Temporal variation of pesticide mixtures in rivers of three agricultural watersheds during a major drought in the Western Cape, South Africa

Lou Curchod,Christelle Oltramare,Marion Junghans,Christian Stamm,Mohamed Aqiel Dalvie,Martin Röösli,Samuel Fuhrimann

doi:10.1016/j.wroa.2019.100039

Abstract

South Africa is the leading pesticide user in Sub-Saharan Africa. However, little is known about the occurrence of pesticide mixtures in surface water and potential environmental risks in Africa.This study investigated the occurrence of pesticides mixtures in three watersheds during a drought year in South Africa. The study was conducted in the Krom River, Berg River and Hex River watersheds within larger agriculture systems in the Western Cape. Pesticide spray records were collected from 38 farms. A total of 21 passive water samplers (styrenedivinylbenzene disks (SDB)) were deployed, each for two weeks per month, over seven sampling rounds during the main pesticide application period between July 2017 and January 2018. Samples were analyzed for 248 pesticide compounds using LC-HR-MS/MS. Pesticide occurrence was analyzed for temporal agreement with pesticide spraying events (Cohen’s κ) and correlation with rainfall patterns and river discharge (Pearson correlation (rp)). Pesticide time-weighted average concentrations were estimated and compared to environmental quality standards (EQS). According to the farm spray records, 96 different pesticides were sprayed during the sampling period and differed considerably between the three study areas, seasons and crops grown. In total, 53 compounds were detected in river water. We detected 39% of compounds from the spraying records and demonstrated close temporal correlations of seasonal patterns for 11 pesticide compounds between reported on spraying records and observations in the streams (κ = 0.90). However, 23 detected pesticides were not found on spray records, many of them being herbicides. Most of the estimated two-week average pesticide concentrations were below 40 ng/L. The insecticides imidacloprid, thiacloprid, chlorpyrifos and acetamiprid and the herbicide terbuthylazine exceeded at least once their EQS 58-fold (EQS 13 ng/L), 12-fold (EQS 10 ng/L), 9-fold (EQS 0.46 ng/L), 5-fold (EQS 24 ng/L) and 3-fold (EQS 220 ng/L), respectively. Our study substantially widens the view on pesticide pollution in surface water compared to previous studies in Sub-Saharan Africa by targeting more than 200 pesticides using passive sampling systems. This broad assessment revealed the presence of 53 compounds, some of them in high concentrations, indicating possible adverse effects on biota and the quality of the ecosystem. Whether the observed concentration levels in the year 2017 were exceptional due to the lowest ever recorded rainfall and river discharge needs to be tested with additional data to better understand how pesticide pollution levels manifest under average rainfall and river discharge conditions.

Highlights

IntroductionA thorough understanding of pesticide occurrence in surface water is of increasing importance as changing environmental conditions due to climate change, such as an increase in temperature or more frequent extreme events, may considerably impact on the use of pesticides and their environmental fate (Bloomfield et al, 2006; Delcour et al, 2015; Rosenzweig et al, 2001)
Our study substantially widened the view on pesticide pollution in surface water compared to previous studies in Sub-Saharan Africa, by including more than 200 pesticide compounds using passive sampling systems in surface water while systematically comparing it with agriculture use
The presented results demonstrate that a wide variety of pesticide compounds is present in streams draining agricultural watersheds

Summary

Introduction

A thorough understanding of pesticide occurrence in surface water is of increasing importance as changing environmental conditions due to climate change, such as an increase in temperature or more frequent extreme events, may considerably impact on the use of pesticides and their environmental fate (Bloomfield et al, 2006; Delcour et al, 2015; Rosenzweig et al, 2001). Pest infestations often coincide with changes in climatic conditions (Delcour et al, 2015; Rosenzweig et al, 2001), implying a potential increase in pesticide use during extreme events such as droughts or heavy rains (Bloomfield et al, 2006; Rosenzweig et al, 2001). A decrease in rainfall may reduce pesticide transport processes such as run-off and lower pesticide concentration in surface water (Bloomfield et al, 2006; Otieno et al, 2013)

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Results

Conclusion