Pesticide residues in EU soils and related risks

Research output: Thesisinternal PhD, WU


Pesticides have contributed significantly to increases in food production over the last few decades. In the European Union (EU), nearly 500 active substances have been approved for use as pesticides. The EU has one of the highest pesticide use in the world, with 374 000 tons being sold annually. Around 90% of pesticide sales can be linked to the agricultural sector. The pesticides used in agriculture, also known as Plant Protection Products (PPP), are applied to soil to prevent or combat the growth of undesired plants that compete with crops for resources, or to crops to combat organisms that can cause damage and reduce crop yields. There are several benefits associated with pesticides, the main ones relate to increasing yields, improving food security, and positively impacting the regional and national economies. On the other hand, intensive and widespread use of pesticides raises serious environmental and human health concerns. This is because substantial amounts of the pesticides applied in agriculture are released into the environment during or after application, and several pesticides (and/or their degradation products) are toxic to non-target-species, persistent in the environment, and accumulate through food chains. The high frequency of reports, increased diversity, and severity of negative effects of some pesticides raise serious concerns about the protection level of the current pesticide regulatory systems. The EU has the strictest system in the world but even it has shortcomings. The main problems relate to the low representativity of pre-approval risk assessments, the effective applicability of precautionary principles, and the limited post-approval monitoring of pesticide risks.

Chapter 2 focuses on the limited post-approval monitoring data point, more specifically in soil, a compartment where pesticide data is particularly scarce and fragmented. We analysed the occurrence and levels of 76 pesticide residues in 317 agricultural topsoil samples from the EU-LUCAS 2015 survey. The compounds were selected based on the most commonly used active substances in Europe and on the findings of previous EU studies on soil contamination by pesticide residues. The soils originated from 11 EU Member States and 6 main cropping systems where pesticide use is assumed to be the highest. We observed that 83% of the tested soils contained pesticide residues and 58% had mixtures of compounds. Glyphosate, AMPA, DDTs, boscalid, epoxiconazole, and tebuconazole were the most frequent compounds found in soil and the ones with the highest concentrations. Occasionally, the measured levels of glyphosate, epoxiconazole, and tebuconazole exceeded predicted environmental concentrations in soil. Also, measured DDT  levels occasionally exceeded the maximum values of the respective countries. Total pesticide content in soil reached values as high as 2.87 mg/kg. This study shed some light on the soil contamination problem and highlighted problems with current risk assessment evaluations.

Chapter 3 starts with a zoom-in of Chapter 2, exploring the distribution of glyphosate and its main metabolite AMPA across the same 317 agricultural topsoil samples and follows with the potential export of these substances by wind and water erosion. We conducted this more targeted study because such results could contribute to the ongoing debate about the approval of glyphosate use in the EU. Glyphosate was present in 21% of the samples and AMPA in 42% of the samples. Both compounds had a maximum concentration of 2 mg/kg. The highest levels of glyphosate and AMPA were found in southern parts of the EU in fields of permanent crops. Glyphosate and/or AMPA contaminated soils occurred often in areas that were highly susceptible to water and wind erosion. Pesticide export can be higher due to water as compared to wind erosion. Maximum export was estimated to be close to 48 g/ha/year for AMPA via water erosion. Our results corroborate the widespread soil contamination by these residues and indicate that particulate transport can contribute to human and environmental exposure to herbicide residues.

Chapter 4 investigates pesticide profiles in soils from conventional and organic farms. This was explored via the analyses of 340 topsoil samples originating from 4 representative EU case study sites: vegetable production in Spain (S-V), orange production in Spain (S-O), grape production in Portugal (P-G), and potato production in the Netherlands (N-P). The organic fields were converted to organic farming more than 5 or  10 years before the soil sampling was conducted.  Over 70% of the soils from the conventional fields had mixtures of pesticide residues, with a maximum of 16 residues per sample. The residues with the highest frequency of detection and the highest content in these soils were glyphosate/AMPA (P-G, N-P, S-O) and pendimethalin (S-V). Total pesticide content in soil reached values up to 0.8 mg/kg for S-V, 2 mg/kg for S-O and N-P, and 12 mg/kg for P-G. Soils from the organic fields presented significantly fewer residues, but mixtures of 2 to 5 residues were rather common. Organic soils presented 70-90% lower pesticide content than the corresponding conventional soils. Prosulfocarb, DDTs, AMPA, and bixafen were the most common compounds in organic soils. DDTs and AMPA had the highest levels. Our results stress the need for regular monitoring of pesticide residues and the necessity of establishing pesticide thresholds for both conventional and organic soils, including maximum levels of total pesticide residues. This will provide clarity to farmers and awareness of the time needed to change from a conventional to an organic farming system as thresholds and targets set for organic farm systems will be more strict than for conventional ones.

Chapter 5 establishes an EU pesticide use and risk baseline and explores the potential of seven pesticide reduction scenarios to achieve the envisioned 50% reduction goals of the Farm to Fork Strategy. To establish the use baseline, we compiled the recommended application rates of all 230 EU-approved, synthetic, open-field use active substances used as herbicides, fungicides, and insecticides. For the risk baseline, we compiled their (eco)toxicological risk/hazard information from PPDB. Our compilation revealed very high use levels of a couple of compounds (the soil sterilam metam and the soil fumigant dazomet) and evidence that all the 230 compounds are potentially harmful to humans and ecosystems. These results emphasize the need for a re-evaluation of pre-market requirements for pesticides. The presented pesticide reduction scenarios provide practical cut-off criteria for the EC, e.g., with regard to pesticide type, presence on the candidate for substitution EC list, or posing a hazard to humans or ecosystems. The 7 scenarios represent a decrease from 21 to 100% in the number of substances on the EU market.  Only the 4 most restrictive scenarios (complete conversion to organic farming; allowing only low hazard pesticides; no/acceptable human health effects; no/low toxicity to the ecosystem) resulted in the targeted 50% reduction in pesticides use and risk. Our results highlight the need for severe restrictions to achieve the Farm to Fork Strategy reduction goals, which could end up covering a combination of the pesticide reduction scenarios presented.

In Chapter 6, we present and discuss the main pesticide findings for soil, based on pesticide properties, pesticide application information, soil sampling time, and, of course, field management type. Implications of achieved results for soil monitoring programs, environmental risk assessment, pesticide approval procedures, and sustainable plant protection are highlighted in detail. Main thesis shortcomings are identified in this chapter as well, and recommendations for future work are outlined. Overall, this PhD thesis enhances our knowledge and adds to the discussion in three main fields: soil contamination,  post-approval pesticide monitoring, and the required measures to achieve the 50% pesticide reduction targets of the EU Farm to Fork Strategy. This thesis corroborates the notion that intensive pesticide use turns soil into sinks and potential sources of pesticide residues. Results show that some pesticides persist in the soil longer than expected from pre-registration studies and that banned compounds may still be found in EU agricultural soils. It is critical to establish better monitoring programs for pesticide residues in soil which should include assessments on the risks of pesticide residue cocktails found in soil systems and elsewhere. Risk assessment procedures must continue to evolve around mixtures, accounting for exposure pulses, chronic pesticide exposure, and indirect effects of pesticides. More sustainable agronomic practices and substantial reductions in the pesticides available on the EU market are urgently needed to meet the envisioned Farm to Fork targets in order to facilitate the transition towards more sustainable food production systems and improve human and environmental health.

Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • Wageningen University
  • Geissen, Violette, Promotor
  • Ritsema, Coen, Promotor
  • Mol, Hans, Co-promotor
Award date13 Jun 2022
Place of PublicationWageningen
Print ISBNs9789464472080
Publication statusPublished - 13 Jun 2022


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