Quantifying Terrestrial Microplastic Transport And Transit Times To Determine Current And Future Risks To Soils And Groundwater Aquifers
Besides the many technological and lifestyle benefits of living in the “plastic age”, the exceeded global production of 10 billion metric tons and increasing levels of pollution by highly durable and persistence mismanaged plastic waste cause widespread environmental and public health concerns. There is growing evidence that microplastics (MP), defined as plastic particles of less than 5mm in size resulting from a variety of different sources and with different chemical properties are now omnipresent, affecting marine environments and the atmosphere. A recent Royal Society evidence report highlighted that a move towards a more circular economy urgently requires understanding and reducing of MPs in soil and groundwater. Terrestrial MP sources are increasing globally, including agricultural inputs from plastic encapsulated seeds and agrochemicals, plastic mulches as well as MPs from solid waste sludges used as fertilizers, car tyre and break wear and tear and atmospheric deposition.
A Subsurface Microplastic Legacy? Globally, scientists, (agro)chemical, manufacturing and water industries, regulators and the wider public are increasingly concerned about the potential environmental risks of MPs for the functioning of soils. Potential risks of MPs that require further investigation include impacts on the soil microbiome and potentially consequences for soil fertility and global food production, as well as MP uptake into plant tissue and thus, also human food webs. In addition, MP leaching from top soils bears the risk of acting as transport vectors for additives as well as hazardous organic pollutants, viruses and antimicrobial resistance polluting highly vulnerable groundwater systems. Our preliminary research reveals that MPs are not only present in groundwater but can even be taken up by groundwater organisms, posing a threat to essential groundwater ecosystem services such as the natural attenuation of pollutants. Unfortunately, current and future MP exposure to soils and groundwater remain uncertain as the wide ranges of MPs stored in soils, their leaching rates as well as travel times towards the groundwater have yet to be determined.
Critical knowledge gaps related to MP transport and residence time in soils and resulting leaching into groundwater hamper the implementation of technological innovations for companies like Syngenta Ltd who are actively developing biodegradable agrochemical encapsulations and seed coatings. Unquantified risks to soils and groundwater furthermore limit informed changes of management practices (e.g., exclusion zones for solid waste sludge or fertiliser applications) and regulation updates which all require mechanistic understanding of subsurface MP transport. processes and time scales. Understanding of soil and aquifer specific transport time scales for MPs of different physical properties is crucial for quantifying the risks posed by subsurface MPs and thus, determining the potential magnitude of the existing legacy and future trajectory of MP in soils and groundwater. Chemical industries such as the fellowship partner Syngenta Ltd urgently require detailed understanding of the transport mechanisms and time scales of soil and groundwater MP fluxes, including aquifer type and land use specific MP transit and peak arrival times for optimising polymer properties of novel biodegradable materials for anticipated travel and residence times.
Project Vision
This fellowship integrates innovations in physical and mathematical MP transport modelling to quantify MP property, soil and hydrology dependent transport and transit times of non-degradable and novel biodegradable MPs (manufactured by Syngenta) in soils and groundwater, following five scientific objectives:
- Objective 1: Identify distribution of MP along vertical soil profiles in different types of agricultural soils under controlled management conditions and agrochemical treatments
- Objective 2: Conduct experiments on undisturbed soil columns quantifying particle property dependent MP breakthrough curves in selected soil types and water flow scenarios to determine MP travel times and property dependent residence time distributions
- Objective 3: Develop porous media particle flow model to simulate MP transport through soil columns and validate particle transport model based on observed column particle breakthrough curves
- Objective 4: Apply validated model for quantifying subsurface travel time distributions and assessing current and future risks to UK groundwater aquifers
- Objective 5: Quantify potential for MP risk reduction through technological innovation and management change scenarios
Funding
This fellowship is funded by the Royal Society.
People Involved
Professor Stefan Krause (Project Lead)
Work Packages
Vertical MP profiles will be characterised for soil types of known management and MP exposure history, utilizing the unique opportunities provided by Syngenta’s controlled field-testing facilities. MPs will be extracted for 10 cm intervals of 200cm long cores for triplicates of 5 different soil types/treatments, using density separation, digestion and fluorescent staining protocols developed in Krause’s lab (Nel et al., 2020 a, b). Extracted MPs will be analysed for particle concentrations and total MP mass per soil volume (Fluorescent Microscopy), polymer type (μ-Raman Spectroscopy) and concentrations of additives containing endocrine disrupting substances Bisphenol A (BPA) and Nonylphenol (NP) (by HPLC).
M1 Sampling and extraction of MPs from a total of 300 soil samples (month 4)
M2 Physical and chemical characterization of extracted MPs (month 12)
Fluorescently labelled (Nel et al., 2020) mixtures of non-degradable and Syngenta manufactured biodegradable MPs will be introduced to glass columns (Figure 2) filled with monoliths of five different agricultural soils obtained from Syngenta’s field-testing facilities. Their vertical transport and depth specific breakthrough will be observed at five depths for three different water flow scenarios (Figure 2), providing unprecedented opportunity to quantify particle property, soil type and hydrology-dependent MP transport in soils and recharge towards groundwater. Experimental MP concentrations will be based on WP1 field results. Information of size fractions for nondegradable and biodegradable MPs will be contributed by Syngenta through a separately funded PhD studentship who will support the project by deriving information of biodegradation and related changes in particle size distributions as an additional direct contribution to the fellowship by Syngenta (see letter of support.
M3 Set-up and installation of soil column experiments, triplicates of 5 soil types x 3 flow conditions (month 13)
M4 Observation of flow/soil/particle property dependent MP breakthrough curves at 5 different depths (month 20)
Building on the existing MP transport modelling expertise of Prof. Krause’s group (Drummond et al., 2020), the 1-dimensional C-RIDE HYDRUS model (Pang & Šimůnek, 2006; Šimůnek, et al., 2006) will be tested for its ability to simulate MP transport in the soil columns of WP2, including at the solid-water interface (Bradford et al., 2015). MP breakthrough curves observed in WP2 will be used to calibrate the model by optimising the particle sorption / detachment parameters of the HYDRUS C-RIDE routines (using a subset of WP2 data) and estimate parameter sensitivities in an uncertainty framework. The full range of WP2 observations will then be used to validate the model for 1-D vertical transport and analyse MP particle size and concentration impacts on MP transport in soils and groundwater leaching, including testing the model capacity to represent potential blocking as well as preferential transport when retention sites are filled.
M5 Model setup and parameterization (month 22)
M6 Model calibration and parameter sensitivity analysis, uncertainty assessment (month 25)
M7 Model validation and assessment of MP transport characteristics (month 28)
Simulated MP size dependent particle transport rates of WP3 will be used in conjunction with UK wide soil property information (from 1km2 UKCEH HOST dataset) to estimate the potential range of MP size dependent transit and residence time distributions in different UK top soils and resulting MP leaching potential towards groundwater. Based on simulated MP size and soil property dependent MP transport from top soils and spatially detailed information of UK groundwater depths (Jackson et al., 2015), the UK-wide risk of groundwater contamination will be quantified for different MP sizes, measured by expected mean residence and breakthrough times. The resulting risk map will identify hotspot areas and MP size fractions of particular risk of accumulation in top soils as well as rates of MP transport to UK groundwater resources and resulting accumulation of MP and additives (using BPA and NP concentration ranges from WP1).
M8 Development of UK-wide MP groundwater recharge scenarios (month 32)
M9 Quantification of MP transport to UK groundwater aquifers and topsoil residence times (month 38)
Scenarios considering technological innovations in biodegradable polymers will be co-created with Syngenta, based on enhanced biodegradation of novel formulations. Utilizing the additional contribution of an independently Syngenta funded PhD studentship, information on time-dependent MP degradation rates will be used to parameterises the HP2 reaction kinetics module (Jaques et al., 2012) in HYDRUS for estimating how particle degradation and changed MP size distributions affect topsoil residence and aquifer transit times. Scenario simulations of the UK-wide WP4 model will reveal to what degree and under what conditions polymer degradation is likely to reduce MP aquifer inputs and at what time scales. The simulation results will inform biodegradable polymer designs as well as soil management recommendations to reduce current and future MP recharge to groundwater.
M10 Development of innovation scenarios (month 42)
M11 Scenario simulations for evaluation of efficiency of different polymer biodegradation options (month 46)
Impact and Outcomes
This Royal Society Industry Fellowship will create technological solutions of high commercial relevance to Syngenta Ltd as well as the wider UK and international agrochemical, water, environmental and manufacturing sectors. The project outcomes, specifically the understanding of exposure rates and residence time distributions in top soils will have direct impact on the assessment of plastic environmental impacts on soil microbiome health, with immediate relevance for agricultural production, food security and safety. The model results of this fellowship will allow manufacturers of agrochemical formulations to make informed decisions on product changes towards faster biodegradable materials. In addition, model derived time scales and quantities of MP recharge towards groundwater support changes in agricultural practice and the design of solid waste and wastewater management by identifying high-risk areas of groundwater MP contamination from topsoil sources and thus, underlining designation of best practice principles for risk zones and potentially exclusion areas for the application of wastewater sludges or inform groundwater pumping and abstraction rates applied by water industries.
Project results will be disseminated through scientific publications in high-impact scientific journals and summary briefs to non-academic stakeholders distributed via the project website and social media channels as well as by presenting research results at international scientific conferences (AGU, EGU, SETAC). The co-creation of this ambitious research fellowship with Syngenta Ltd offers pathways into direct industry decision making processes on material designs and thus, immediate impact of the created research innovations beyond the scientific community.
For the applicant, the Royal Society Industry Fellowship provides a unique opportunity to transform his research on the environmental MP cycle, extending his aquatic and terrestrial research towards inclusion of subsurface MP transport and transformation processes which remain the last frontier in environmental MP research. The developed physical and mathematical modelling techniques will provide opportunity for wide-ranging industry applications in the agro-chemical, water and energy sectors, thus, supporting his long-term strategy of pioneering technological innovation that enable private and public sectors to improve water resource management and the safe use of plastics.
English