Contaminant mass discharge estimation in groundwater based on multi-level point measurements: a numerical evaluation of expected errors

Different methods for the field-scale estimation of contaminant mass discharge in groundwater at control planes based on multi-level well data are numerically analysed for the expected estimation error. We consider "direct" methods based on time-integrated measuring of mass flux, as well as "indirect" methods, where estimates are derived from concentration measurements. The appropriateness of the methods is evaluated by means of modelled data provided by simulation of mass transport in a three-dimensional model domain. Uncertain heterogeneous aquifer conditions are addressed by means of Monte-Carlo simulations with aquifer conductivity as a random space function. We investigate extensively the role of the interplay between the spatial resolution of the sampling grid and aquifer heterogeneity with respect to the accuracy of the mass discharge estimation. It is shown that estimation errors can be reduced only if spatial sampling intervals are in due proportion to spatial correlation length scales. The ranking of the methods with regard to estimation error is shown to be heavily dependent on both the given sampling resolution and prevailing aquifer heterogeneity. Regarding the "indirect" estimation methods, we demonstrate the great importance of a consistent averaging of the parameters used for the discharge estimation.     

Changes in air saturation and air-water interfacial area during surfactant-enhanced air sparging in saturated sand

Reduction in the surface tension of groundwater, prior to air sparging for removal of volatile organic contaminant from aquifer, can greatly enhance the air content and the extent of influence when air sparging is implemented. However, detailed information on the functional relationship between water saturation, air-water contact area induced by air sparging and the surface tension of water has not been available. In this study, the influence of adding water-soluble anionic surfactant (sodium dodecyl benzene sulfonate) into groundwater before air sparging on the air-water interfacial area and water saturation was investigated using a laboratory-scale sand packed column. It was found that water saturation decreases with decreasing surface tension of water until it reaches a point where this trend is reversed so that water saturation increases with further decrease in the surface tension. The lowest water saturation of 0.58 was achieved at a surface tension of 45.4 dyn/cm, which is considered as the optimum surface tension for maximum de-saturation for the initially water-saturated sand used in this study. The air-water contact area generated in the sand column due to air sparging was measured using a gaseous interfacial tracer, n-decane, and was found to monotonically increase with decreasing water saturation. The results of this study provide useful design information for surfactant-enhanced air sparging removal of volatile contaminants from aquifers.     

Geochemical and Pb isotopic evidence for sources and dispersal of metal contamination in stream sediments from the mining and smelting district of Príbram, Czech Republic

Stream sediments from the mining and smelting district of Príbram, Czech Republic, were studied to determine the degree, sources and dispersal of metal contamination using a combination of bulk metal and mineralogical determinations, sequential extractions and Pb isotopic analyses. The highest metal concentrations were found 3-4 km downstream from the main polymetallic mining site (9800 mg Pb kg(-1), 26 039 mg Zn kg(-1), 316.4 mg Cd kg(-1), 256.9 mg Cu kg(-1)). The calculated enrichment factors (EFs) confirmed the extreme degree of contamination by Pb, Zn and Cd (EF>40). Lead, Zn and Cd are bound mainly to Fe oxides and hydroxides. In the most contaminated samples Pb is also present as Pb carbonates and litharge (PbO). Lead isotopic analysis indicates that the predominant source of stream sediment contamination is historic Pb-Ag mining and primary Pb smelting (206Pb/207Pb=1.16), while the role of secondary smelting (car battery processing) is negligible.     

Levels of trace elements in green turtle eggs collected from Hong Kong: Evidence of risks due to selenium and nickel

Concentrations of 22 trace elements were determined in green turtle (Chelonia mydas) eggs collected from Hong Kong. Concentrations of selenium, lead and nickel in these eggs were generally higher than those reported in other studies. The predicted no effect concentrations (PNEC; ng/g wet weight) of Pb (1000), Se (340 and 6000 for the worst-case and best-case scenarios, respectively) and Ni (17) in the green turtle eggs were estimated. Hazard quotients (HQs) estimate that Se (HQs: 0.2-24.5) and Ni (HQs: 4.0-26.4) may pose some risks to the turtles. Our study also found that concentrations of Ag, Se, Zn, Hg and Pb in the shell of the turtle eggs were significantly correlated with levels in the whole egg contents (yolk+albumen). Once the precise relationships of specific elements are established, egg-shell concentrations may be used as a non-lethal, non-invasive, surrogate for predicting whole egg burden of certain contaminants in marine turtles.     

Fixation of metals in soil constituents and potential remobilization by hyperaccumulating and non-hyperaccumulating plants: results from an isotopic dilution study

In this study isotopic dilution methods were used to investigate the hypothesis that access to metals associated with specific chemical components in the soil that are not available to non-accumulator species could be involved in hyperaccumulation. The hyperaccumulator Thlaspi caerulescens and a non-accumulator species, Brassica napus, were grown in Cd and Zn enriched soil components calcite, goethite, charcoal and cryptomelane. The metal enriched components were aged to allow transformation of a proportion of added metals to non-labile forms. Results from the isotopic dilution L value method showed that despite taking up more metals, T. caerulescens accessed the same pool of metals as B. napus. Hence differential access to different solid-phase pools of metals appears to be an unlikely mechanism underlying metal hyperaccumulation. For all components except charcoal, L values for Cd and Zn were greater than the corresponding E values suggesting that E values may tend to underestimate the bioavailable fraction of metals in soils.     

Microbial activity in an acid resin deposit: biodegradation potential and ecotoxicology in an extremely acidic hydrocarbon contamination

Acid resins are residues produced in a recycling process for used oils that was in use in the forties and fifties of the last century. The resin-like material is highly contaminated with mineral oil hydrocarbons, extremely acidic and co-contaminated with substituted and aromatic hydrocarbons, and heavy metals. To determine the potential for microbial biodegradation the acid resin deposit and its surroundings were screened for microbial activity by soil respiration measurements. No microbial activity was found in the core deposit. However, biodegradation of hydrocarbons was possible in zones with a lower degree of contamination surrounding the deposit. An extreme acidophilic microbial community was detected close to the core deposit. With a simple ecotoxicological approach it could be shown that the pure acid resin that formed the major part of the core deposit, was toxic to the indigenous microflora due to its extremely low pH of 0-1.     

Characteristics and risks of drivers with low annual distance driven

OBJECTIVES: It has been noted by several authors that risk (defined only in terms of total expected numbers of crash involvements per total distance driven) paints a misleading picture of crash liability, particularly for the young and the old, as their high risk is associated with risky driving patterns typical of people who drive low annual kms. This article sets out to analyze these driving patterns of low-km drivers and to evaluate the risk of these patterns. As licensing programs tend to focus on young and old drivers, who tend to drive lower annual distances, income and employment data are also analyzed for low-km drivers. This is to provide a better picture for policy makers of the sort of people and the sorts of transportation requirements that their policies may affect. METHODS: Crash data and travel data were disaggregated by driver characteristics and by driving conditions (road type, day and night, weekend and weekday) and combined to form estimates of risk for typical driving patterns of driver groups. Characteristics of driving patterns and of the drivers themselves were derived for groups defined by age and by the amount of annual driving undertaken. RESULTS: Older drivers who drive less tend to have higher risk per km mainly due to their predominantly urban trips. Nevertheless, because older drivers on average manage to reduce their risk per distance driven by choosing driving patterns that are safer than the driving patterns of other age groups, the risk of older drivers as a group is not overestimated. CONCLUSION: Despite being quite different from one another, the low- and high-km driving patterns of younger drivers were found to impose identical risks.     

Effects of sulfur dioxide and nitric oxide on mercury oxidation and reduction under homogeneous conditions

This paper is particularly related to elemental mercury (Hg0) oxidation and divalent mercury (Hg2+) reduction under simulated flue gas conditions in the presence of nitric oxide (NO) and sulfur dioxide (SO2). As a powerful oxidant and chlorinating reagent, Cl2 has the potential for Hg oxidation. However, the detailed mechanism for the interactions, especially among chlorine (Cl)-containing species, SO2, NO, as well as H2O, remains ambiguous. Research described in this paper therefore focused on the impacts of SO2 and NO on Hg0 oxidation and Hg2+ reduction with the intent of unraveling unrecognized interactions among Cl species, SO2, and NO most importantly in the presence of H2O. The experimental results demonstrated that SO2 and NO had pronounced inhibitory effects on Hg0 oxidation at high temperatures when H2O was also present in the gas blend. Such a demonstration was further confirmed by the reduction of Hg2+ back into its elemental form. Data revealed that SO2 and NO were capable of promoting homogeneous reduction of Hg2+ to Hg0 with H2O being present. However, the above inhibition or promotion disappeared under homogeneous conditions when H2O was removed from the gas blend.     

The influence of rear turn-signal characteristics on crash risk

IntroductionThe relationship between the relative risk of a rear-end collision during a turn, merge, or lane change maneuver and the characteristics of the rear turn-signal configuration was examined using crash data from seven states in the United States.MethodRear turn-signal characteristics—including color, optics, separation, and light source—were identified for 55 vehicle models and used in a logistic regression analysis to model the odds of a rear-end collision. Additional variables including driver demographics (gender, age), vehicle age, and light condition were also modeled. Risk was assessed using a contrast group of striking vehicles in similar collisions.ResultsThe results suggest that the odds of being the struck vehicle were 3% to 28% lower among vehicles equipped with amber versus red turn signals. Although the analysis suggests that there may be a safety benefit associated with amber rear turn signals, it is unclear whether turn-signal color alone is responsible.Impact on IndustryThe results suggest that aspects of a vehicle's rear signal characteristics may influence crash risk.     

Heavy trucks, conspicuity treatment, and the decline of collision risk in darkness

IntroductionThe influence of amendments to Federal Motor Vehicle Safety Standard (FMVSS) 108, requiring conspicuity treatments on heavy tractors and trailers, was determined in analyses of the odds of fatal collisions in darkness.MethodComparisons were made between crashes in which conspicuity treatment was likely relevant, and those in which it was likely irrelevant.ResultsOver 23 years, the odds that a fatal collision involving a heavy truck occurred in darkness declined by 58% among relevant crashes, while little decline was observed for irrelevant crashes. Disaggregation into crash types revealed the largest declines occurred in fatal rear-end and angle collisions. A parallel analysis of light vehicles also found declines but no differences among crash type. Similar trends were also observed for nonfatal rear end collisions.ConclusionThe results suggest that detection failure may have contributed to the risk of striking a tractor-semitrailer in darkness, and that conspicuity treatments have reduced this risk.Impact on IndustryConspicuity treatments appear to reduce risk of collision into heavy trucks in darkness. It is likely that this benefit would also extend to other vehicles that are not included in the FMVSS 108 regulation (e.g., buses, single unit trucks, recreational vehicles), although many are so equipped, regardless of the regulation.