No abstract

Two headwaters located in southwest France were monitored for 3 and 2 years (Auvézère and Aixette watershed, respectively) with two sampling strategies: grab and passive sampling with polar organic chemical integrative sampler (POCIS). These watersheds are rural and characterized by agricultural areas with similar breeding practices, except that the Auvézère watershed contains apple production for agricultural diversification and the downstream portion of the Aixette watershed is in a peri-urban area. The agricultural activities of both are extensive, i.e., with limited supply of fertilizer and pesticides. The sampling strategies used here give specific information: grab samples for higher pesticide content and POCIS for contamination background noise and number of compounds found. Agricultural catchments in small headwater streams are characterized by a background noise of pesticide contamination in the range of 20-70 ng/L, but there may also be transient and high-peak pesticide contamination (2000-3000 ng/L) caused by rain events, poor use of pesticides, and/or the small size of the water body. This study demonstrates that between two specific runoff events, contamination was low; hence the importance of passive sampler use. While the peak pesticide concentrations seen here are a toxicity risk for aquatic life, the pesticide background noise of single compounds do not pose obvious acute nor chronic risks; however, this study did not consider the risk from synergistic "cocktail" effects. Proper tools and sampling strategies may link watershed activities (agricultural, non-agricultural) to pesticides detected in the water, and data from both grab and passive samples can contribute to discussions on environmental effects in headwaters, an area of great importance for biodiversity.

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The nitrous oxide determination, after acetylene inhibition of its reduction, was made by GPC. We observed no N20 production in the water. The dénitrification rate, expressed as mg of nitrates. M~ 2 . d~ can be considered as a constant and is independent of the temperature variation, nitrate and dissolved oxygen concentrations of the sediment-water system.On a annual cycle, whithout the marked influence of environmental factors, the mean rate of dénitrification in the river Charente is about 682 mg.m -2 . d~ '.The comparison of the field measurements and evaluation by calculation of the dénitrification phenomenon shows that it is preferable to estimate the dénitrification rate by direct titration instead of nitrogen form analysis.

This paper describes an automatized multi-step methodology in order to identify the relationships between integrative pesticide quantifications and land-use on a given watershed. This methodology contains multivariate statistical analyses such as hierarchical cluster analysis (HCA) and principal component analysis (PCA), which are commonly used for the interpretation of complex geospatial datasets. A large amount of pesticide concentration data were collected along 1-year monitoring in 2016, for 50 sites located on the Adour Garonne basin (South-West France). For those sampling sites, concentrations of 37 selected pesticides were investigated during six periods of 14-days immersion of integrative samplers. Specifically, the sampling devices used were Polar Organic Chemical Integrative Sampler (POCIS), providing time-weighted average concentration estimates. For each studied site, the associated watershed and its land-use repartition were determined based on the Corine Land Cover 2012 and geographical information system (GIS) aggregation of data. The HCA clustered the 50 sites into five groups with similar main land uses. After that, the datasets of pesticide integrated concentration and land use repartition were analyzed in a PCA. The key variables (pesticide distribution and concentrations) responsible for sampling site discrimination showed consistent patterns of distribution with specific land uses. In order to confirm these observations, pesticide fingerprints (based on the waffle method) of sites with contrasted land use relative to the surface areas were compared. These fingerprints confirmed that there was different and specific patterns, visible at a glance, of pesticide occurrence in surface water, in relation with their initial use at the catchment level. This method allowed identifying sources of contamination that could be interesting to prevent or contain pesticide pollutions beyond simply acting on the most at-risk areas.