à la dénitrification a mis en évidence l'importance de considérer la dénitrification comme un processus à plusieurs étapes, sous peine de le sous-estimer.Denitrification, reducing nitrate (NO3-) to dinitrogen (N2), is considerate like a purification process in environment. However, several intermediates, such as nitrite (NO2-) and nitrous oxide (N2O) lead to negative effects on the environment when accumulated. In order to regulate their production, resulting largely of the anthropic activities, it is necessary to improve understanding of the processes which produce and reduce them. This study aims to test, using a coupled approach, the measurements of N2O isotopomers, d15N and d18O of N2O and d15N of NO2-, in order to trace the total denitrification. The originality of this work lies on the confrontations of experiments lead in laboratory with in-situ measurements. In-situ studies showed that the measurements of the natural isotopic abundances alone does not reveal the N2O origin, and should be coupled to the determination of N2O-SP values. Experiments conducted with benthic denitrifying communities under controlled laboratory conditions explained the occurrence of high N2O-SP values in the environment. Thus, high N2O-SP values can be either the result of (1) N2O production by nitrification, firstly, or (2) N2O reduction to N2 during denitrification, where the N2O reduction to N2 occurs. Finally, the study on the impact of d15N-NO2- values on that of e15N related to denitrification has highlighted the importance to consider denitrification like a process with several steps, otherwise the importance of this process will be underestimated.PARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

Benthic nitrate removal capacity in marine mangroves of Guadeloupe, Lesser Antilles

International audienceMangrove sediments are known to be potentially active reducing zones for nitrogen removal. The goal of this work was to investigate the potential for nitrate reduction in marine mangrove sediments along a canal impacted by anthropogenic activity (Guadeloupe, Lesser Antilles). To this end, the effect of nitrate concentration, organic carbon load and the hydraulic retention time was assessed as factors affecting these nitrate reduction rates. Nitrate reduction potential was determined using flow-through reactors in marine mangrove sediments collected along ‘The Canal des Rotours’ in Guadeloupe. Potential nitrate reduction rates, in the presence of indigenous organic carbon, generally increased upon increasing nitrate supply from around 120 nmol cm−3 h−1 (low nitrate) up to 378 nmol cm−3 h−1 (high nitrate). The potential for nitrate reduction increased significantly with the addition of mangrove leaves, whereas the addition of simple, easily degradable carbon (acetate) resulted in an almost five-fold increase in nitrate reduction rates (up to 748 nmol cm-3 h-1). The hydraulic retention time also had an impact on the nitrate reducing capacity due to an increased contact time between nitrate and the benthic microbial community. Marine mangrove sediments have a high potential to mitigate nitrogen pollution, mainly governed by the presence of large amounts of degradable carbon in the form of litter. The mangrove sediments from this Caribbean island, currently exposed to a small tidal effect could increase its nitrate elimination capacities due to prolonged water retention via engineering

Using oxygen isotopes of phosphate to trace phosphorus sources and cycling in lake Geneva

National audienceThis poster deals with the use of the oxygen isotopic composition (δ18Op) of dissolved inorganic phosphate (DIP) as a tracer for phosphate sources. Recent studies have shown that δ18Op values can be used as a tracer to identify the sources and cycling of P in different environments including marine and freshwater systems (Paytan and McLaughlin., 2006). Several studies were made for the purification and the analysis of phosphates oxygen isotopic composition since the pioneering works of Tudge 1960. Thus, many methods have been developed for the purification of phosphate from apatite. However, they are not easily applied to DIP purification. In 2004, Karen McLaughlin et al. propose a new method for purifying phosphate by precipitation as cerium phosphate. Currently, this method was never repeated. Here we present the first results of reproducibility and repeatability tests of this method with the goal in mind to apply this one to trace phosphate sources in Lake Geneva