Pesticides in a tropical Costa Rican stream catchment: from monitoring and risk assessment to the identification of possible mitigation options
OPEN ACCESS
Loading...
Author / Producer
Date
2021
Publication Type
Doctoral Thesis
ETH Bibliography
yes
Citations
Altmetric
OPEN ACCESS
Data
Rights / License
Abstract
Pesticides are applied at high rates around the globe to protect crops from pest infestation. As a consequence, a broad spectrum of pesticides is found in surface waters. Together with their transformation products (TP), they can elicit adverse effects on aquatic organisms. Threats due to pesticide contamination to aquatic organisms is an especially concerning issue in tropical regions in Low- and Middle-Income Countries. These countries represent the most intensively used agricultural areas of the world and heavy rainfalls are expected to favor the transport of pesticides from the fields into surface water. Additionally, owing to the low level of economic development, environmental monitoring, risk assessment and implementation of risk mitigation strategies receive little attention. One country illustrating this situation is Costa Rica. This thesis set out to 1) monitor agricultural-driven pesticide pollution in tropical Costa Rican streams in order to describe the occurrence, concentrations and distribution of pesticides and pesticide transformation products (PPTP); 2) assess the risks by these PPTP to aquatic biota; and 3) identify relevant pathways of PPTP-transport from the field into the streams as a basis to propose mitigation options to reduce pesticide inputs.
1) The Tapezco river catchment was selected as study site. It is an area with intensive agriculture, characterized by horticultural fields cultivated partially on steep slopes, to grow a variety of different crops. This catchment is located in the central highland plateau in the province of Alajuela and an essential production area for vegetables. Five sites were sampled over a period of two and a half months in 2015 and eight were sampled over a period of four and a half months in 2016. For obtaining PPTP concentration data, three passive samplers were employed. Two were well-known sorbent-based passive samplers, namely styrene-divinylebenzene reverse phase sulfonated (SDB) disks and polydimethylsiloxane (PDMS) sheets, yielding biweekly time-integrated averaged concentrations. The third passive sampler was a water level proportional sampling system (WLPSS), yielding biweekly water level-weighted concentrations. After collecting the samplers in the fields, they were extracted in the laboratory. The SDB disk and the WLPSS water sample extracts were analyzed via high-resolution liquid chromatography tandem mass spectrometry and screened for 258 polar and semi-polar PPTP, including herbicides, insecticides, fungicides and some of their TP. The PDMS sheet extracts were analyzed for 18 non-polar insecticides via atmospheric pressure chemical ionization gas chromatography tandem mass spectrometry. With the SDB disks and the WLPSS, a broad PPTP spectrum was detected throughout the catchment. Despite the different sampling principles, the majority of the PPTP were detected with both sampler types,chemicals that had the highest median water concentrations were identified as such with both the SDB disks and the WLPSS. However, for seven of the pesticides, the concentrations determined with the WLPSS exceeded those of the SDB disks. This finding points to the WLPSS to collect pesticide peaks during heavy rainfall events, linked with water level rises, in a more pronounced fashion than the SDB disks. However, the majority of the WLPSS samplers employed did not sample in the optimal range, i.e. they were completely filled prior to sample collection, calling for a need to further optimize sampler operation. The PDMS approach allowed detection of additional, non-polar pesticides. Based on the reliability and the high share of retrieved samples, chemical concentrations determined from extracts of the SDB disks and PDMS sheets were further processed as measured environmental concentrations (MEC) for a risk assessment.
2) Two MEC-based approaches were used for risk assessment, allowing for identification of risks for both single PPTP as well as PPTP mixtures. The first approach compares the MEC to chemical-specific Environmental Quality Standards (EQS) for primary producers, invertebrates and vertebrates while the second uses the MEC to calculate so-called Toxic Units (TU),focusing on the chemicals’ toxicity to invertebrates. Three indices that rely on macroinvertebrate diversity and abundance in the streams were additionally applied to investigate the state of water quality: the Species at Risk (SPEARpesticide), the Costa Rican Biological Monitoring Working Party (BMWP-CR) and the Ephemeroptera, Plecoptera and Tricoptera (EPT)-taxa richness indices. With the EQS and the TU approaches, the quality of the water was indicated as poor at all sites and sampling periods and only few of the PPTP explained the overall risk. Invertebrates were the most affected, without any time window to recover from pesticide stress. The SPEARpesticide, BMWP-CR and the EPT indices indicated that, despite the continuous pesticide pollution stress, the water quality appeared to improve at the most downstream sites, which maybe due to a large river stretch upstream with a high share of natural forest.
3) To identify potential risk mitigation measures, an analysis of the most important pathways for PPTP transport from the fields into the streams was performed within the Tapezco river catchment. Analysis focused on the pesticides that dominated the aquatic biota health risks in addition to three of their TP and three pesticides with high application rates. The first pathway encompassed direct inputs via handling at four headwater sub-catchments, identified by concentration peaks unrelated to water level increases. Such direct inputs were indicated for several pesticides. The second type of pesticide transport was related to surface run-off, leading to concentrations being positively correlated with water level increases and potentially influenced by hydrological and topographical variables. Linear regression modelling with data from all eight sampling sites revealed that for a selection of insecticides (particularly for acephate, cyhalothrin, and thiamethoxam) the flux increased at sites with fields with high average slopes while for other PPTP (2,6 dichlorbenzamide, boscalid, carbofuran, diazinon,diuron TP, linuron and prometryn + terbutryn) flux decreased in areas with a high share of forested buffer zones. These trends, however, did not hold true for all pesticides. Different input patterns were, e.g. observed for the fungicide carbendazim. The third pathway of PPTP transport considered was via exfiltration of contaminated groundwater through the river bank,which was likely to lead to constant inputs, showing an inverse relationship with water levels due to dilution. Such inputs were principally possible for several PPTP based on their detection in groundwater samples, even though their contribution would be expected to be much lower,compared to the other pathways. Though the data set was limited and more research would be needed to more precisely delineate the input pathways, several mitigation strategies could be proposed: offering workshops about improved pesticide handling; avoid cultivation of those crops that demand high use of herbicides and insecticides on fields with steep slopes; and installation of stream buffer zones with natural forest.
Permanent link
Publication status
published
External links
Editor
Contributors
Examiner : Eggen, Hendrikus I.L.
Examiner : Stamm, Christian H.
Examiner: Wehrli, Bernhard
Examiner : Schäfer, Ralf B.
Examiner : De Voogt, Pim
Book title
Journal / series
Volume
Pages / Article No.
Publisher
ETH Zurich
Event
Edition / version
Methods
Software
Geographic location
Date collected
Date created
Subject
high-resolution liquid chromatography tandem mass spectrometry; Gas chromatography; Pyrethroids; Passive sampling; Environmental quality; EPT taxa; SPEAR; tropics
Organisational unit
01714 - DR Umweltnaturwissenschaften / DR Environmental Sciences
02721 - Inst. f. Biogeochemie u. Schadstoffdyn. / Inst. Biogeochem. and Pollutant Dynamics
Notes
Funding
Related publications and datasets
Is cited by: https://doi.org/10.25678/0004P2Is cited by: https://doi.org/10.25678/0004Q3Is cited by: https://doi.org/10.25678/0004R4