22
Oct
Flooding Transports Pesticides from Streams to Soil and Plants, Threatens Terrestrial Food Webs
(Beyond Pesticides, October 22, 2024) A recent study, published through the American Chemical Society, analyzes pesticide contamination in riparian soil and plants as a result of flooding from streams in Germany. The authors hypothesize, and then prove, that frequently flooded sites have higher levels of pesticides present due to the pesticides in surface waters contaminating the soil. Results show that the plant vegetation in the contaminated soil then takes up the pesticides, which bioaccumulate and lead to higher contamination that can further cascade throughout the ecosystem and affect terrestrial food webs.
“[O]ur study provides evidence from the field that nontarget plant species typical for riparian stream sites receive considerable pesticide exposure via flooding events,†the authors share. This exposure, and subsequent bioaccumulation in plants, threatens the food web, as many riparian plants are a vital food source for insects.
According to the authors, flooding events, and their impact on pesticide contamination within soil and plants, are rarely investigated. This study, “measur[ing] 98 pesticides and metabolites in plants and root-zone soils sampled at five streams situated in an area in Southwest Germany characterized by intensive agricultural land use,†highlights the differences in contamination between frequently flooded and rarely flooded areas and the greater impacts on the environment and resident organisms.
At each of the five streams in the Upper Rhine Valley, samples were collected from paired sites with different flooding frequencies. Regularly flooded sites are closer to each stream’s usual water level while rarely flooded sites are more elevated. Samples of the plant vegetation and soil were collected during October 2022, while the water-level data utilized was for the year prior to sampling (October 2021 – October 2022). The water-level data, from the Agency for the Environment in Rhineland-Palatinate, provides data on the frequency of site flooding, permitting the authors to calculate the average flooded days per year for each sampling site.
The vegetation collected at each site includes “five plant species typical for riparian sites, namely, stinging nettle (Urtica dioica), blackberry (Rubus sect. Rubus), ivy (Hedera helix), ground ivy (Glechoma hederacea), and alder (Alnus glutinosa),†the authors state. The soil attached to the roots of the collected plants was also used as root-zone soil samples. All samples of plant material and soil were freeze-dried before being filtered and analyzed using liquid chromatography and mass spectrometry techniques.
In analyzing the amount of pesticide contamination within the samples, the authors find that the regularly flooded sites were flooded at a frequency that was 10 times higher than the rarely flooded sites and exhibit not only higher concentrations of individual pesticides but also a higher number of pesticides present. Â Â
Of the 98 pesticides and metabolites screened, 33 are quantified in plant material from regularly flooded sites while 27 pesticides are detected in the rarely flooded site samples. At the regularly flooded site, up to 17 individual pesticides are found in a single plant sample. Within the soil from the regularly flooded sites, 39 pesticides are detected while the soil from rarely flooded sites shows only 25 pesticides present.
“The average pesticide concentration over all 98 compounds was up to 3 times higher at regularly flooded sites compared to rarely flooded sites in both soil and plants,†the authors report. In just the soil of the regularly flooded sites, about 10 times higher average pesticide concentrations are seen than in the soil of the rarely flooded sites. “Our results suggest a pathway for pesticides via flooding from the aquatic system to the terrestrial soil,†the authors conclude.
Nine pesticides are detected at high frequencies in both the soil and plants of the regularly flooded sites, including the fungicides cyflufenamid, fluopyram, metrafenone, spiroxamine, boscalid, and azoxystrobin, and the herbicides prosulfocarb, flufenacet, and pendimethalin. Of the nine pesticides, the six fungicides are detected more frequently and with higher concentrations in the soil than the three herbicides while the opposite is seen in plant material. “All of these pesticides are regularly detected in surface waters of the area,†the authors say, which “represent a usual contamination profile driven by flooding events.â€
Within the plants from the regularly flooded sites, pesticide concentrations are more than twice as high as those in rarely flooded sites. According to the authors, “Four out of the nine pesticides (cyflufenamid, prosulfocarb, flufenacet, and pendimethalin) were generally detected at higher mean concentrations in plant material compared to soil material,†which highlights bioaccumulation within plant tissue.
In analyzing the different parts of the plants, the study finds that the leaves contain the highest average pesticide concentration. Higher levels are also observed in woody plants, such as the alder and blackberry, when compared to herbaceous plants. To explain these differences, the authors state, “It has been suggested that the lipid content of the roots, evapotranspiration rates, and other physiological factors influence the species-specific contamination profile.†This indicates that some species, and even parts of the plants within all species, are more likely to have contaminants present. (See studies here, here, and here.)  Â
Contamination of nontarget plants through flooding can greatly impact the species present, but also threaten the herbivores that feed on them as well. The five species included in this study provide a food source for many organisms, such as various butterflies, moths, and beetles. “The results of the present study indicate that different ecological niches within the same (micro)habitat are exposed to different concentrations of the same compound, posing different threats to taxa occupying these different ecological niches,†the authors highlight.
With the pattern of higher frequency and concentrations of pesticides at the regularly flooded sites, present in both plants and root-zone soil, it suggests that flooding is a vector for the aquatic−terrestrial transport of contamination in small streams. “Our study provides evidence from the field that flooding events, which may increase due to climate change, can transport pesticides to riparian soils and plants with potential cascading effects on terrestrial food webs,†the authors state.
Bioaccumulation and biomagnification of pesticides has been documented as a threat to biodiversity. As the authors point out, “[E]ven low pesticide concentrations at the base of the food web may translate via biomagnification to increased levels at higher trophic levels of the food chain, leading to cascading effects.†They continue in summarizing, “The transfer of flood-driven contaminants from water to land can result in their accumulation in the soil, and thus enter the food web via plant absorption potentially triggering bottom-up effects at higher trophic levels of the terrestrial food web.â€
The authors demonstrate how surface waters, which are often polluted with chemicals such as those used in agriculture, can transport contamination back into adjacent terrestrial ecosystems. One study, that followed the extreme flooding of the rivers Elbe and Vltava in August 2002, finds similar results of a 10-fold increase in concentration of pesticides in flooded soil compared to that in nonflooded soil. Another study corroborates the higher concentrations of pesticides within plants due to the soil-to-plant uptake for various nontarget plant species, including the stinging nettle.
Riparian soil can act as a pesticide sink and is a potential source for contaminated plants in those areas that can lead to implications for the entire terrestrial food web. This study highlights the importance of the “widespread distribution of small surface waters in agriculturally dominated areas and the predicted increases in flooding events due to climate change†that can impact riparian zones but also calls for additional studies and analyses on the widespread impacts of flooding.
The runoff of pesticides from agricultural land into surface waters is well documented and continues to be a concern not only in Germany but across the globe. (See previous coverage of pesticides in rivers and streams in the United States here.) This contamination not only threatens aquatic life but terrestrial organisms, as shown by the authors’ results. The only solution to prevention of this harmful contamination that can impact all species including humans is the elimination of the source pesticides, according to environmental and public advocates. Chemical-intensive agriculture relies on petrochemical pesticides and synthetic fertilizers, but a safer alternative via organic agriculture exists.
With organic land management, the threats to biodiversity and climate change are mitigated. In adopting organic standards, which are continuously improved upon through the National Organic Standards Board (NOSB), less harmful chemicals will pollute waterways and be able to impact not only human health but the health of all organisms and the environment.
Learn more about transitioning your community to organic land management, as well as how to make The Safer Choice within your home. The Daily News Blog provides the latest information on pesticide policies and science, and you can sign up for Action of the Week and Weekly News Updates here to stay engaged and informed. Play a part in the organic solution and join Beyond Pesticides as a member today.
On the necessity, viability, productivity, and profitability of organic land management, attend the 41st National Forum: Imperatives for a Sustainable Future—Reversing the existential crises of pesticide-induced illness, biodiversity collapse, and the climate emergency.
All unattributed positions and opinions in this piece are those of Beyond Pesticides.
Source:
Fiolka, F. et al. (2024) Flooding as a Vector for the Transport of Pesticides from Streams to Riparian Plants, American Chemical Society ES&T Water. Available at: https://pubs.acs.org/doi/abs/10.1021/acsestwater.4c00571.
