Using applied research to reduce uncertainty in health risk assessment: Five case studies involving human exposure to chromium in soil and groundwater

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.

Influence of organic ligands on Cr desorption from hydroxy-Cr intercalated montmorillonite

Retention of metals as hydroxy precipitates on mineral surfaces is an important process maintaining environmental quality. Hydroxy precipitates of Cr(III) are particularly important because appreciable mobilisation of intercalated, hydrolysed Cr from montmorillonite is currently thought to occur only via oxidative dissolution. The present study examines the potential of oxalate, tartrate, and citrate to desorb Cr from both hydroxy-Cr and hydroxy Al-Cr precipitates sorbed to montmorillonite. For all intercalated clays there was an initial very rapid dissolution, followed by a slow but sustained Cr release that more closely followed a parabolic rate law, the latter indicative of transport-controlled dissolution. The initial rapid dissolution persisted longer for the pure Cr clay ( approximately 48 h) than for the coprecipitated Al-Cr clay ( approximately 24 h). At the end of the 30 d reaction, however, most Cr remained sorbed to the montmorillonite in all systems. Citrate, for example, solubilised only approximately 10% of the total sorbed Cr from the pure Cr clays. Aluminum presence decreased Cr solubilisation rate due to Al accumulation at the polymer margins. Moreover, Al presence maintained the integrity of the intercalated polymers and prevented c-axis expansion beyond 1.4 nm, thus restricting the supply of organic ligand to the inter-layer region. Oxalate, tartrate, and citrate were shown to be effective chelators of sorbed Cr and these ligands may therefore play an important role in the mobilisation and cycling of this metal in soils and sediments.