Monitoring of selected pesticide concentrations in South African aquatic systems using the Chemcatcher® passive sampler

Abstract

A passive sampler, namely the Chemcatcher®, was used as the main tool in this study to monitor pesticide concentrations in surface water. This was achieved by equipping the Chemcatcher® with either SDB‐XC or C18 receiving phase disks, which are suitable for the sampling of polar organic pesticides such as atrazine, chlorpyrifos and terbuthylazine. These three pesticides as well as azinphos‐methyl and imidacloprid were the target analytes for this study which all have different applications in agriculture. A laboratory study which aimed at calibrating the passive sampler was performed whereby the flow rate was varied between 0 and 0.1 m.s‐1 whilst other variables remained constant. The laboratory sampling was carried out in a metal (31 cm x 70 cm x 20 cm) tank equipped with an electrical pump used for circulating the water. To avoid pesticide depletion, the spiked water was renewed every 6th day. In all sampling, the receiving phases were ultrasonically extracted separately in acetone and acetonitrile and grab water samples were treated using an optimised solid phase extraction (SPE) method which employed a C18 cartridge. After the second field sampling campaign, the initial gas chromatograph coupled to a mass selective detector (GC‐MSD) method was optimised for the separation of atrazine and terbuthylazine peaks which were co‐eluting and this was achieved by adding a slower temperature ramp between the retention time of atrazine and terbuthylazine. The optimised method yielded a lower limit of detection (LOD) of 0.030 µg.L‐1 which meant lower concentrations could be detected, compared to the 0.057 µg.L‐1 which was obtained with the non‐optimised method. The results which were based on the GC‐MSD and ultra‐performance liquid chromatograph coupled to a time‐of‐flight mass spectrometer (UPLC‐TOFMS) analysis of the pesticides showed that there was a linear uptake of the pesticides as the period of deployment or sampling increases and the system of static renewal to avoid analyte depletion was found to be important. Sampling rate, which indicates the volume of water that the passive sampler interacts with per unit time, differed per pesticide and showed a dependence on the water flow rate. The sampling rates obtained for all the target analytes were in the range of 0.14‐ 0.28 and 0.54‐1.09 L.day‐1 for flow rates of 0.0 and 0.1 m.s‐1 , respectively. Three field sampling campaigns were carried out in Delmas, Mpumalanga, South Africa before and after the spraying season, with the passive samplers equipped with either the C18 or SDB‐XC receiving phase disks. For the first sampling campaign, none of the target analytes were detected as it was not pesticide spraying or raining season, but pollutants such as hydrocarbons were detected as the river is located near a road. Only two of the five target pesticides: atrazine and terbuthylazine, were detected at quantifiable levels from the second and third sampling campaign, which varied between the SDB‐XC and the C18 receiving phase whereby terbuthylazine was accumulated more on the SDB‐XC and atrazine more on the C18. This was dependent on the similarity of the polarity of the receiving phases to that of the pesticides. Upon analysis of the receiving phases and through the use of the laboratory based sampling rates, the obtained concentration of terbuthylazine in the water was 0.261‐0.358 µg.L‐1 and 0.515‐545 µg.L‐1 for the second and third sampling campaigns, whilst that of atrazine was 0.109‐0.127 µg.L‐1 and 0.245‐0.263 µg.L‐1 , respectively. The two pesticides were only detected in the grab water samples of the third field sampling (after spraying) at concentrations of 0.533 µg.L‐1 for terbuthylazine 0.173 µg.L‐1 for atrazine. The advantage of passive sampling over grab sampling was evident, as the use of the receiving phases allowed for concentration of the pesticides (specifically for the second field campaign), which showed that the pesticides were indeed present in the water. This showed the importance of passive sampling and of this study. Based on the results obtained in this study, the Chemcatcher® passive sampler and other passive samplers, have the potential to be used in assessments that aim to monitor pesticide levels in water and thereby leading to the development of more efficient agricultural techniques or programs that limit the amount of pesticides that end up in surface water.