Guidance on the selection of efficient computational methods for multimedia fate models

How good are the predictions of mobility of aged polychlorinated biphenyls (PCBs) in soil? Insights from a soil column experiment

A column leaching experiment was performed to evaluate the influence of some relevant environmental factors (soil/water contact time, temperature, saturation) on mobility of aged polychlorinated biphenyls (PCBs) in soil together with transport mediated by dissolved organic carbon (DOC) and mobile organic carbon (OC) coated fine particles/colloids. Consecutive fractions of leachates were collected after a variable pre-equilibration time (2, 5, 7, 48 days), using leaching solutions with different DOC content (tap water vs. Aldrich humic acid), in saturated vs. field capacity conditions and at different temperatures (25 °C vs. 15 °C). The data obtained were compared to the predicted values using a multimedia model (SoilPlusVeg) to evaluate model behaviour. Contact time and temperature determined a relevant effect on DOC and particle/colloid availability, with significant variations in leachate concentrations (up to 1 order of magnitude), typically overlooked by most environmental fate models. Results obtained at different temperatures show a modulation of the DOC/particles production with temperature and therefore the role of temperature changes in the environmental scenarios (e.g. seasonal variations). Transport of PCBs enhanced by Aldrich DOC was not linearly correlated to chemical hydrophobicity but revealed a threshold to ~Log K_OW 6.5, likely because of the slow sorption kinetics of more hydrophobic chemicals. Additionally, variation of the saturation conditions (e.g. drying-wetting cycles) can determine contamination peaks at the beginning of an irrigation/rainfall event because of the soil/water equilibration. Model simulations, even when including DOC in the water phase, but not accounting for the particle/colloidal transport and sorption/desorption kinetics, mismatched the ratio of dissolved vs. DOC-associated and particle-associated PCBs and substantially underpredicted concentrations, especially for the high chlorinated congeners. The results indicated that some of the common assumptions and paradigms in fate modelling of such hydrophobic compounds should be revisited and models updated.

Pesticide fate in cultivated mountain basins: The improved DynAPlus model for predicting peak exposure and directing sustainable monitoring campaigns to protect aquatic ecosystems

Agricultural activities can involve the use of plant protection products (PPPs) and the use of such chemicals can occur near surface waters bodies, thus creating a potential for adverse effects on aquatic ecosystems. In mountain watersheds, where runoff fluxes are particularly rapid due to side slopes, exposure is generally characterized by short but intense concentration peaks. Monitoring campaigns are often inadequate or too expensive to be carried out and modelling tools are therefore vital for exposure assessment and their use is encouraged by current legislation. However, currently adopted models and scenarios (e.g., FOCUS for PPPs) are often too conservative and/or "static" to accurately capture exposure variability, and the need for more realistic and dynamic tools is now one of the major challenges for risk assessment. In a previous work, the new fate model DynAPlus was developed to improve pesticide fate predictions in cultivated mountain basins and was successfully evaluated against chlorpyrifos water concentrations measured in a mountain stream in Northern Italy. However, the need for some model improvements (e.g., the inclusion of dissolved organic matter and macrophytes in water) was highlighted. In this work, DynAPlus was improved by replacing the water-sediment unit with ChimERA fate, a recently-published model capable of predicting bioavailable chemical concentrations in shallow water environments accounting for the presence and temporal variations of particulate/dissolved organic carbon and primary producers. The model was applied to preliminarily characterize the risk associated to the use of four PPPs (two insecticides and two fungicides) in a sub-basin of the Adda River (Valtellina Valley, Northern Italy), surrounded by apple orchards. Results revealed the potential magnitude of exposure peaks for the four PPPs and suggested that monitoring campaigns should prioritize, in the selected case study, chlorpyrifos, etofenprox and fluazinam. The potential role of DynAPlus in providing more realistic exposure predictions for ecological risk assessment, as well as for planning efficient monitoring campaigns and help pesticide management practices, was also stressed.

Improving the SoilPlusVeg model to evaluate rhizoremediation and PCB fate in contaminated soils

Tools to predict environmental fate processes during remediation of persistent organic pollutants (POPs) in soil are desperately needed since they can elucidate the overall behavior of the chemical and help to improve the remediation process. A dynamic multimedia fate model (SoilPlusVeg) was further developed and improved to account for rhizoremediation processes. The resulting model was used to predict Polychlorinated Biphenyl (PCB) fate in a highly contaminated agricultural field (1089 ng/g d.w.) treated with tall fescue (Festuca arundinacea), a promising plant species for the remediation of contaminated soils. The model simulations allowed to calculate the rhizoremediation time (about 90 years), given the available rhizoremediation half-lives and the levels and fingerprints of the PCB congeners, to reach the legal threshold, to show the relevance of the loss processes from soil (in order of importance: degradation, infiltration, volatilization, etc.) and their dependence on meteorological and environmental dynamics (temperature, rainfall, DOC concentrations). The simulations showed that the effective persistence of PCBs in soil is deeply influenced by the seasonal variability. The model also allowed to evaluate the role of DOC as a possible enhancer of PCB degradation as a microorganism "spoon feeder" of PCBs in the soil solution. Additionally, we preliminary predicted how the contribution of PCB metabolites could modify the PCB fingerprint and their final total concentrations. This shows that the SoilPlusVeg model could be used in selecting the best choices for a sustainable rhizoremediation of a POP contaminated site.

Do environmental dynamics matter in fate models? Exploring scenario dynamics for a terrestrial and an aquatic system

Nowadays, there is growing interest in inserting more ecological realism into risk assessment of chemicals. On the exposure evaluation side, this can be done by studying the complexity of exposure in the ecosystem, niche partitioning, e.g. variation of the exposure scenario. Current regulatory predictive approaches, to ensure simplicity and predictive ability, generally keep the scenario as static as possible. This could lead to under or overprediction of chemical exposure depending on the chemical and scenario simulated. To account for more realistic exposure conditions, varying temporally and spatially, additional scenario complexity should be included in currently used models to improve their predictive ability. This study presents two case studies (a terrestrial and an aquatic one) in which some polychlorinated biphenyls (PCBs) were simulated with the SoilPlusVeg and ChimERA models to show the importance of scenario variation in time (biotic and abiotic compartments). The results outlined the importance of accounting for planetary boundary layer variation and vegetation dynamics to accurately predict air concentration changes and the timing of chemical dispersion from the source in terrestrial systems. For the aquatic exercise, the results indicated the need to account for organic carbon forms (particulate and dissolved organic carbon) and vegetation biomass dynamics. In both cases the range of variation was up to two orders of magnitude depending on the congener and scenario, reinforcing the need for incorporating such knowledge into exposure assessment.

SoilPlusVeg: An integrated air-plant-litter-soil model to predict organic chemical fate and recycling in forests

Current modelling approaches often ignore the dynamics of organic chemicals uptake/release in forest compartments under changing environmental conditions and may fail in accurately predict exposure to chemicals for humans and ecosystems. In order to investigate the influence of such dynamics on predicted concentrations in forest compartments, as well as, on air-leaf-litter fluxes, the SoilPlusVeg model was developed including a forest compartment (root, stem, leaves) in an existing air-litter-soil model. The accuracy of the model was tested simulating leaf concentrations in broadleaf woods located in Northern Italy and resulted in satisfying model performance. Illustrative simulations highlighted the "dual behaviour" of both leaf and litter compartments. Leaves appeared to behave as "filters" of air contaminants but also as "dispensers", being deposition flux exceeded by volatilization flux in some periods of the day. Similarly, litter seemed to behave as a dynamic compartment which could accumulate and then release contaminants recharging air and vegetation. In just 85days, litter could lose due to volatilization, diffusion to depth and infiltration processes, from 6% to 90% of chemical amount accumulated over 1year of exposure, depending on compound physical and chemical properties. SoilPlusVeg thus revealed to be a powerful tool to understand and estimate chemical fate and recycling in forested systems.

European environmental scenarios of chemical bioavailability in freshwater systems

In exposure prediction for environmental risk assessment, the transition to more dynamic and realistic modelling approaches and scenarios has been recently identified as a major challenge, since it would allow a more accurate prediction of bioavailable concentrations and their variations in space and time. In this work, an improved version of the multimedia model ChimERA fate, including a phytoplankton compartment and equations to calculate phytoplankton, detritus and dissolved organic matter variations in time, was developed. The model was parameterized to simulate five dynamic scenarios for shallow meso-eutrophic water bodies based on a latitudinal gradient (in Europe); such scenarios include seasonal profiles of water temperature, phytoplankton biomass, detritus, and dissolved organic matter. Model runs were performed for each scenario for 8 hydrophobic chemicals (PCB congeners), with the aim of investigating the influence of scenario characteristics and compound properties on bioavailable concentrations. The key processes were adsorption/uptake by phytoplankton and deposition to sediment of detritus-bound chemicals. The northern scenarios ("Scandinavia" and "UK") showed the highest bioavailable concentrations, with annual maximum/minimum concentration up to 25; in contrast, for example, maximum concentrations in the "Mediterranean" scenario were lower by a factor of 2 to 9 with respect to the northern ones (depending on chemical hydrophobicity), due to the generally higher biomass and carbon levels, and showed only limited seasonal variability (up to a factor of 4). These results highlight the importance of including biomass and organic carbon dynamics in both modelling approaches and scenarios for the evaluation of exposure concentrations in aquatic environments.

Importance of environmental and biomass dynamics in predicting chemical exposure in ecological risk assessment

In ecological risk assessment, exposure is generally modelled assuming static conditions, herewith neglecting the potential role of emission, environmental and biomass dynamics in affecting bioavailable concentrations. In order to investigate the influence of such dynamics on predicted bioavailable concentrations, the spatially-resolved dynamic model "ChimERA fate" was developed, incorporating macrophyte and particulate/dissolved organic carbon (POC/DOC) dynamics into a water-sediment system. An evaluation against three case studies revealed a satisfying model performance. Illustrative simulations then highlighted the potential spatio-temporal variability of bioavailable concentrations after a pulsed emission of four chemicals in a system composed of a pond connected to its inflow and outflow streams. Changes in macrophyte biomass and POC/DOC levels caused exposure variations which were up to a factor of 4.5 in time and even more significant (several orders of magnitude) in space, especially for highly hydrophobic chemicals. ChimERA fate thus revealed to be a useful tool to investigate such variations and to identify those environmental and ecological conditions in which risk is expected to be highest.

Evaluating the temporal variability of concentrations of POPs in a glacier-fed stream food chain using a combined modeling approach

Falling snow acts as an efficient scavenger of contaminants from the atmosphere and, accumulating on the ground surface, behaves as a temporary storage reservoir; during snow aging and metamorphosis, contaminants may concentrate and be subject to pulsed release during intense snow melt events. In high-mountain areas, firn and ice play a similar role. The consequent concentration peaks in surface waters can pose a risk to high-altitude ecosystems, since snow and ice melt often coincide with periods of intense biological activity. In such situations, the role of dynamic models can be crucial when assessing environmental behavior of contaminants and their accumulation patterns in aquatic organisms. In the present work, a dynamic fate modeling approach was combined to a hydrological module capable of estimating water discharge and snow/ice melt contributions on an hourly basis, starting from hourly air temperatures. The model was applied to the case study of the Frodolfo glacier-fed stream (Italian Alps), for which concentrations of a number of persistent organic pollutants (POPs), such as polychlorinated biphenyl (PCBs) and p,p'-dichlorodiphenyldichloroethylene (p,p'-DDE) in stream water and four macroinvertebrate groups were available. Considering the uncertainties in input data, results showed a satisfying agreement for both water and organism concentrations. This study showed the model adequacy for the estimation of pollutant concentrations in surface waters and bioaccumulation in aquatic organisms, as well as its possible role in assessing the consequences of climate change on the cycle of POPs.

Investigating the need for complex vs. simple scenarios to improve predictions of aquatic ecosystem exposure with the SoilPlus model

A spatially-explicit version of the recent multimedia fate model SoilPlus was developed and applied to predict the runoff of three pesticides in a small agricultural watershed in north-eastern Italy. In order to evaluate model response to increasing spatial resolution, a tiered simulation approach was adopted, also using a dynamic model for surface water (DynA model), to predict the fate of pesticides in runoff water and sediment, and concentrations in river water. Simulation outputs were compared to water concentrations measured in the basin. Results showed that a high spatial resolution and scenario complexity improved model predictions of metolachlor and terbuthylazine in runoff to an acceptable performance (R(2) = 0.64-0.70). The importance was also shown of a field-based database of properties (i.e. soil texture and organic carbon, rainfall and water flow, pesticides half-life in soil) in reducing the distance between predicted and measured surface water concentrations and its relevance for risk assessment.

Integration of a dynamic organism model into the DynA Model: development and application to the case of DDT in Lake Maggiore, Italy

The Single Organism (SO) model was developed to investigate the influence of temporal dynamics of aquatic organism properties on their exposure to organic chemicals in water. SO was then integrated with an existing dynamic surface-water model (DynA), to form the coupled water-bioaccumulation model EcoDynA. In order to evaluate the model performance, the results produced by EcoDynA were compared to the p,p'-DDT concentrations measured in specimens of whitefish of different age and sex caught in Lake Maggiore after the discovery of a DDT spill. The comparison showed a good agreement. Other satisfying results were obtained comparing model results with p,p'-DDT concentration values measured in another species of whitefish which were available in the literature. A preliminary sensitivity analysis confirmed that accounting for dynamics of parameters such as organism lipid fraction and feeding rate is necessary to obtain accurate exposure predictions.