Temporal variability of nitrate transport through hydrological response during flood events within a large agricultural catchment in south-west France

The temporal variability of nitrate transport was monitored continuously in a large agricultural catchment, the 1110 km2 Save catchment in south-west France, from January 2007 to June 2009. The overall aim was to analyse the temporal transport of nitrate through hydrological response during flood events in the catchment. Nitrate loads and hysteresis were also analysed and the relationships between nitrate and hydro-climatological variables within flood events were determined. During the study period, 19 flood events were analysed using extensive datasets obtained by manual and automatic sampling. ThemaximumNO3 − concentration during flood varied from 8.2 mg l−1 to 41.1 mg l−1 with flood discharge from 6.75 m3 s−1 to 112.60 m3 s−1. The annual NO3 − loads in 2007 and 2008 amounted to 2514 t and 3047 t, respectively, with average specific yield of 2.5 tkm−12 yr−1. The temporal transport of nitrate loads during different seasonal flood events varied from 12 t to 909 t. Nitrate transport during flood events amounted to 1600 t (64% of annual load; 16% of annual duration) in 2007 and 1872 t (62% of annual load; 20% of annual duration) in 2008. The level of peak discharge during flood events did not control peak nitrate concentrations, since similar nitrate peaks were produced by different peak discharges. Statistically strong correlations were found between nitrate transport and total precipitation, flood duration, peak discharge and total water yield. These four variables may be the main factors controlling nitrate export from the Save catchment. The relationship between nitrate and discharge (hysteresis patterns) investigated through flood events in this study was mainly dominated by anticlockwise behaviour

Bioturbation experiments in the Venice Lagoon

Short experiments (14–21 days) were carried out during autumn 1998 and spring 1999 at one selected site of the Venice Lagoon to measure bioturbation activities and mixing rates, as well as to obtain quantitative information on benthos functionality. Fluorescent sediment particles (luminophores, 63–350 μm) were introduced as pulse inputs at the sediment surface. The concentration–depth profiles of the tracer were simulated with a new advection– diffusion–non local model applied under non-steady state conditions. This allowed the quantification of the mixing parameters associated with different mechanisms: biodiffusion (Db), bioadvection (W) and non-local mixing (Ke,z1, z2). A parameter RS (removed sediment) was also calculated to account for the flux of sediment due to nonlocal transport. Results show that bioturbation was dominated by biodiffusion in autumn and by bioadvection in spring. Mean mixing parameters Db, W, and RS changed from 3.09 to 0.87 cm2 y−1, from 0.93 to 15.50 y−1 and from 5.85 to 7.79 g cm−2 y−1, respectively

On modeling chronic detachment of periphyton in artificial rough, open channel flow

Periphyton communities, which are native to river beds, serve as a functional indicator of river health but remain one of the least-studied communities despite the significant increase in the examination of aquatic microbial communities in recent years. In this study, we tested the relevance of three formulations of the chronic detachment term in a simple model for the biomass dynamics of periphyton. Numerical simulations of the periphyton biomass dynamics were performed by using three different descriptors for the flow conditions: the discharge Q, the friction velocity u⁄, and the roughness Reynolds number k+ = u⁄ks/m (where m is water kinetic viscosity and ks is the Nikuradse equivalent sand roughness). Comparisons of numerical simulation results with experimental data from literature revealed chronic detachment to be better simulated by taking the roughness Reynolds number as the external variable of detachment. These results support the idea that transport phenomena that occur in the nearbed layer, e.g. chronic detachment of periphyton matter or vertical transport of nutrients and pollutants in submerged aquatic canopies, are not related to a single turbulence descriptor such as the friction velocity u⁄. Its description requires at least two descriptors, here the friction velocity u⁄ and the Nikuradse equivalent sand roughness ks, which depend on the initial form and dimensions of the colonized substratum, and its changes owing to the thickness, resistance, and composition of the epilithic matter

Cadmium transport in sediments by tubificid bioturbation: An assessment of model complexity

Biogeochemistry of metals in aquatic sediments is strongly influenced by bioturbation. To determine the effects of biological transport on cadmium distribution in freshwater sediments, a bioturbation model is explored that describes the conveyor-belt feeding of tubificid oligochaetes. A stepwise modelling strategy was adopted to constrain the many parameters of the model: (i) the tubificid transport model was first calibrated on four sets of microspheres (inert solid tracer) profiles to constrain tubificid transport; (ii) the resulting transport coefficients were subsequently applied to simulate the distribution of both particulate and dissolved cadmium. Firstly, these simulations provide quantitative insight into the mechanism of tubificid bioturbation. Values of transport coefficients compare very well with the literature, and based on this, a generic model of tubificid bioturbation is proposed. Secondly, the application of the model to cadmium dataset sheds a light on the behaviour of cadmium under tubificid bioturbation. Cadmium enters the sediment in two ways. In one pathway, cadmium enters the sediment in the dissolved phase, is rapidly absorbed onto solid particles, which are then rapidly transported to depth by the tubificids. In the other pathway, cadmium is adsorbed to particles in suspension in the overlying water, which then settle on the sediment surface, and are transported downwards by bioturbation. In a final step, we assessed the optimal model complexity for the present dataset. To this end, the two-phase conveyor-belt model was compared to two simplified versions. A solid phase-only conveyorbelt model also provides good results: the dissolved phase should not be explicitly incorporated because cadmium adsorption is fast and bioirrigation is weak. Yet, a solid phase-only biodiffusive model does not perform adequately, as it does not mechanistically capture the conveyor-belt transport at short time-scales

Dynamics of Nitrogen loads in surface water of an agricultural watershed by modelling approach, the Save, Southwest France.

Agriculture is known to have a great impact of nutrients enrichment on continental water resources. In south-West of France (Gascogne region), water resource are essentially surface water and shallow aquifer. Nitrogen dynamic in river is complex and highly variable throughout season and year, depending on hydrology, landuse, removal in stream. In this context, agricultural impacts on nitrogen concentration are a matter of concern for agricultural decision-maker. In order to introduce sustainable land use concepts in this hilly, clayey and agricultural shallow soil context, the hydrological simulation model SWAT2005 has been tested as a valuable tool to evaluate the consequences of such land use changes on water and nutrient balance components. This semi-distributed hydrological model coupled with agronomical model EPIC is able to simulate the impact of each agricultural landuse at the outlet of the Save catchment (1100 km2). Hydrological parameters model are calibrated based on 14-year historical record (1994–2008). Nitrogen losses have been measured during 2 years (2006-2008) at the outlet and are used to validate the model calibration. Agricultural data at communal scale coupled with Spot image analyses have been used to evaluate agricultural distribution and pressure in SWAT. The aim of this modelling exercise is to simulate nitrogen cycle in whole agricultural Hydrological Response Units (HRU), depending on plant growth and culture rotation, to simulate accurately nitrate load in river. The ability of SWAT to reproduce nitrogen transfert and transformation at this scale and in this agricultural context will be evaluated by a discussion of importance of each nitrogen cycle process in nitrogen losses. SWAT could be a useful tool to test agricultural scenario to improve the nitrogen management in river

Longitudinal transformation of nitrogen and carbon in the hyporheic zone of an N-rich stream: A combined modelling and field study

A combined modelling and field study approach was used to examine biogeochemical functioning of the hyporheic zone in two gravel bars in an N-rich fourth-order stream (River Hers, south-west France). Surfacewater and interstitial water were sampledmonthly (August 1994–January 1995), the latter in a network of 29 piezometers in the first gravel bar and 17 in the second. In both gravel bars, the hyporheic zone was created only by advected channelwater without any connectionwith groundwater. Longitudinal chemical profiles of Dissolved Organic Carbon (DOC), nitrate (NO3–N), ammonium (NH4–N) and Dissolved Oxygen (DO) were established for both gravel bars. Ambient and potential denitrification weremeasured in the laboratory during the same period using the acetylene inhibition technique. Factors limiting denitrification were also examined by testing the separate effects of nitrate or nitrate + carbon additions. A 1D reactive-transport model was used to simulate longitudinal transformation of nitrogen in the hyporheic zone, and to estimate the role of organic matter (DOC and POC) in the biogeochemical functioning of the hyporheic zone. Denitrification measurements with nitrate and nitrate + carbon additions both showed increased denitrification, suggesting that denitrification might not be C-limited at this site. Observations and model results showed the hyporheic zone to be a sink of DOC and nitrate, but DOC consumption appeared insufficient to explain nitrate depletion measured in the two gravel bars. Field data were better modelled when an additional DOC source from the POC fraction degraded by anaerobic respiration was included in the model

Modelling of trace metal transfer in a large river under different hydrological conditions (the Garonne River in southwest France)

The modelling of trace metals (TM) in rivers is highly dependent on hydrodynamics, the transport of suspended particulate matter (SPM) and the partition between dissolved and particulate phases. A mechanistic, dynamic and distributed model is proposed that describes the fate of trace metals in rivers with respect to hydrodynamics, river morphology, erosion-sedimentation processes and sorption–desorption processes in order to identify the most meaningful parameters and processes involved at the reach scale of a large river.The hydraulic model is based on the 1-D Saint Venant equation integrating real transects to incorporate the river's morphology. The transport model of dissolved species and suspended sediments is based on advection–dispersion equations and is coupled to the one-dimensional transport with in flowand storage (OTIS) model, which takes transient storage zones into account. The erosion and sedimentation model uses Partheniades equations. Finally, the transfer of trace metals is simulated using two parameters,namely the partition coefficient(Kd)and the concentration of TM in the eroded material. The model was tested on the middle course of the Garonne River,southwest France,over an 80km section under two contrasting hydrological conditions (80m3 s-1 and 800m3 s-1) based on measurements (hydrology, suspended sediments, particulate and dissolved metals fractions) taken at 13 sampling stations and tributaries. The hydrodynamic model was calibrated with discharge data for the hydraulic model,tracer experiments for the dissolved transport model and SPM data for the erosion-sedimentation model. The TM model was tested on two trace metals: arsenic and lead. Arsenic was chosen for its large dissolved fraction, while lead was chosen for its very important particulate fraction, thus providing contrasting elements. The modelling of TM requires all four processes to be simulated simultaneously. The presented model requires the calibration of ten parameters divided in four submodels during two hydrological conditions(lowand high flow). All parameters could be explained by the physical properties of the case study, suggesting that the model could be applied to other case studies. The strategy of using different datasets under different hydrological conditions highlights: (a) the importance of transient storage in the study case, (b) a detailed description of the erosion and sedimentation processes of SPM, and (c) the importance of TM eroded from the sediment as a secondary delayed source for surface water

Modelling river discharge at sub-daily time-step: comparison of the performances of the conceptual SWAT model and the process-oriented MARINE model

Due to global change, the frequency of intense rainfall events and consequent flash floods are expected to increase in the next decades across the Mediterranean coastal basins. To date, few distributed models are able to simulate hydrological processes at basin-scale at a reasonable time scale to describe these flash events with accurate details. The MARINE model is one of them: it is a process-oriented fully distributed model operating dynamically at the rainfall event time-scale. Both infiltration and saturation excess are represented along with subsurface, overland and channel flows. It does not describe ground-water processes since the model's purpose is to simulate individual flood events during which ground-water processes are considered negligible. The SWAT model is a conceptual semi-distributed model assuming several simplifications in equations that dynamically simulates above- and below-ground processes. It has been recently upgraded to sub-daily time-step calculations. Considering the 1400 km² Têt Mediterranean river basin (southwestern France) as a case-study, the objective of this study was to assess and compare the performances of these two models when simulating the discharge at sub-daily time-step. We first calibrated the two models based on the same input dataset (topography, land-use, soil classes, and meteorological stations’ grid). We then compared the performances of the two models on a number of selected flood events. This ongoing work will contribute to assess the ability of the SWAT model to simulate discharge at sub-daily time-step

Spatio-temporal analysis of factors controlling nitrate dynamics and potential denitrification hot spots and hot moments in groundwater of an alluvial floodplain.

Nitrate (NO3−) contamination of freshwater systems is a global concern. In alluvial floodplains, riparianareas have been proven to be efficient in nitrate removal. In this study, a large spatio-temporal datasetcollected during one year at monthly time steps within a meander area of the Garonne floodplain (France)was analysed in order to improve the understanding of nitrate dynamic and denitrification process infloodplain areas. The results showed that groundwater NO3−concentrations (mean 50 mg NO3−L−1) wereprimarily controlled by groundwater dilution with river water (explaining 54% of NO3−variance), butalso by nitrate removal process identified as denitrification (explaining 14% of NO3−variance). Dilutionwas controlled by hydrological flow paths and residence time linked to river-aquifer exchanges and floodoccurrence, while potential denitrification (DEA) was controlled by oxygen, high dissolved organic car-bon (DOC) and organic matter content in the sediment (31% of DEA variance). DOC can originate bothfrom the river input and the degradation of organic matter (OM) located in topsoil and sediments of thealluvial plain. In addition, river bank geomorphology appeared to be a key element explaining potentialdenitrification hot spot locations. Low bankfull height (LBH) areas corresponding to wetlands exhibitedhigher denitrification rates than high bankfull height (HBH) areas less often flooded. Hydrology deter-mined the timing of denitrification hot moments occurring after flood events. These findings underlinethe importance of integrating dynamic water interactions between river and aquifer, geomorphology, anddual carbon source (river and sediment) when assessing nitrate dynamics and denitrification patterns infloodplain environments