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Pesticides in air and in precipitation and effects on plant communities

3. Materials and methods

The localities were chosen so they were at some distances from agricultural areas. They are all situated in a clearing in a forest.

Three localities away from agricultural areas

The Gadevang locality (12° 16.29´Ø, 55° 58.01´N) is placed about 1 km
West, 5 km South, 5 km East and more than 10 km North of the nearest agricultural area. The Lorup locality (11° 29.86´Ø, 55° 23.24´N) is situated about 0.5 km in all directions from smaller fields, but 2-3 km from larger connected agricultural areas in all directions. The Gisselfeld locality (11° 55.20´Ø,55° 15.50´N) is placed about 1 km West, 3 km East and 2,5 km North of sprayed areas. Samples were taken in March/April, when the first spraying is conducted and until November/December, when the last spraying is done before it begins to freeze.

3.1 Sampling and analysis of rainwater

2 weeks collection periods

The rainwater is sampled through a glass funnel (20 cm in diameter), which is placed about 2 meters above ground. From the funnel, the water runs through a Teflon tube into a 2 liter glass bottle, which is insulated to prevent fluctuations in the temperature and photochemical reactions. Four bottles were placed at each locality, each collects water from two funnels to get a sample that is large enough for analysis. The bottles were acidified to avoid/delay microbiological degradation of whatever pesticides, collected with the rainwater. The samples were normally collected after 2 weeks. In the laboratory the samples were kept at approximately –18^oC until sample preparation and analysis.

With the described experimental design it was not possible to avoid degradation of the pesticides from the time they were collected in the bottle to the time they arrived in the laboratory. Pesticide residues due to rainfall in the beginning of the collection period will result in lower concentrations due to degradation compared to pesticides from rainfall happening just before the samples are collected and brought to the laboratory.

Minimum concentrations depending on storage periodes on locations

Therefore the concentrations listed in the tables in the result section, are minimum concentrations. Stability tests were performed to study the keeping
qualities of the rainwater samples when they were frozen before extraction. The amount of pesticides being degraded at -18° C, is negligible compared to the amount degraded in the collection period.

Solid phase extraction followed by gas- or liquid chromatography with mass spectrometric detection

The method for analysing isoproturon in the samples is based on a pre-
concentration by solid phase extraction. The phenoxyalkanoic acid
herbicides in the samples are also determined by a concentration step with solid phase extraction followed by derivatization with
pentafluorobenzylbromide, which produces the corresponding pentafluorobenzylesters. The samples are analysed with GC-MS in SIM and SCAN mode (see appendix I-III). The water samples collected in 1998 are analysed with the use of LC-MS (see appendix IV). With this method it is not necessary to derivatize the phenoxyalkanoic acid herbicides. The detection limit in using the LC-MS has been 0.01m g/L. Together with the rainwater samples, samples spiked with the pesticides concerned, were analysed.

For the GC-MS-analysis, several water samples were spiked in the concentration range 0.05µg/L-1.00µg/L, r² values were ³ 0.95 and the detection limits differed from 0.01µg/L and 0.09µg/L. The detection limits were set (Miller and Miller, 1988), every time a set of samples was analysed by quantifying the spiked samples together with the collected rain water samples. The samples were scanned to identify, if it was isoproturon, mecoprop, MCPA or dichlorprop etc. The results from the tests were transferred to SAS, which calculates the detection limits and the concentrations of pesticides in the samples based on the calibration data.

3.2 The pesticides

MCPA, dichlorprop, mecoprop and

The phenoxyalkanoic acid herbicides; MCPA, dichlorprop and mecoprop
together with isoproturon were chosen in the beginning as model substances. isoproturon The phenoxyalkanoic acid herbicides were extensively used both in the spring and in the autumn when the project began in 1996. In the beginning of 1997 the Danish Environmental Protection Agency announced that products containing phenoxyalkanoic acid herbicides in general were prohibited for autumn use henceforward and only allowed for spring for a few applications.

In 1996, isoproturon was approved for use both in the spring and in the autumn, but from 1997 only the autumn application was permitted.

The substances have been found in precipitation from neighbouring countries (Kirknel and Felding, 1996)

DNOC in 1998

In 1998, the analysis program was extended to include bentazone and 2 of its metabolites, DNOC, and 2 metabolites of isoproturon. DNOC has not been used in Denmark during the last 10 years, but was included because it was found in the samples that were analysed at DMU in 1997.

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