Flood event impact on pesticide transfer in a small agricultural catchment (Moutousse at Aurade, south west France)

In this paper, pesticide transfer dynamic is studied during two flood events in a small experimental catchment close to Toulouse (south west France). Thirteen pesticide molecules (herbicides, fungicides) have been analysed by multi-residue technique on filtered and unfiltered waters. The results show very high pesticide concentrations in the different fractions compared to low flow periods and to the data collected by the French institutional networks in charge of the pesticide river water pollution survey. Several molecules present concentration higher than 0.1 mgL-1 and even higher than 1 mgL-1 in the unfiltered waters. In the suspended matters the concentrations vary respectively between 0.1 and 30 mg g-1 according to the molecules and can represent 40 to 90% of the total concentration for low soluble molecules. All the molecule concentrations and fluxes increase during the flood flows and have positive relationships with the stream discharge, but hysteresis between rising and falling periods can be observed for some molecules. Pesticide concentrations in unfiltered waters and partitioning between dissolved and particulate fractions (Kd¼[diss]/[part]) are controlled by dissolved organic carbon and total suspended matter. A good negative relationship can be established between logKd and logKow for 6 molecules

Continuous measurement of nitrate concentration in a highly event-responsive agricultural catchment in south-west of France: is the gain of information useful?

A nitrate sensor has been set up to measure every 10 min the nitrate signal in a stream draining a small agricultural catchment dominated by fertilized crops during a 2-year study period (2006–2008) in the south-west of France. An in situ sampling protocol using automatic sampler to monitor flood events have been used to assume a point-to-point calibration of the sensor values. The nitrate concentration exhibits nonsystematic concentration and dilution effects during flood events. We demonstrate that the calibrated nitrate sensor signal gathered from the outlet is considered to be a continuous signal using the Nyquist–Shannon sampling theorem. The objectives of this study are to quantify the errors generated by a typical infrequent sampling protocol and to design appropriate sampling strategy according to the sampling objectives. Nitrate concentration signal and flow data are numerically sampled to simulate common sampling frequencies. The total fluxes calculated from the simulated samples are compared with the reference value computed on the continuous signal. Uncertainties are increasing as sampling intervals increase; the method that is not using continuous discharge to compute nitrate fluxes bring larger uncertainty. The dispersion and bias computed for each sampling interval are used to evaluate the uncertainty during each hydrological period. High underestimation is made during flood periods when high-concentration period is overlooked. On the contrary, high sampling frequencies (from 3 h to 1 day) lead to a systematic overestimation (bias around 3%): highest concentrations are overweighted by the interpolation of the concentration in such case. The in situ sampling protocol generates less than 1% of load estimation error and sample highest concentration peaks. We consider useful such newly emerging field technologies to assess short-term variations of water quality parameters, to minimize the number of samples to be analysed and to assess the quality state of the stream at any time

Assessing the risk caused by diffuse Cd and Pb pollution: critical load modelling for forest and arable lands

International audienceWe use critical load modelling to assess the risks caused by diffuse Cd and Pb pollution on two sites located on forest and arable lands respectively. The objectives of this work are: (i) to test the proposed critical load method on arable land and, (ii) to compare critical loads with current measured Cd and Pb fluxes. For the two sites, computed critical concentrations of Cd and Pb in soil water regarding the protection of soil organisms (2.3 and 4.9 mg.m-3 respectively for the forest, 0.7 and 4.5 mg.m-3 respectively for the arable site) are above current concentrations. Modelled critical loads (15.6 and 46.4 g.ha-1.a-1 for Cd and Pb respectively) are lower than current inputs for the forested site. However the arable site would be in exceedance with current inputs larger than modelled critical loads (1.3 and 7.1 g.ha-1.a-1 for Cd and Pb respectively). These results are going to be checked thanks to long-term monitoring and ecotoxicological tests

Pesticide transfers in a small agricultural catchment (Aurade, south west of France) during flood flows: partitioning into dissolved and particulate fractions

Pesticide transfer dynamic is studied during 2 flood events in a small experimental catchment close to Toulouse (South West of France). 16 to 25 pesticide molecules (herbicides, fungicides) have been analysed by multi-residue technique on filtered and unfiltered waters and on bottom sediments. The results show very high pesticide concentrations in the different fractions compared to low flow periods and to the data collected by the French institutional networks in charge of the pesticide river water pollution survey. Several molecules present concentration higher than 0.1 ug/L and even higher than 1 ug/L in the unfiltered waters. In the suspended matters and in the bottom sediments, the concentrations vary respectively between 0.1-30 ug/g and 0.01 -1 ug/g according to the molecules. All the molecule concentrations increase during the flood flows and present positive relationship with the stream discharge, but hysteresis between rising and falling periods can be observed for some molecules. Pesticide concentrations in unfiltered waters are generally controlled by dissolved organic carbon (DOC) content and total suspended matter (TSM) concentration. Partitioning between dissolved and particulate (Kd) fractions is also controlled by DOC, and TSM. A good negative relationship can be established between log Kd and log Kow for 6 molecules