Groundwater Contamination Risk Assessment Research Articles

A reduced complexity model for probabilistic risk assessment of groundwater contamination

We present a model of reduced complexity for assessing the risk of groundwater pollution from a point source. The progress of contamination is represented as a sequence of transitions among coarsely resolved states corresponding to simple statements like “a spill has occurred.” Transitions between states are modeled as a Markov jump process, and a general expression for the probability of aquifer contamination is obtained from two basic assumptions: that the sequence of transitions leading to contamination is Markovian and that the time when a given transition occurs is independent of its end state. Additionally, we derive an asymptotic value for the probability of contamination that is equivalent to the so‐called rare event approximation. First we develop the model for sites in statistically homogeneous natural porous media, and then we extend it to highly heterogeneous media composed of multiple materials. Finally, we apply the model to a simple example to illustrate the method and its potential.

Proposed Methodology of Vulnerability and Contamination Risk Mapping for the Protection of Karst Aquifers in Slovenia

On the basis of work accomplished by the European COST Action 620, a comprehensive approach to groundwater vulnerability and contamination risk assessment is proposed, taking into account the special characteristics of Slovene karst aquifer systems. The Slovene Approach is consistent with national environmental legislation and enables comparison across European countries. The method integrates temporal hydrological variability in the concept of groundwater vulnerability and offers a new possibility to combine surface and groundwater source and resource protection, which required the development of a new K factor (karst groundwater flow within the saturated zone). The risk analysis considers intrinsic vulnerability, contamination hazards and the importance of the source or resource. It has been first applied to the Podstenjsek springs catchment in southwestern Slovenia and validated by means of two multi-tracer tests with a total of six injection points. The resulting vulnerability, hazard and risk maps are plausible, and the validation confirmed the vulnerability assessment at the representative sites that were selected for tracer injection. The maps provide improved source protection zones and make it possible to identify land mismanagement and to propose better practices for future planning.

Stochastic Risk Assessment of Groundwater Contamination under Uncertainty: A Canadian Case Study

This study presents a stochastic approach for risk assessment of groundwater contamination through incorporating a stochastic subsurface contaminant transport and fate modeling system within a general risk assessment framework. The uncertainties associated with soil properties (e.g. soil porosity and permeability) were addressed through the probabilistic method, and the resulting uncertainties in risks of groundwater contamination were then identified under different remediation scenarios. The method was applied to a petroleum-contaminated site in western Canada. Three remediation scenarios with different clean-up efficiencies (0, 60, and 90%) were examined for a planning period of 10 years to study the risks introduced by xylene. The concept of hazard index was applied to characterize the associated non-carcinogenic risk. Three risky zones with different mean hazard index distributions were identified, and the probabilities of violating assumed hazard index criteria at the study site were also presented. The obtained results indicated that the proposed method could effectively identify risky zones with different risk levels and violation probabilities under various remediation actions, and they are directly useful for the decision maker to gain insight into the study site and to make remediation decisions.

Areal identification of groundwater nitrate contamination sources in periurban areas

Background, Aims and Scope The purpose of this study is to verify a method for groundwater contamination risk assessment in urban and periurban areas using Geographic Information Systems (GIS). The method is based on agricultural hazard evaluation. In urban and periurban areas, the problem of dealing with NO3 concentrations mainly entails identifying nitrate sources from the excessive use of fertilizers, or leaks from the sewage network and old septic systems. Residential areas coexisting with farming, and high and increasing nitrate concentrations, were identified in a densely inhabited area (>2 million inhabitants in about 1,000 km2) with an alluvial aquifer, located in southern Italy.

Joint propagation of variability and imprecision in assessing the risk of groundwater contamination

Estimating risks of groundwater contamination often require schemes for representing and propagating uncertainties relative to model input parameters. The most popular method is the Monte Carlo method whereby cumulative probability distributions are randomly sampled in an iterative fashion. The shortcoming of the approach, however, arises when probability distributions are arbitrarily selected in situations where available information is incomplete or imprecise. In such situations, alternative modes of information representation can be used, for example the nested intervals known as “possibility distributions”. In practical situations of groundwater risk assessment, it is common that certain model parameters may be represented by single probability distributions (representing variability) because there are data to justify these distributions, while others are more faithfully represented by possibility distributions (representing imprecision) due to the partial nature of available information. This paper applies two recent methods, designed for the joint-propagation of variability and imprecision, to a groundwater contamination risk assessment. Results of the joint-propagation methods are compared to those obtained using both interval analysis and the Monte Carlo method with a hypothesis of stochastic independence between model parameters. The two joint-propagation methods provide results in the form of families of cumulative distributions of the probability of exceeding a certain value of groundwater concentration. These families are delimited by an upper cumulative distribution and a lower distribution respectively called Plausibility and Belief after evidence theory. Slight differences between the results of the two joint-propagation methods are explained by the different assumptions regarding parameter dependencies. Results highlight the point that non-conservative results may be obtained if single cumulative probability distributions are arbitrarily selected for model parameters in the face of imprecise information and the Monte Carlo method is used under the assumption of stochastic independence. The proposed joint-propagation methods provide upper and lower bounds for the probability of exceeding a tolerance threshold. As this may seem impractical in a risk-management context, it is proposed to introduce “ a-posteriori subjectivity” (as opposed to the “ a-priori subjectivity” introduced by the arbitrary selection of single probability distributions) by defining a single indicator of evidence as a weighted average of Plausibility and Belief, with weights to be defined according to the specific context.

Probabilistic risk and uncertainty analysis for bioremediation of four chlorinated ethenes in groundwater

Groundwater contamination risk assessment for health-threatening compounds should benefit from a stochastic environmental risk assessment which considers the effects of biological, chemical, human behavioral, and physiological processes that involve elements of biotic and abiotic aquifer uncertainty, and human population variability. This paper couples a complex model of chemical degradation and transformation with movement in an aquifer undergoing bioremediation to generate a health risk analysis for different population cohorts in the community. A two-stage Monte Carlo simulation has separate stages for population variability and aquifer uncertainty yielding a computationally efficient and conceptually attractive algorithm. A hypothetical example illustrates how risk variance analysis can be conducted to determine the distribution of risk, and the relative impact of uncertainty and variability in different sets of parameters upon the variation of risk values for adults, adolescents, and children. The groundwater example considers a community water supply contaminated with chlorinated ethenes. Biodegradation pathways are enhanced by addition of butyrate. The results showed that the contribution of uncertainty to the risk variance is comparable to that of variability. Among the uncertain parameters considered, transmissivity accounted for the major part of the output variance. Children were the most susceptible and vulnerable population cohort.

An integrated fuzzy-stochastic modeling approach for risk assessment of groundwater contamination

An integrated fuzzy-stochastic risk assessment (IFSRA) approach was developed in this study to systematically quantify both probabilistic and fuzzy uncertainties associated with site conditions, environmental guidelines, and health impact criteria. The contaminant concentrations in groundwater predicted from a numerical model were associated with probabilistic uncertainties due to the randomness in modeling input parameters, while the consequences of contaminant concentrations violating relevant environmental quality guidelines and health evaluation criteria were linked with fuzzy uncertainties. The contaminant of interest in this study was xylene. The environmental quality guideline was divided into three different strictness categories: “loose”, “medium” and “strict”. The environmental-guideline-based risk (ER) and health risk (HR) due to xylene ingestion were systematically examined to obtain the general risk levels through a fuzzy rule base. The ER and HR risk levels were divided into five categories of “low”, “low-to-medium”, “medium”, “medium-to-high” and “high”, respectively. The general risk levels included six categories ranging from “low” to “very high”. The fuzzy membership functions of the related fuzzy events and the fuzzy rule base were established based on a questionnaire survey. Thus the IFSRA integrated fuzzy logic, expert involvement, and stochastic simulation within a general framework. The robustness of the modeling processes was enhanced through the effective reflection of the two types of uncertainties as compared with the conventional risk assessment approaches. The developed IFSRA was applied to a petroleum-contaminated groundwater system in western Canada. Three scenarios with different environmental quality guidelines were analyzed, and reasonable results were obtained. The risk assessment approach developed in this study offers a unique tool for systematically quantifying various uncertainties in contaminated site management, and it also provides more realistic support for remediation-related decisions.

A GIS‐Based Ground Water Contamination Risk Assessment Tool for Pesticides

AbstractA process‐based preferential flow transport model was implemented in a geographic information system to locate areas in the landscape with high risk of contamination by agrochemicals, especially pesticides. Protecting ground water resources necessitates a reliable ground water quality monitoring strategy. It is valuable to be able to focus monitoring on areas with the highest risk of contamination because monitoring ground water is an expensive activity, especially at the landscape scale. The objective of this project was to develop a tool that quantifiably estimates distributed ground water contamination risk in order to develop reliable, cost‐effective ground water observation networks. The tool is based on a mechanistic model of chemical movement via preferential flow and uses land cover data, information about chemical properties, and modeled recharge to estimate the concentration of chemical reaching the ground water at each point in the landscape. The distributed risk assessment tool was tested by comparing the model‐predicted risk with observed concentrations from 40 sampling wells in Cortland County, New York, for atrazine (pesticide) and nitrate, the latter assumed to be an indicator of agricultural pollution. The tool predictions agreed well with observed nitrate concentrations and pesticide detections. An Internet‐based version of this tool is currently being developed for ready application to New York State.

Pesticide adsorption in the vadose zone: a case study on Eocene and Quaternary materials in Northern France.

We present a set of adsorption coefficients measured on various Eocene and Quaternary materials sampled from the vadose zone of a catchment in Northern France for three herbicides, atrazine, isoproturon and metamitron. Some vadose zone materials were found to have higher adsorption coefficients than the topsoil. The adsorption coefficients were strongly dependent on the clay content of the material. From 83% to 97% of the variability in the adsorption coefficients could be explained by a linear relationship to the clay content. Adsorption coefficients normalized for clay content, Kclay, ranged between 1.6 and 17.6 litre kg(-1) for atrazine. Neglecting the adsorption properties of the vadose zone and relying exclusively on Koc values to predict mobility may bias regional or local risk assessment of groundwater contamination by pesticides. More information on the adsorption properties of geological materials should be collected to improve our ability to predict pesticide concentrations in groundwaters.

GIS Techniques for Mapping Groundwater Contamination Risk

The groundwater contamination risk map of a sample alluvial area was produced by using the Ilwis Geographical Information System (GIS) to construct and to overlay thematic maps. The risk map has been derived from the vulnerability map, the hazard map, where the potential contaminating sources were identified, and the socio-economic value of the groundwater resource, represented by the wells. The groundwater quality map allowed the reliability of hazard and risk maps to be tested. The final map shows interesting results and stresses the need for the GIS to test and improve on the groundwater contamination risk assessment methods. The Environmental Protection Agency, in the United States, has identified in 1993 more than 200 chemical compounds in groundwater (EPA, 1993), some of these are extremely hazardous to human health. Pollutants reach the aquifers from ordinary human activities (agriculture, farming, industry) or from occasional sources. In consequence the groundwater protection is a prior environmental concern in many countries, for example in Europe, where more than 50% of the water supply is obtained from groundwater (Lobo-Ferreira, 1997). The correct way to solve this problem is to prevent the contamination assessing and managing the groundwater pollution risk at regional scale. In this paper the groundwater contamination risk map of a sample alluvial area was produced by using the Ilwis Geographical Information System to construct and overlay thematic maps. The adopted groundwater contamination risk assess- ment method follows the approach of previous studies published in Italy (Civita, 1995; Civita and De Maio, 1997; Corniello and Ducci, 1997) and is optimised by developing new analytical procedures. Moreover, the use of the GIS has allowed: high spatial resolution; numeric checking of calculated parameters and easy matrix operations.

Risk Assessment of Groundwater Contamination and Current Applications in the Decision-Making Process

Risk Assessment of Groundwater Contamination and Current Applications in the Decision-Making Process

Groundwater contamination risk assessment: initial methodology for highly developed areas. Case study in the Province of Milan (Italy)

ABSTRACTMaps of factors contributing to groundwater contamination risk have been produced for a sector of the province of Milan to support decision‐making in land planning and management.Risk assessment has considered factors related to: natural vulnerability, hazard due to human activities and costs resulting from pollution. In order to evaluate existing hazard, a specific census was carried out over the area and industrial, agricultural and quarrying activities were mapped. Industrial activities were first divided into 4 groups according to national waste type regulation.The number of data and the density of mapped factors necessitated the use of a land information system for data storage and management and map design.

Pesticide transport modelling in soil for risk assessment of groundwater contamination

The risk of groundwater contamination with pesticides applied to soil surface depends on the soil properties, the agricultural practices, the climatic influences, and on the properties of the pesticides themselves. The EXSOL model was developed for the simulation of the transport and fate of pesticides and organics in soils. The dynamics of mobility, accumulation and degradation can be studied under various soil and climatic conditions. Transient water flow is provided from a simulation model of the field water balance. The percentages of the herbicide 2,4,5‐trichlorophenoxyacetic acid in a luvisol soil after a single application in summer are compared with model calculations using sorption coefficients from laboratory column studies. The calculated percentages lie within the measured range, except for those in the deeper soil layer. The underestimation can be explained with preferential flow in macropores which may have occurred during the heavy rainfall six days after application.