ALBERT

Beschreibung: The transfer of dissolved organic matter (DOM) at soil–river interfaces controls the biogeochemistry of micropollutants and the equilibrium between continental and oceanic C reservoirs. Then determining the transfer mechanisms of DOM is of main importance for ecological and geochemical reasons. Is stream DOM the result of the flushing of pre-existing soil DOM reservoirs activated by the modification of water flow paths? The evolution of the chemical composition of stream DOM investigated by thermally assisted hydrolysis and methylation (THM) using tetramethylammonium hydroxide (TMAH) coupled to a gas chromatograph and mass spectrometer (THM-GC-MS) during inter-storm conditions and five storm events with a high-frequency sampling gives new insights on this question. In inter-storm conditions, stream DOM is inherited from the flushing of soil DOM, while during storm events, the modification of the distribution of chemical biomarkers allows the identification of three additional mechanisms. The first one corresponds to the destabilization of microbial biofilms by the increase in water velocity resulting in the fleeting export of a microbial pool. The second mechanism corresponds to the erosion of soils and river banks leading to a partition of organic matter between particles and dissolved phase. The third mechanism is linked to the increase in water velocity in soils that could induce the erosion of macropore walls, leading to an in-soil partitioning between soil microparticles and dissolved phase. The contribution of this in-soil erosive process would be linked to the magnitude of the hydraulic gradient following the rise of water table and could persist after the recession, which could explain why the return to inter-storm composition of DOM does not follow the same temporal scheme as the discharge. Those results are of main importance to understand the transfer of nutrients and micropollutants at the soil–river interfaces during the hot moments that are storm events.

Beschreibung: High-frequency, long-term and multisolute measurements are required to assess the impact of human pressures on water quality due to (i) the high temporal and spatial variability of climate and human activity and (ii) the fact that chemical solutes combine short- and long-term dynamics. Such data series are scarce. This study, based on an original and unpublished time series from the Kervidy-Naizin headwater catchment (Brittany, France), aims to determine solute transfer processes and dynamics that characterise this strongly human-impacted catchment. The Kervidy-Naizin catchment is a temperate, intensive agricultural catchment, hydrologically controlled by shallow groundwater. Over 10 yr, five solutes (nitrate, sulphate, chloride, and dissolved organic and inorganic carbon) were monitored daily at the catchment outlet and roughly every four months in the shallow groundwater. The concentrations of all five solutes showed seasonal variations but the patterns of the variations differed from one solute to another. Nitrate and chloride exhibit rather smooth variations. In contrast, sulphate as well as organic and inorganic carbon is dominated by flood flushes. The observed nitrate and chloride patterns are typical of an intensive agricultural catchment hydrologically controlled by shallow groundwater. Nitrate and chloride originating mainly from organic fertilisers accumulated over several years in the shallow groundwater. They are seasonally exported when upland groundwater connects with the stream during the wet season. Conversely, sulphate as well as organic and inorganic carbon patterns are not specific to agricultural catchments. These solutes do not come from fertilisers and do not accumulate in soil or shallow groundwater; instead, they are biogeochemically produced in the catchment. The results allowed development of a generic classification system based on the specific temporal patterns and source locations of each solute. It also considers the stocking period and the dominant process that limits transport to the stream, i.e. the connectivity of the stocking compartment. This mechanistic classification can be applied to any chemical solute to help assess its origin, storage or production location and transfer mechanism in similar catchments.

Beschreibung: Assessing the impact of human pressures on water quality is difficult. First, there is a high temporal and spatial variability of climate and human activity. Second, chemical elements have their own characteristics mixing short and long term dynamics. High frequency, long-term and multi-element measurements are required. But, such data series are scarce. This paper aims at determining what the hydro-chemical particularities of a livestock farming catchment are in a temperate climatic context. It is based on an original and never published time series, from Kervidy-Naizin headwater catchment. Stream chemistry was monitored daily and shallow groundwater roughly every four month, for 10 yr and five elements (nitrate, sulphate, chloride, and dissolved organic and inorganic carbon). The five elements present strong but different seasonal patterns. Nitrate and chloride present a seasonal flush, all along or at the beginning of the wet season, respectively. Sulphate, organic and inorganic carbon present storm flushes, with constant or decreasing peaks throughout the wet season. These depicted nitrate and chloride patterns are typical of a livestock farming catchment. There, nitrate and chloride coming from organic fertilisation have been accumulating over years in the shallow groundwater. They are seasonally flushed when the groundwater connects to the stream. Sulphate, organic and inorganic carbon patterns do not seem specific to agricultural catchments. These elements are produced each year and flushed by storms. Finally, a generic classification of temporal patterns and elements is established for agricultural catchments. It is based on the distance of the source component to the stream and the dominant controlling process (accumulation versus production). This classification could be applied to any chemical element and help assessing the level of water disturbances.

Beschreibung: Monitoring the isotopic composition (δ13CDOC) of dissolved organic carbon (DOC) during flood events can be helpful for locating DOC sources in catchments and quantifying their relative contribution to DOC stream flux. High-resolution (〈 hourly basis) δ13CDOC data were obtained on six successive storm events occurring during the high-flow period in a small headwater catchment from western France. Intra-storm δ13CDOCvalues exhibit a marked temporal variability, with some storms showing large variations (〉2‰), and others yielding a very restricted range of values (80% of the stream DOC flux flows through the most superficial soil horizons of the riparian domain and (ii) the soil DOC flux is comprised of DOC coming ultimately from both riparian and upland domains. Based on its δ13C fingerprint, we find that the upland DOC contribution decreases from ca. 30% of the stream DOC flux at the beginning of the high-flow period to

Beschreibung: The transfer of dissolved organic matter (DOM) at soil–river interfaces controls the biogeochemistry of micropollutants and the equilibrium between continental and oceanic C reservoirs. Understanding the mechanisms controlling this transfer is fundamental to ecology and geochemistry. DOM delivery to streams during storms is assumed to come from the flushing of preexisting soil DOM reservoirs mobilized by the modification of water flow paths. We tested this hypothesis by investigating the evolution of the composition of stream DOM during inter-storm conditions and five storm events monitored with high-frequency sampling. The composition of DOM was analyzed using thermally assisted hydrolysis and methylation (THM) with tetramethylammonium hydroxide (TMAH) coupled to a gas chromatograph and mass spectrometer. In inter-storm conditions, stream DOM is derived from the flushing of soil DOM, while during storm events, the modification of the distribution of chemical biomarkers allows the identification of three additional mechanisms. The first one corresponds to the destabilization of microbial biofilms due to the increase in water velocity, resulting in the fleeting export of a microbial pool. The second mechanism corresponds to the erosion of soils and river banks, leading to a partition of organic matter between particulate and dissolved phases. The third mechanism is linked to the increase in water velocity in soils that could induce the erosion of macropore walls, leading to an in-soil partition between soil microparticles and dissolved phase. The contribution of this in-soil erosive process would be linked to the magnitude of the hydraulic gradient following the rise of the water table and could persist after the recession, which could explain why the return to inter-storm composition of DOM does not follow the same temporal scheme as the discharge. These results are the most important factors in understanding the transfer of nutrients and micropollutants at the soil–river interfaces during the hot moments that are storm events.

Beschreibung: Monitoring the isotopic composition (δ13CDOC) of dissolved organic carbon (DOC) during flood events can be helpful for locating DOC sources in catchments and quantifying their relative contribution to stream DOC flux. High-resolution (〈 hourly basis) δ13CDOC data were obtained during six successive storm events occurring during the high-flow period in a small headwater catchment in western France. Intra-storm δ13CDOC values exhibit a marked temporal variability, with some storms showing large variations (〉 2 ‰), and others yielding a very restricted range of values (〈 1 ‰). Comparison of these results with previously published data shows that the range of intra-storm δ13CDOC values closely reflects the temporal and spatial variation in δ13CDOC observed in the riparian soils of this catchment during the same period. Using δ13CDOC data in conjunction with hydrometric monitoring and an end-member mixing approach (EMMA), we show that (i) 〉 80% of the stream DOC flux flows through the most superficial soil horizons of the riparian domain and (ii) the riparian soil DOC flux is comprised of DOC coming ultimately from both riparian and upland domains. Based on its δ13C fingerprint, we find that the upland DOC contribution decreases from ca.~30% of the stream DOC flux at the beginning of the high-flow period to 〈 10% later in this period. Overall, upland domains contribute significantly to stream DOC export, but act as a size-limited reservoir, whereas soils in the wetland domains act as a near-infinite reservoir. Through this study, we show that δ13CDOC provides a powerful tool for tracing DOC sources and DOC transport mechanisms in headwater catchments, having a high-resolution assessment of temporal and spatial variability.