Crystalline geological bedrock headwater stream contamination in an agricultural-extensive rural watershed: keys factors for the behaviour of neutral and ionic pesticides using a passive sampling approach

The Aixette Watershed, located in southwestern France, was semi-continuously monitored (every 14 days) over 3 years at four different sampling spots with Polar Organic Chemical Integrative Samplers (POCIS), using the Oasis® HLB configuration (for 64 neutral pesticides/metabolites) and the Oasis® MAX configuration (for 15 ionic pesticides/metabolites). This watershed is characterised by extensive agricultural practices (bovine and ovine production) with breeding areas and cereal growing for cattle feeding. The downstream of the Aixette River is located in a peri-urban area. The neutral pesticide contamination could be qualified as background contamination noise, whereas ionic pesticide contamination revealed contamination peaks. The detection frequencies for neutral compounds were higher than those for ionic compounds. Different contamination behaviours were shown by a wider distribution of the time-weighted average concentration (TWAC) obtained for the ionic compounds (median and average TWAC were significantly different). The difference in the trends between neutral and ionic compounds could be explained by their large affinity to water, in accordance with their solubility or logP. Nevertheless, the trend of ionic compounds could not be linked to rainfall but to the flowrate of the Aixette river. This means than contamination with ionic pesticides or metabolites is mainly linked to drainage water from groundwater compared to runoff water, which is consistent with the hydrogeologic behaviour on crystalline geological bedrock and the leaching of ionic compounds from the surface to the aquifer.