Flood survey of nitrate behaviour using nitrogen isotope tracing in the critical zone of a French agricultural catchment

Measurements of δ15N–NO3– were taken in a highly flood-responsive agricultural catchment in the southwest of France to trace the sources and transfer pathways of nitrates during flood events. From January to March 2013, surface water samples were collected every week at the outlet, and four floods were sampled with a high resolution. Sampling was also performed in surface waters and sand lenses from the rest of the basin to trace nitrate sources and processes spatially. Nitrate extractions were performed using a method based on the solubility difference between inorganic salts and organic solutions. The δ15N values were in the range of surface water contaminated by N-fertilisers. Depending on the hydroclimatic event, nitrates resulted from a combination of sources and processes. At the start of the floods, the values of δ15N–NO3– and nitrate concentrations were low, demonstrating the dilution of water with rainwater. During a second phase, the nitrate concentration and the δ15N were higher. Deeper waters and soil solutions were the second source of nitrates. When the water level was low, both nitrate concentration and isotopic composition were high. These values reflected the denitrification processes that occurred in the soil under anaerobic conditions. An analysis of δ15N–NO3– in stream water in a small agricultural catchment was efficient at determining the origin of nitrates during flood events using a simple method

Geochemical composition of fluvial sediments in the Milo River basin (Guinea): is there any impact of artisanal mining and of a big African city, Kankan?

For the first time, the bed sediments of the Milo River and its tributaries were sampled and analyzed to characterize their geochemical signature in terms of major, trace and rare earth elements and to determine the potential impacts of artisanal mining and of the anthropogenic activities of Kankan city. 24 sites have been sampled along the Milo river from upstream to downstream and on its tributaries during one sampling campaign to assess the spatial variations of the geochemical signature. During one year, the bed sediments have been sampled monthly on the Milo river upstream and downstream Kankan to determine the seasonal variations of their chemical composition and to investigate the potential impacts of Kankan on the fluvial sediment quality. The geochemical signatures of the sediments are mainly controlled by the geological substratum and the lateritic soil cover, showing that there is no impact of the artisanal mining and very few impacts of Kankan, except for As and Sb on two small tributaries draining the urban agglomeration. All the sediments are more concentrated and naturally enriched in Zr, Hf, U and Th compared to PAAS, due to the regional geochemical background. REE distribution patterns show LREE and HREE enrichments and positive Ce and Eu anomalies. The La/Yb ratios are positively correlated to Al and Fe-oxides and negatively to silica. The non-residual fractions (labile) extracted with EDTA represent less than 20% of the total concentration, showing that the threats for aquatic ecosystems and human populations are limited. This study can represent a reference with regard to the potential development of industrial mining and increase of population due the abundant mineral resources within this catchment

Combined investigations on long-term hydrochemical monitoring and high frequency measurements in the Critical Zone from the Auradé catchment (SW, France)

The Critical Zone (CZ) is now well identified as the land-atmosphere interface under the influence of many human pressures rendering up vulnerable for future generations. Although many investigations have been undergone over the last 30 years in the different compartments of the CZ, it remains important to understand the overall functioning of this area in a context of global change. A long-term hydrological and chemical monitoring was performed since 30 years for nitrates and discharge, and for 10 years for major elements at the stream outlet of a small agricultural carbonated catchment (Auradé site). This catchment is part of the observatories network OZCAR infrastructure and since 1992 it was a pilot for improving agricultural practices. Two time scales were investigated based on a discrete sampling during low water flow and hydrological events, and since 2006 on high frequency datas (every 10mn) for pH, conductivity, nitrate, temperature. . . using a multiparameter probe. The long-term trends indicated mostly a decreasing in nitrate, Ca and Mg concentrations namely and an increase in DOC, which can be related to the influence of the environmental practices (fertilizers inputs, vegetative filter strip etc..), but more recently to the changes in temperature and hydrological patterns (decreasing discharge and occurrence of rare but intensive events). The high frequency measurements on short-term events allowed: (i) to highlight the mechanisms involved in flux exportations (nycthemeral cycle for nitrates as ex.), (ii) to reconstruct the chemical patterns by correlating the parameters to major elements, and finally (iii) to have a better and more precise approach of the contribution of weathering and land use on the hydrochemical functioning of the CZ, particularly on the disturbance of carbon cycle by anthropogenic fingerprints

Long and short-term trends of stream hydrochemistry and high frequency surveys as indicators of the influence of climate change, agricultural practices and internal processes (Aurade agricultural catchment, SW France)

The hydrochemical time series of stream water from a cultivated catchment were investigated at different time scales and survey frequencies. A 35-year time series of nitrate concentration and discharge, a 15-year time series of major elements and dissolved organic carbon (DOC) concentrations were analysed from a yearly to a daily/hourly basis during discharge recession after storm event periods, to determine the origin of elements, the time trends and the main controlling factors of the trends. A significant decrease over time of nitrate, base cations and other major anions was observed. These trends were controlled by agricultural practice changes (decrease of N-fertiliser input, grass-band set up) and discharge increase, especially in the last years of the period. On the other hand, K and DOC increased over the 15-year period. This increase might result from both 1) organic matter eroded from the soil surface by runoff during flood events and 2) an increase in mineralisation with increasing temperature. Seasonal variations and nycthemeral cycles indicated either calcite precipitation and nitrification processes and/or evapotranspiration, water and/or vegetation uptake during the day with increasing temperature. This paper highlights that the hydrochemical parameters measured at various time scales and frequencies can be used as powerful indicators of catchment internal processes, and of changes in agricultural management and climate change. Particularly, the multivariate high-resolution survey has shown its ability to evidence very tenuous processes not detectable by discrete sampling. The recent observed changes in hydrology argue for the need to continue the hydrochemical survey over decades

Role of Pond Sediments for Trapping Pesticides in an Agricultural Catchment (Auradé, SW France): Distribution and Controlling Factors

In agricultural areas, ponds are suitable wetland environments to dissipate and reduce the occurrence of pesticides in aquatic environments. However, their impact at a catchment scale is still poorly understood. This study aims to determine how these organic contaminants were trapped in a pond located in an agricultural critical zone from SW France (Auradé catchment). The spatial distribution of pesticide concentrations and their different controlling factors were investigated in waters and sediments collected during two distinct seasons. The results highlighted (i) the link between the presence of the molecules and the agricultural practices upstream, (ii) the influence of hydrological/seasonal conditions, especially on hydrophobic molecule accumulation such as tebuconazole, (iii) the key role of clay content in sediments on the control of moderately hydrophilic pesticides (metolachlor and boscalid), but also the unexpected role of coarse particles for boscalid; and (iv) the influence of sediment depth on pesticide storage. Nevertheless, other physico-chemical parameters, such as mineralogical composition of sediment, needed to be considered to explain the pesticide patterns. This study brings a new hypothesis to be investigated in the future about pesticide behaviour in such pond environments

A Forty-Year Karstic Critical Zone Survey (Baget Catchment, Pyrenees-France): Lithologic and Hydroclimatic Controls on Seasonal and Inter-Annual Variations of StreamWater Chemical Composition, pCO2, and Carbonate Equilibrium

The long-term trends and seasonal patterns of stream water chemical composition in a small remote forested karst catchment, were investigated from 1978 to 2018. Calcium, magnesium, and bicarbonates, the dominant ions, increased over the period together with temperature, while sulfates decreased. Carbonate and sulfate mineral dissolution was the main source of these elements. These trends and the seasonal opposite patterns of discharge vs. temperature, calcite saturation index vs. pCO2 and bicarbonate vs. sulfates, suggested the influence of discharge, of reduced long-range atmospheric pollution, and of increasing air temperature on biological activity and carbonate dissolution. Furthermore, the hydrological regime controlled the seasonal stream water chemical composition and fluxes by: (i) a dilution during the high discharge period, (ii) a change in the contribution rate of the waters draining different lithological areas in the catchment, e.g., the increased sulfates to bicarbonates ratio during summer low flows, with a maximum alkalinity decrease of 24%, and (iii) a “piston” and a “flushing” effects of dissolved elements stored in soils and epikarst with the first autumn heavy rains. Long-term stream water hydrochemical surveys of karst system have proved to be powerful indicators of biogeochemical processes, water sources and pathways under variable natural and anthropogenic environmental pressure conditions

Trace Element and Pesticide Dynamics During a Flood Event in the Save Agricultural Watershed: Soil-River Transfer Pathways and Controlling Factors

Agricultural practices are the main source of water contamination in rural areas. Rainfall events, and subsequently, soil leaching and storm runoff are mainly controlling the transfer of pollutants from diffuse sources in watersheds during floods. These periods are also very important to better understand their dynamics, particularly their different soil-river transfer pathways (surface runoff SR, subsurface runoff SSR, and groundwater flow GF). This study focuses on riverine transfers of both pesticides and trace elements. High-resolution monitoring of water discharge and water sampling were performed during a flood event that occured in May 2010 in an agricultural catchment of SWFrance. Chemical composition of major and trace elements, silica, alkalinity, pH and conductivity, DOC and POC, TSM, and commonly used pesticides were analyzed with a high sampling frequency. The different stream flow components (SR, SSR, and GF) were assessed using two independent hydrograph separation methods: a hydrological approach based on Maillet’s formula (1905) for the recession period and a chemical approach based on physico-chemical tracers, TSM for SR and PO43− for GF. Both methods exhibited important contributions of SR (33 %) and SSR (40 %) to the total riverine pollutant transfers. The contribution of different components was also visible using concentration-discharge relationships which exhibited hysteresis phenomenon between the rising and the falling limbs of the hydrograph. Higher concentrations during the rising period(clockwise hysteresis) were characteristic of pollutants mainly exported by SR (trifluralin, Cd). Anticlockwise hysteresis with higher concentration during the recession period showed pollutants mainly exported by SSR (metolachlor, Cu). Moreover, significant relationships were highlighted between the controlling factors (DOC,POC, and TSM) and SR, SSR, and GF contributions:DOC and the complexed pollutants were highly correlated to SSR while POC, TSM, and the adsorbed pollutants were linked to SR. During the flood, Kd of most pollutants increased, particularly at the beginning, and therefore,future studies should investigate their availability to living organisms and thus their toxicity. An additional characteristic equation between Kd and Kow of the different pesticides was proposed to help future management, modelling,and estimation of pollutant transfers during floods

Baget catchment: a Critical Zone Observatory of a karstic area in the Pyrenees mountains, South West of France.

International audienc

Forty years’ fluctuations of streamwater chemistry in the Baget karstic catchment (Ariege, Pyrennees mountains, SW France): impact of atmospheric deposition on carbonate dissolution.

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Une étude de la phénologie de la végétation dominante dans une tourbière à sphaignes de montagne avec Sentinel-2 et des observations

International audienceAbstract Peatlands store more than a third of the global soil organic carbon stock. Bryophytes, more specifically Sphagnum mosses, play a major role in the carbon and water cycles of these ecosystems. There is a need to include Sphagnum mosses into Earth system models to better simulate the dynamics of peatlands in a changing environment. Leaf area index (LAI) is a key plant trait that characterizes the plant photosynthesizing capacity. Moreover, LAI is a variable calculated by land surface models used in climate models, allowing control of the exchange of matter and energy between vegetation and the environment. There is extremely little data on Sphagnum LAI and none on its seasonal change. We monitored Sphagnum mosses LAI phenology in a mountainous peatland site (altitude of 1,343 m) from June to December 2021 using two methods: 2D scans of monthly Sphagnum moss samples and analysis of Sentinel‐2 images. LAI derived from field campaigns and the remote sensing approach show a strong seasonality, with high peak values reaching 10 and 7 m 2 ·m −2 , respectively. The Sentinel‐2 images were also used to derive common vegetation indices. The moisture soil index effectively discriminates Sphagnum ‐dominated areas in the peatland. Satellite‐derived LAI of Sphagnum mosses is directly correlated to gross primary production monitored by gas exchange measurements ( R 2 = 0.83) but also to physical drivers of the environment such as air temperature ( R 2 = 0.74) or water table depth ( R 2 = 0.61) over the 2017–2021 period. It is therefore highly suitable to investigate ecosystemic functions