Dynamic modeling of chemical fate and transport in multimedia environments at watershed scale-I: theoretical considerations and model implementation

Forests effects on the environmental fates of organic pollutants in a tropical watershed

Poly-brominated diphenyl ethers (PBDEs), dioxins, furans and current-use pesticides (CUPs) are common organic pollutants that have received global scrutiny due to their association with adverse environmental and health impacts. However, there is limited previous work assessing their environmental fates in the context of tropical multi-use watersheds. The aim of this study was to estimate the effect of forests on long-term environmental fate for some PBDEs, dioxins, furans and CUPs for a case study watershed of a tropical developing country (the Rio Cobre River drainage basin, Jamaica). Specifically, a dynamic, 16-compartment environmental multimedia model, RioShed, was developed and applied to calculate compartmental concentrations, as well as some long-term environmental fate metrics. Results indicate that the presence of tropical forests, especially those that are evergreen, reduced the atmospheric concentrations, atmospheric long-range transport potential, and the overall persistence of the study pollutants. Reductions in atmospheric concentrations by tropical forests were most enhanced for the more polar CUPs. Forest parameters that notably influenced soil concentration and/or overall persistence included the canopy drip parameter, the leaf area index and the wax erosion rate. The results of this research are expected to inform land-use and environmental management of the study area and similar tropical regions.

A Comparative Assessment of Analytical Fate and Transport Models of Organic Contaminants in Unsaturated Soils

Analytical models for the simulation of contaminants’ fate and transport in the unsaturated zone are used in many engineering applications concerning groundwater resource management and risk assessment. As a consequence, several scientific studies dealing with the development and application of analytical solutions have been carried out. Six models have been selected and compared based on common characteristics to identify pros and cons as well as to highlight any difference in the final output. The analyzed models have been clustered into three groups according to the assumptions on contaminant source and physico-chemical mechanisms occurring during the transport. Comparative simulations were carried out with five target contaminants (Benzene, Benzo(a)pyrene, Vinyl Chloride, Trichloroethylene and Aldrin) with different decay’s coefficient, three types of soil (sand, loam and clay) and three different thicknesses of the contaminant source. The calculated concentration at a given depth in the soil for the same contamination scenario varied greatly among the models. A significant variability of the concentrations was shown due to the variation of contaminant and soil characteristics. As a general finding, the more advanced is the model, the lower the predicted concentrations; thus, models that are too simplified could lead to outcomes of some orders of magnitude greater than the advanced one.

Development and evaluation of a dynamic multimedia model (ECORAME) for local scale assessment of aquatic ecological exposure to chemicals originating from sources in environmental media

Use of multimedia models (MMMs) has been limited in exposure assessment for aquatic ecosystems at local scale typically due to their coarse spatial resolution and inability to predict the individual concentrations of multiple streams within a watershed cell. An MMM named ECORAME is presented which overcomes the limitations by treating each water segment as an independent cell rather than a compartment within a watershed cell. This offers two advantages for exposure assessment, i.e., i) the spatial resolution for water is readily adjustable and ii) multiple water streams within one watershed cell could be handled individually. Model evaluation with respect to polycyclic aromatic hydrocarbons (PAHs) demonstrated that ECORAME's prediction of relative concentration agreed with measured values within a factor of five or less. A case study of PAHs using ECORAME shows that the concentration can change by more than 10 fold over the 40km main stream stretch of the Han River in Seoul, Korea. The concentration difference among multiple streams in the same watershed cell could be substantial (greater than 100 fold). Besides a need of finer spatial resolutons than those typically used in MMMs, the results strongly suggest that exposure prediction capability for individual streams in the same watershed is necessary for local scale assessment. As demostrated with ECORAME, the need can be effectively met by handling the water segments as individual cells in future MMMs.

Modeling complexity in simulating pesticide fate in a rice paddy

Modeling approaches for pesticide regulation are required to provide generic and conservative evaluations on pesticide fate and exposure based on limited data. This study investigates the modeling approach for pesticide simulation in a rice paddy, by developing a component-based modeling system and characterizing the dependence of pesticide concentrations on individual fate processes. The developed system covers the modeling complexity from a "base model" which considers only the essential processes of water management, water-sediment exchange, and aquatic dissipation, to a "full model" for all commonly simulated processes. Model capability and performance were demonstrated by case studies with 5 pesticides in 13 rice fields of the California's Sacramento Valley. With registrant-submitted dissipation half-lives, the base model conservatively estimated dissolved pesticide concentrations within one order of magnitude of measured data. The full model simulations were calibrated to characterize the key model parameters and processes varying with chemical properties and field conditions. Metabolism in water was identified as an important process in predicting pesticide fate in all tested rice fields. Relative contributions of metabolism, hydrolysis, direct aquatic photolysis, and volatilization to the overall pesticide dissipation were significantly correlated to the model sensitivities to the corresponding physicochemical properties and half-lives. While modeling results were sensitive to metabolism half-lives in water for all fields, significances of metabolism in sediment and water-sediment exchange were only observed for pesticides with pre-flooding applications or with rapid dissipation in sediment. Results suggest that, in addition to the development of regional modeling scenarios for rice production, the registrant-submitted maximum values for the aquatic dissipation half-lives could be used for evaluating pesticide for regulatory purposes.

Environmental modeling and exposure assessment of sediment-associated pyrethroids in an agricultural watershed

Synthetic pyrethroid insecticides have generated public concerns due to their increasing use and potential effects on aquatic ecosystems. A modeling system was developed in this study for simulating the transport processes and associated sediment toxicity of pyrethroids at coupled field/watershed scales. The model was tested in the Orestimba Creek watershed, an agriculturally intensive area in California' Central Valley. Model predictions were satisfactory when compared with measured suspended solid concentration (R(2) = 0.536), pyrethroid toxic unit (0.576), and cumulative mortality of Hyalella azteca (0.570). The results indicated that sediment toxicity in the study area was strongly related to the concentration of pyrethroids in bed sediment. Bifenthrin was identified as the dominant contributor to the sediment toxicity in recent years, accounting for 50-85% of predicted toxicity units. In addition, more than 90% of the variation on the annual maximum toxic unit of pyrethroids was attributed to precipitation and prior application of bifenthrin in the late irrigation season. As one of the first studies simulating the dynamics and spatial variability of pyrethroids in fields and instreams, the modeling results provided useful information on new policies to be considered with respect to pyrethroid regulation. This study suggested two potential measures to efficiently reduce sediment toxicity by pyrethroids in the study area: [1] limiting bifenthrin use immediately before rainfall season; and [2] implementing conservation practices to retain soil on cropland.

Multimedia transport and risk assessment of organophosphate pesticides and a case study in the northern San Joaquin Valley of California

This paper presents a framework for cumulative risk characterization of human exposure to pesticides through multiple exposure pathways. This framework is illustrated through a case study of selected organophosphate (OP) pesticides in the northern San Joaquin Valley of California. Chemical concentrations in environmental media were simulated using a multimedia environmental fate model, and converted to contamination levels in exposure media. The risk characterization in this study was based on a residential-scale exposure to residues of multiple pesticides through everyday activities. Doses from a mixture of OP pesticides that share a common mechanism of toxicity were estimated following US Environmental Protection Agency guidelines for cumulative risk analysis. Uncertainty in the human exposure parameters was included in the Monte Carlo simulation in order to perform stochastic calculations for intakes and corresponding risks of OP pesticides. Risk of brain acetylcholinesterase inhibition was reported as margins of exposure (MOEs) of the 99.9th population percentile for two age groups living in the northern San Joaquin Valley during 1992-2005. Diet was identified as the dominant exposure pathway in cumulative exposure and risk, while the temporal trend and spatial variation in total MOE levels were associated with exposures to contaminated drinking water and ambient air. Uniformly higher risks were observed for children because of their greater inhalation and ingestion rates per body weight, relative to adults. The results indicated that exposures for children were about twice of those estimated for adults. Concerns over children's exposure to OP pesticide through food and water ingestion were suggested based on the spatiotemporal variations predicted for the subchronic MOEs at the 99.9th percentile of exposure in the study area.

Transport and dynamics of toxic pollutants in the natural environment and their effect on human health: research gaps and challenge

The source-pathway-receptor (SPR) approach to human exposure and risk assessment contains considerable uncertainty when using the refined modelling approaches to pollutant transport and dispersal, not least in how compounds of concern might be prioritized, proxy or indicator substances identified and the basic environmental and toxicological data collected. The impact of external environmental variables, urban systems and lifestyle is still poorly understood. This determines exposure of individuals and there are a number of methods being developed to provide more reliable spatial assessments. Within the human body, the dynamics of pollutants and effects on target organs from diffuse, transient sources of exposure sets ambitious challenges for traditional risk assessment approaches. Considerable potential exists in the application of, e.g. physiologically based pharmacokinetic (PBPK) models. The reduction in uncertainties associated with the effects of contaminants on humans, transport and dynamics influencing exposure, implications of adult versus child exposure and lifestyle and the development of realistic toxicological and exposure data are all highlighted as urgent research needs. The potential to integrate environmental with toxicological models provides the next phase of research opportunity and should be used to drive empirical and model assessments.

Dynamic modeling of chemical fate and transport in multimedia environments at watershed scale-II: trichloroethylene test case

A multimedia environmental fate model was developed to study the temporal dynamics and spatial distribution of a chemical pollutant at watershed scale. The theoretical considerations and implementation of the model were described in the accompanying paper (Part I). This paper presents the result of a test simulation on the transport of trichloroethylene (TCE) in the Connecticut River Basin. The simulation results were reported as time series of concentrations and inter-media transport fluxes in the compartments of atmosphere, plant, soil, surface water, and sediment. Predicted concentrations from the test simulation were compared with published field data or predictions by validated models. The temporal trends in TCE predictions were evaluated by comparing the simulation results with monthly TCE concentrations in various environmental compartments and monthly fluxes of inter-media transport processes. Results indicated that the simulation results were in reasonable agreement with reported data in the literature. The results also revealed that the mass transport of TCE from the atmosphere compartment to soil and surface water was a major route of TCE dispersion in the environment.