This influence of pluviometry on pesticide contamination of rainwater and atmosphere
Contamination of rainwater by pesticides was investigated by analyzing samples from 5 locations in Brittany (France). These sites were chosen on the basis of various characteristics including agricultural practices around the site, the direction of prevailing winds (west to east) and the proximity to heavily populated areas. The sites chosen were: Ile de Ouessant (a natural reserve without pesticide application), Landivisiau (a semi-natural zone), Plouay (a rural site with intensive agriculture, mainly cereals and vegetables), Ploufragan (a suburban site) and Rennes (an urban area surrounded by intensive agriculture and orchards). The pesticides to be monitored included the compounds most commonly used in Brittany. New molecules were added to this list to take into account new homologues or uses. Some organochlorines and pesticides with specific application modes were thus included in the list. Analytical techniques involved solid-liquid extraction procedures followed by GC-NPD, GC-ECD or GC-MS after prior derivatisation. Derivatisation was performed with heptafluorobutyric acid to detect urea and sulfonylurea herbicides. Quantification limits were about 5 ng/L for GC-ECD or GC-NPD, and 50 ng/L for molecules detected by GC-MS after derivatisation. Rain episodes were selected according to the prevailing meteorological conditions and the air mass trajectory. Only precipitation events greater than 6 mm were considered, in order to collect sufficient volume for analyses. Thirty-five samples (7 per location) were collected between 15 March and 15 July 2000, corresponding to various weather fronts. Six pesticides were detected: atrazine and its metabolite de-ethylatrazine (DEA), alachlor, desmethryne, lindane and its isomer (beta HCH). Among those detected most concentrations were below 100 ng/L but higher concentrations were occasionally detected during and after spraying in agricultural areas. Lindane was detected at low concentration (<10 ng/L) in several samples collected during different periods and locations. These results demonstrate chronic atmospheric contamination due to the presence of this substance even though its use has been banned in the Economic European Community (EEC) (e.g. in 1999 in France). The most frequently detected compound was atrazine, detected in 60 % of the samples. The presence of atrazine and its metabolite DEA in rainfall indicate chronic contamination of the atmosphere outside of spraying time (mid-March to mid-April). Detection of these compounds was generally limited to agricultural sites, and could also indicate transfer from agricultural areas during soil preparation for maize sowing. These hypotheses will be tested by monitoring atrazine in rainwater during a complete agricultural cycle. Important contamination by atrazine was detected during spraying time at agricultural locations, where concentrations ranged from 10 to 60 ng/L. Detection of alachlor was limited to application periods and rural sites, at levels ranging from 20 to 240 ng/L. However, amounts of alachlor were also detected in Ouessant, even though its use is banned on this isle. This presence was presumably due to the specific conditions of formation of the weather fronts that resulted in rainfall. Concentrations of pesticides in rainwater obtained in 2000 were generally lower than those described in literature or in a previous study done in Brittany. Alachlor and atrazine concentrations were compared to those of a previous study conducted in Brittany in 1996. The two sites chosen were Rennes in 2000 and Le Rheu in 1996. These two locations, separated by 5 km, were not significantly different since Rennes is directly downwind of Le Rheu. Pluviometry was very important in the year 2000, showing a 30 % increase in rainfall compared to the average from 1961-1990. The concentrations of pesticides in both years were also very different. During a year of normal rainfall (e.g., 1996) concentrations of pesticides in application periods were high, but they remained low during wet years (e.g., 2000). These differences could not be explained by a dilution effect - when fall-out was calculated (mg/ha) to correct for the dilution effect, the differences remained important. Maize surfaces (132 000 ha in 1996 compared to 128 000 ha in 2000) and the quantities of pesticides applied (580 tonnes in 1994 in Brittany compared to 350 tonnes for atrazine, and 263 tonnes in 1998 compared to 119 tonnes in 1994 for alachlor) were too similar to explain the differences observed in 1998. Photodegradation processes, which are more important during normal or dry years, should also favour higher concentrations in 2000 than in 1996. Rain concentrations and fallout observed in 2000 remained lower than those obtained in 1996. Thus, it can be concluded that atmospheric contamination is lower for wet years (e.g., 2000) than for normal or dry years (e.g., 1996). Soil humidity, high for wet years, favours the vertical migration of these substances to lower soil layers, reducing pesticide concentrations available to be transferred to the atmosphere.