Use of fluorescence to trace DOM sources in a headwater agricultural catchment. In : European Geosciences Union General Assembly, Vienna, Austria, 02 - 07 May 2010

High Dissolved Organic Carbon concentration [DOC] in stream modifies the physical, biological and chemical quality of natural waters, particularly via the transport of mineral or organic pollutants in agricultural catchments. In headwater agricultural catchment, in French Brittany, Valley BottomWetlands (VBW) are the main contributors of dissolved organic matter (DOM) during storm events. These areas may be intensively cultivated under maize, wheat or meadows with farming waste spreading in spring season or serve as intensively grazing pastures. In / this study, the potential of EEM fluorescence spectroscopy for the determination of farming waste impact on the quality of DOM in the streams was studied. Recent studies showed that ratio tryptophane/ fulvic-humic like can be a good tracer of DOM from animal waste. In headwater agricultural catchments, two sources of DOM can be transfered to the stream during storm events: DOM from soil and DOM from farming waste recently spreading on soil or in intensively grazing pastures. 15 headwater catchments were studied during 2007. Land / occupation (forest, agricultural surface), linear of hedges, and agricultural practices (organic fertilization, crop rotation) were characterized in the Valley Bottom Wetlands by remote detection analysis and farming survey. [DOC] was analyzed during three storm events between February and May 2007. EEM fluorescence spectroscopy with regional integration approach was carried out on water samples. EEM was divided into biochemical (bio) and geochemical (geo) fluorescent regions and the fluorescence were also integrated in reference wavelengths range of fluorophores of tryptophan (TRY), fulvic like (FL) and humic like (HL). The quantitative analysis included the integration of fluorescent volume beneath each region and zone. Ratios TRY:HL and bio:geo were also calculated. The fluorescence DOM quality differences, among three storm events, were analysed by principal component analysis (PCA) on the DOC normalised fluorescence dataset with R® software (package ade4). Ratio TRY:HL / and bio:geo were always higher in one catchment with important maize production and cow manure fertilization in the Valley Bottom Wetlands. In all the other catchments, the DOM fluorescence properties during Storm S2 showed a major contribution of highly humified DOM probably due to flushing of soils. The DOM fluorescence properties shift towards less humified and higher ratio Bio/Geo or TRY/HL during S1 in six catchments suggested a contribution from DOM issued directly from recent farming wastes. During S3, a small increase of the ratio / bio:geo and TRY/HL suggested also a transfer of DOM from farming waste but less pronounced than during S1. In four catchments, the ratio were very low during all the events and these catchments were the less impacted by intensive agricultural practices in the Valley BottomWetlands. Results demonstrate that fluorescence spectroscopy, by coupling with regional integration approach, are capable to reveal chemical changes in the DOM quality and depict the anthropogenic loads versus contribution from soils on fresh water streams