Sources of uncertainty in pesticide fate modelling

There is worldwide interest in the application of probabilistic approaches to pesticide fate models to account for uncertainty in exposure assessments. The first steps in conducting a probabilistic analysis of any system are: (i) to identify where the uncertainties come from; and (ii) to pinpoint those uncertainties that are likely to affect most of the predictions made. This article aims at addressing those two points within the context of exposure assessment for pesticides through a review of the different sources of uncertainty in pesticide fate modelling. The extensive listing of sources of uncertainty clearly demonstrates that pesticide fate modelling is laced with uncertainty. More importantly, the review suggests that the probabilistic approaches, which are typically being deployed to account for uncertainty in the pesticide fate modelling, such as Monte Carlo modelling, ignore a number of key sources of uncertainty, which are likely to have a significant effect on the prediction of environmental concentrations for pesticides (e.g. model error, modeller subjectivity). Future research should concentrate on quantifying the impact these uncertainties have on exposure assessments and on developing procedures that enable their integration within probabilistic assessments.     

Using a linked soil model emulator and unsaturated zone leaching model to account for preferential flow when assessing the spatially distributed risk of pesticide leaching to groundwater in England and Wales

Although macropore flow is recognized as an important process for the transport of pesticides through a wide range of soils, none of the existing spatially distributed methods for assessing the risk of pesticide leaching to groundwater account for this phenomenon. The present paper presents a spatially distributed modelling system for predicting pesticide losses to groundwater through micro- and macropore flow paths. The system combines a meta version of the mechanistic, dual porosity, preferential flow pesticide leaching model MACRO (the MACRO emulator), which describes pesticide transport and attenuation in the soil zone, to an attenuation factor leaching model for the unsaturated zone. The development of the emulator was based on the results of over 4000 MACRO model simulations. Model runs describe pesticide leaching for the range of soil types, climate regimes, pesticide properties and application patterns in England and Wales. Linking the MACRO emulator to existing spatial databases of soil, climate and compound-specific loads allowed the prediction of the concentration of pesticide leaching from the base of the soil profile (at 1 m depth) for a wide range of pesticides. Attenuation and retardation of the pesticide during transit through the unsaturated zone to the watertable was simulated using the substrate attenuation factor model AQUAT. The MACRO emulator simulated pesticide loss in 10 of 12 lysimeter soil-pesticide combinations, for which pesticide leaching was shown to occur and also successfully predicted no loss from 3 soil-pesticide combinations. Although the qualitative aspect of leaching was satisfactorily predicted, actual pesticide concentrations in leachate were relatively poorly predicted. At the national scale, the linked MACRO emulator/AQUAT system was found to predict the relative order of, and realistic regional patterns of, pesticide leaching for atrazine, isoproturon, chlorotoluron and lindane. The methodology provides a first-step assessment of the potential for pesticide leaching to groundwater in England and Wales. Further research is required to improve the modelling concept proposed. The system can be used to refine regional groundwater monitoring system designs and sampling strategies and improve the cost-effectiveness of the measures needed to achieve 'good status' of groundwater quality as required by the Water Framework Directive.     

A fugacity based continuous and dynamic fate and transport model for river networks and its application to Altamaha River

In this paper, a continuous and dynamic fugacity-based contaminant fate and transport model is developed. The dynamic interactions among all phases in the physical domain are addressed through the use of the fugacity approach instead of the use of concentration as the unknown variable. The full form of Saint Venant equations is used in order to solve for the hydrodynamic conditions in the river network. Then a fugacity-based advection-dispersion equation is modeled to examine the fate and transport of contaminants in the river network for all phases.The fugacity-based, dynamic and continuous contaminant fate and transport model developed here is applied to Altamaha River in Georgia, USA to demonstrate its use in environmental exposure analysis. Altamaha River is the largest river system east of Mississippi which offers habitat for many species, including about 100 rare endangered species, along its 140 mile course. Polychlorinated biphenyls (PCBs), a highly hydrophobic and toxic chemical ubiquitous in nature, and atrazine, the most commonly-used agricultural pesticide are modeled as contaminants in this demonstration. Through this approach the concentration distribution of PCBs and atrazine in the water column of Altamaha River as well as the sediments can be obtained with relative ease, which is an improvement over concentration based analysis of phase distribution of contaminants.     

Impacts of climate change on the fate and behaviour of pesticides in surface and groundwater--A UK perspective

Over the last two decades significant effort has been dedicated to understanding the fate and transport of pesticides in surface water and groundwater and to use this understanding in the development of environmental policy and regulation. However, there have been few studies that have investigated the relationships between pesticides and climate change, and where this work has been undertaken it has principally been in relation to the impacts of climate change on agricultural production rather than in the context of environmental protection. This study addresses that gap by reviewing how climate change may impact the fate and transport of pesticides in surface and groundwaters as a pre-cursor to quantitative studies. In order to structure the review, we have adopted a source-pathway-receptor approach where climate sensitivities of pesticide source terms, environmental pathways and receptors are reviewed. The main climate drivers for changing pesticide fate and behaviour are thought to be changes in rainfall seasonality and intensity and increased temperatures, but the effect of climate change on pesticide fate and transport is likely to be very variable and difficult to predict. In the long-term, indirect impacts, such as land-use change driven by changes in climate, may have a more significant effect on pesticides in surface and groundwaters than the direct impacts of climate change on pesticide fate and transport. The review focuses on climate change scenarios and case studies from the UK; however, the general conclusions can be applied more widely.     

Three-dimensional modelling of mercury cycling in the Gulf of Trieste

The Gulf of Trieste (Northern Adriatic) is subject to mercury pollution from a former mercury mine in Idrija, located along a river which transports mercury-contaminated sediments into the Gulf. Concentrations in suspended and bottom sediments are up to two orders of magnitude higher than in the central and southern Adriatic. Extensive research has been carried out on measurements and modelling of the transport and fate of mercury in the Gulf. Two- and three-dimensional models have been developed to include the influence of the significant advective transport due to currents. Wind, thermohaline forcing, and the Soca river momentum are the most important forcing factors. A two-dimensional model simulated the transport of non-methylated and methylated mercury in dissolved, particulate and plankton fractions. Mercury processes included the input of atmospheric mercury, sedimentation, reduction, methylation and demethylation. The model simulations gave basically what were proper trends of the phenomena; quantitatively the measured and computed results are mainly within a factor of three. To simulate the non-uniform distribution of parameters over the depth, an existing three-dimensional (3D) hydrodynamic and transport-dispersion (TD) model, PCFLOW3D, was adapted and applied. As it was found that most mercury transport is related to suspended sediment particles, a new 3D sediment transport module was also developed and included in the model. Three cases are presented: one describing the simulation of TD of dissolved total mercury; another the simulation of the TD of particulate mercury in the Gulf during a river flood; and the third simulating sediment transport in the Gulf during a period of strong ENE wind. Comparison with measurements was only partly possible, but mainly the computed and measured results were within a factor of two and proper trends of the phenomena were obtained by the simulations. The combination of modelling and measurements has resulted in some interesting conclusions about the phenomenon of the transport and fate of mercury in a coastal sea.     

北京城市扩展的环境效应模拟与评价

基于L THIA GIS模型,以北京为案例研究区,应用水文土壤类型、土地利用类型以及长期日降水量数据,有效模拟了长时间段内以城市扩展为主要特征的土地利用变化对水文过程以及城市非点源污染的宏观影响态势,挖掘出区域内潜在的长期污染问题及其空间分布。结果表明,虽然1996—2004年间连续干旱导致年径流量整体减少,但城八区年径流量却呈增加趋势,各分区年径流量变化幅度与耕地向建成区转化的比例具有一致性,说明保护并合理利用耕地对减少城市径流有重要意义;城市建设用地的单位面积产流量远远大于农业用地和生态用地,林地向城市建设用地转变所产生的水文效应敏感性最强,其次依次为：水域、草地、农村居民点和农业用地,林地、水域、草地等生态用地需要特别加强保护;通过L THIA模型和GIS平台,实现了区域非点源污染的分布式模拟,结果显示城市扩展对城市非点源污染的影响大于降水变化的影响,根据模拟结果和下垫面（土地利用）对城市非点源污染的影响,初步将北京划分为4类城市非点源污染产生的敏感性区域,为土地利用管理和环境规划提供指引。     

Mechanisms of Groundwater Pollution by Pesticides

An increasing number of pesticide compounds are being detected in groundwater supplies worldwide. There has, nevertheless, been little research into the mechanisms by which such pollution occurs and the routes via which pesticides move from the land surface into aquifers. In view of the potential time-lag in the response of groundwater systems to pollution inputs, such knowledge is essential if the current situation is to be appraised adequately and if any necessary controls on pesticide use are to be introduced.
From basic data on the physicochemical properties of pesticide compounds and on groundwater flow and pollutant transport in the principal British aquifers, some useful observations can be made about the relative significance of different mechanisms of potential groundwater pollution. These mechanisms and their implications for field research on pesticide pollution of groundwater are discussed. This problem is compared and contrasted to that of nitrate pollution from agricultural land-use practices.     

The significance of colloids in the transport of pesticides through Chalk

Agrochemical contamination in groundwater poses a significant long term threat to water quality and is of concern for legislators, water utilities and consumers alike. In the dual porosity, dual permeability aquifers such as the Chalk aquifer, movement of pesticides and their metabolites through the unsaturated zone to groundwater is generally considered to be through one of two pathways; a rapid by-pass flow and a slower 'piston-flow' route via the rock matrix. However, the dissolved form or 'colloidal species' in which pesticides move within the water body is poorly understood. Following heavy rainfall, very high peaks in pesticide concentration have been observed in shallow Chalk aquifers. These concentrations might be well explained by colloidal transport of pesticides. We have sampled a Chalk groundwater beneath a deep (30 m) unsaturated zone known to be contaminated with the pesticide diuron. Using a tangential flow filtration technique we have produced colloidal fractions from 0.45 microm to 1 kDa. In addition, we have applied agricultural grade diuron to a typical Chalk soil and created a soil water suspension which was also subsequently fractionated using the same filtration system. The deep groundwater sample showed no evidence of association between colloidal material and pesticide concentration. In comparison, despite some evidence of particle trapping or sorption to the filters, the soil water clearly showed an association between the <0.45 microm and <0.1 microm colloidal fractions which displayed significantly higher pesticide concentrations than the unfiltered sample. Degradation products were also observed and found to behave in a similar manner to the parent compound. Although relatively large colloids can be generated in the Chalk soil zone, it appears transport to depth in a colloidal-bound form does not occur. Comparison with other field and monitoring studies suggests that rapid by-pass flow is unlikely to occur beneath 4-5 m. Therefore, shallow groundwaters are most at risk from rapid transport of high concentrations of pesticide-colloidal complexes. The presence of a deep unsaturated zone will mean that most of the colloidal-complexes will be filtered by the narrow Chalk pores and the majority of pesticide transport will occur in a 'dissolved' form through the more gradual 'piston-flow' route.     

Integrated modelling of eutrophication and organic contaminant fate & effects in aquatic ecosystems. A review

Eutrophication and contamination with micropollutants have been major problems in water quality management. Both problems have been subjected to extensive research and modelling but traditionally are treated separately. Traditional simulation models for aquatic systems can be categorised as eutrophication models, contaminant fate models, food web models and food chain bioaccumulation models. Because they are single issue models, many interactions and feedbacks between the food web, nutrient and toxicant cycles are missed. Linking these models is essential to evaluate the fate and risks of contaminants in systems with changing nutrient loading, to assess the natural attenuation of contaminants or to understand the selfpurifying capacity of ecosystems. Combination of the single issue models requires inclusion of 'interaction processes' to account for the coupling between the (sub-) model types, such as organic carbon cycling. toxicity, transport and accumulation of organic contaminants in the food chain, and bottom up versus top down control of primary production and nutrient cycling. This review first provides a brief overview of traditional approaches in modelling eutrophication, contaminant fate, food web dynamics and food chain bioaccumulation. Second, five existing integrated eutrophication, fate and/or effects models are reviewed. Third, the gaps and limitations in modelling the four types of interaction processes are discussed. It is concluded that these models are invaluable tools to focus attention to feedback mechanisms that are often overlooked, to identify dominating processes in ecosystems, to formulate counterintuitive hypotheses on ecosystem functioning, or to assess short term risks of acutely toxic stressors. However, the potential of integrated models for long term simulations of contaminant exposure. food chain bioaccumulation and effects to higher trophic levels remains limited, mainly because of principal limitations in food web modelling. In contrast, the potential of integrated models for long term simulations of contaminant fate is better because the environmental distribution of contaminants is mainly determined by the major abiotic compartments and by biotic compartments at the base of the food chain.     

Soil erosion by rainfall and runoff—state of the art

The processes of soil detachment and sediment transport by rainfall and overland flow and resulting sediment transport are described. The results of past research on the role of raindrop impact in detaching soil, experimental studies of erosion, and erosion modelling are presented. Studies of soil loss from experimental field plots led to the development of the Universal Soil Loss Equation and to its subsequent modification for use in quantifying erosion by individual storms. Systems to produce rainfall for erosion studies in the laboratory are described. A system which is currently being calibrated and used for erosion control system testing at Drexel University's Geosynthetic Research Institute (GRI) is described. Preliminary spatial rainfall distributions and runoff measurements are presented. Experimental results obtained by others on the performance of natural and geosynthetic erosion control systems are discussed along with a proposed experimental program for the GRI system.     

Verification of a theoretical model for pesticidal pollution

Lin, Sahi and Eyring's model for the accumulation of pesticide residues in soil, which may relate to pollution by pesticides, has been further developed to estimate maximum pesticide residues in soil and vegetation as a function of time for different orders of pesticide decomposition reactions. The developed model has also been verified from some experimental data available in the literature on solbar, carbofuron, phorate, picloram, linuron and 2,4,5-T pesticides for first or second order reactions using correlation methods. From this model, the eventual persistence of pesticide residues in soil and vegetation may be predicted.     

A globally applicable location-specific screening model for assessing the relative risk of pesticide leaching

A screening model of pesticide leaching loss is described which forms part of a multi-criteria risk-based indicator system called PRoMPT (Pesticide Risk Management and Profiling Tool). The leaching model evaluates pesticide fate in soil for any application rate and time of application (including multiple applications), for any land-based location in the world. It considers a generic evaluative environment with fixed dimensions and soil properties. The soil profile is conceptualised as a number of discrete layers. Equilibrium partitioning between adsorbed and dissolved chemical (based on the organic carbon-water partition coefficient [K(OC)]) is assumed in each time step, in each layer. Non-leaching losses are described using first order kinetics. Drainage is assumed to be uniform throughout the soil profile but varies temporally. The drainage rate, which can be augmented by evapotranspiration-adjusted irrigation, is derived from long-term mean monthly water balance model calculations performed for 30 arc-minute grid cells across the entire ice-free land surface of the earth. Although, such predictions are approximate, they do capture the seasonality and relative magnitude of drainage and allow the model to be applied anywhere, without the need for extensive data compilation. PRoMPT predictions are shown to be consistent with those made by more sophisticated models (PRZM, PELMO and PEARL) for the FOCUS groundwater scenarios.     

Parameter uncertainty analysis of non-point source pollution from different land use types

Land use type is one of the most important factors that affect the uncertainty in non-point source (NPS) pollution simulation. In this study, seventeen sensitive parameters were screened from the Soil and Water Assessment Tool (SWAT) model for parameter uncertainty analysis for different land use types in the Daning River Watershed of the Three Gorges Reservoir area, China. First-Order Error Analysis (FOEA) method was adopted to analyze the effect of parameter uncertainty on model outputs under three types of land use, namely, plantation, forest and grassland. The model outputs selected in this study consisted of runoff, sediment yield, organic nitrogen (N), and total phosphorus (TP). The results indicated that the uncertainty conferred by the parameters differed among the three land use types. In forest and grassland, the parameter uncertainty in NPS pollution was primarily associated with runoff processes, but in plantation, the main uncertain parameters were related to runoff process and soil properties. Taken together, the study suggested that adjusting the structure of land use and controlling fertilizer use are helpful methods to control the NPS pollution in the Daning River Watershed.     

Characterization of the nonpoint source pollution into river at different spatial scales

The objectives of this study were to investigate the effects of rainfall and underlying surface conditions on nonpoint source (NPS) pollution loads and to identify the uncertainty in NPS pollution loads at different spatial scales in the Fuxi River basin, China. Data on monitored daily flow rates and concentrations of ammonium nitrogen, total nitrogen, total phosphorus and permanganate index at the sub‐basin and basin scales were collected for a period from 2013 to 2015. Dynamic time warping distance and information measures were used to characterize pollution loads and determine the uncertainties. The results indicate that, at both sub‐basin and basin scales, NPS pollution loads increased nonlinearly with rainfall until it reached 38.4 mm, and subsequently, the NPS pollution loads stabilized. The underlying surface conditions affected the NPS pollution loads more profoundly than rainfall. Additionally, the uncertainty in NPS pollution loads increased with the spatial scales.     

Nutrients transport in the bare urban redeveloped soil system during runoff process

Stormwater pollution in redeveloped soils mixed with construction wastes imposes a serious impact to receiving waters. The transport and distribution of rainfall water and nutrients in the surface-flow and subsurface-flow (including side-flow and down-flow) of bare redeveloped soil runoff system were determined. Results indicated that the flow patterns and pollutants transport of surface-flow were different from those in subsurface-flow. The flowrate of surface-flow increased at the beginning and reached to a stable state, and then disappeared immediately after the rainfall stopped. While the flow rate of subsurface-flow was persisted and decreased gradually artery the rainfall. Most of the pollutants were carried in the surface-flow, which directly results in polluting the surface-receiving waters. And the pollutants distribution percent in side-flow was higher than that in down-flow. Results obtained in this study will provide information for the management of stormwater pollution in the bare redeveloped soil mixed with construction wastes.     

垃圾渗滤液非均质砂箱动态淋溶模拟试验

通过构建二维非均质砂箱试验模拟垃圾渗滤液污染物在土壤-地下水环境中的运移,得出了渗滤液中Cl^-、SO4^2-、总硬度（CaCO3）在地下环境中的迁移和时空分布规律,同时引入一维土柱试验的结果与二维试验结果作了比较,表明二维砂箱中由于所模拟地下水的作用而使两者的污染物运移规律差异较大,这为预测和控制渗滤液污染土壤．地下水提供了一定的理论依据和试验方法。     

重金属离子和LNAPLs在非饱和土中的运移规律研究

以重金属和轻非水相流体(LNAPLs)为例,介绍了清华大学开展非饱和土中污染物运移规律研究的部分工作成果。利用土工离心机对污染物的运移过程模拟,取样得到污染物的浓度分布特征。同时采用数值分析手段对污染物的长期运移规律和其影响因素进行分析。研究表明:离心模型试验可以有效模拟污染物的运移过程,离心模型试验和数值分析相结合,是研究污染物在非饱和土中运移规律的有效手段。     

黏土垫层水力–力学–化学耦合模型研究

在城市固废堆场建造运维、污染场地土壤、地下水围封阻隔等环境岩土、污染防治领域中,均会涉及到土体力学行为、孔隙水流动以及污染物运移之间的耦合作用问题。将土体的固结变形分为由力学荷载引起的力学固结变形及由化学荷载引起的化学–渗透固结变形,引入广义达西定律,并考虑土体物理特性和输运性质的动态变化,通过理论推导建立了适用于堆场黏土防渗垫层的水力–力学–化学耦合模型,模型最大的特点是实现了土体固结变形、孔隙流体流动与污染物运移过程之间的全耦合,各模型参数均能够体现耦合效应的影响。采用多场耦合有限元分析软件COMSOL Multiphysics对所建模型进行数值验证和求解,模拟结果表明:所建模型可从机理上描述水力–力学–化学全耦合过程,模拟结果与Peters和Smith模拟结果吻合较好,能够正确揭示超孔隙水压力、污染物浓度时空分布及土体固结变形的演化规律。     

Development of a biodegradation model for the prediction of metabolites in soil

The awareness of air, soil and water pollution has driven the search for better methods for the assessment of the environmental fate of industrial chemicals. This paper is focused on the simulation of formation and transformation of metabolites in soil. The key challenges in the development of a simulator for predicting metabolic fate of chemicals in soil are the complexity of the soil compartment and incompleteness of metabolic information. Based on the collected data for metabolic fate of 183 chemicals a set of soil specific transformations were defined and used to develop a simulator for metabolism in soil. The analysis of outliers showed that the low predictability for some chemicals is due to: 1) incomplete documented metabolic pathways with missing intermediates and/or 2) reactions of condensation that are not simulated in the current version of the model. Hence, further improvement of the model requires expanding the metabolism database and further refinement of the logic of metabolic transformations used in the simulator.     

Contamination by organochlorine pesticides in the aquifer of the Ring of Cenotes in Yucatán,México

Banned or restricted organochlorine pesticides in many countries worldwide are still in use in developing countries for agricultural and livestock activities, as well as for vector control in public health campaigns. The present study was conducted to map estimated concentrations of organochlorine pesticides in a karstic region in the Yucatan Peninsula, Mexico, known as ‘Ring of Cenotes’. Water samples from 20 sinkholes (‘cenotes’) were collected during the dry and rainy seasons in 2010–11, analysed by gas chromatography and maps of pesticide concentrations were produced using geographical information system. Results show the presence of banned pesticides, all of them exceeding the limits stated by the Mexican Official Norm. The number and concentration of pesticides during the dry season were qualitatively and quantitatively higher than in the rainy season. The spatial distribution of pesticide concentrations shows that causes of pesticide pollution in the aquifer of the Ring of Cenotes are multifactorial.