Modelling of diffusion-limited retardation of contaminants in hydraulically and lithologically nonuniform media

A new reactive transport modelling approach and examples of its application are presented, dealing with the impact of sorption/desorption kinetics on the spreading of solutes, e.g. organic contaminants, in groundwater. Slow sorption/desorption is known from the literature to be strongly responsible for the retardation of organic contaminants. The modelling concept applied in this paper quantifies sorption/desorption kinetics by an intra-particle diffusion approach. According to this idea, solute uptake by or release from the aquifer material is modelled at small scale by a "slow" diffusion process where the diffusion coefficient is reduced as compared to the aqueous diffusion coefficient due to (i) the size and shape of intra-particle pores and (ii) retarded transport of solutes within intra-particle pores governed by a nonlinear sorption isotherm. This process-based concept has the advantage of requiring only measurable model parameters, thus avoiding fitting parameters like first-order rate coefficients.In addition, the approach presented here allows for modelling of slow sorption/desorption in lithologically nonuniform media. Therefore, it accounts for well-known experimental findings indicating that sorptive properties depend on (i) the grain size distribution of the aquifer material and (ii) the lithological composition (e.g. percentage of quartz, sandstone, limestone, etc.) of each grain size fraction. The small-scale physico-chemical model describing sorption/desorption is coupled to a large-scale model of groundwater flow and solute transport. Consequently, hydraulic heterogeneities may also be considered by the overall model. This coupling is regarded as an essential prerequisite for simulating field-scale scenarios which will be addressed by a forthcoming publication.This paper focuses on mathematical model formulation, implementation of the numerical code and lab-scale model applications highlighting the sorption and desorption behavior of an organic contaminant (Phenanthrene) with regard to three lithocomponents exhibiting different sorptive properties. In particular, it is shown that breakthrough curves (BTCs) for lithologically nonuniform media cannot be obtained via simple arithmetic averaging of breakthrough curves for lithologically uniform media. In addition, as no analytical solutions are available for model validation purposes, simulation results are compared to measurements from lab-scale column experiments. The model results indicate that the new code can be regarded as a valuable tool for predicting long-term contaminant uptake or release, which may last for several hundreds of years for some lithocomponents. In particular, breakthrough curves simulated by pure forward modelling reproduce experimental data much better than a calibrated standard first-order kinetics reactive transport model, thus indicating that the new approach is of high quality and may be advantageously used for supporting the design of remediation strategies at contaminated sites where some lithocomponents and/or grain size classes may provide a long-term pollutant source.     

Assessing the recovery of lakes in southeastern Canada from the effects of acidic deposition

Reductions in North American sulfur dioxide (SO2) emissions promoted expectations that aquatic ecosystems in southeastern Canada would soon recover from acidification. Only lakes located near smelters that have dramatically reduced emissions approach this expectation. Lakes in the Atlantic provinces, Quebec and Ontario affected only by long-range sources show a general decline in sulfate (SO4(2-)) concentrations, but with a relatively smaller compensating increase in pH or alkalinity. Several factors may contribute to the constrained (or most likely delayed) acidity response: declining base cation concentrations, drought-induced mobilization of SO4(2-), damaged internal alkalinity generation mechanisms, and perhaps increasing nitrate or organic anion levels. Monitoring to detect biological recovery in southeastern Canada is extremely limited, but where it occurs, there is little evidence of recovery outside of the Sudbury/Killarney area. Both the occurrence of Atlantic salmon in Nova Scotia rivers and the breeding success of Common Loons in Ontario lakes are in fact declining although factors beyond acidification also play a role. Chemical and biological models predict that much greater SO2 emission reductions than those presently required by legislation will be needed to promote widespread chemical and latterly, biological recovery. It may be unrealistic to expect that pre-industrial chemical and biological conditions can ever be reestablished in many lakes of southeastern Canada.     

Monoterpene emissions and carbonyl compound air concentrations during the blooming period of rape (Brassica napus)

An increasing percentage of agricultural land in Germany is used for oil seed plants. Hence, rape has become an important agricultural plant (in Saxony 1998: 12% of the farmland) in the recent years. During flowering of rape along with intensive radiation and high temperatures, a higher production and emission of biogenic VOC was observed. The emissions of terpenes were determined and more importantly, high concentrations of organic carbonyl compounds were observed during this field experiment. All measurements of interest have been carried out during two selected days with optimal weather conditions. It is found that the origin or the mechanism of formation of different group of compounds had strong influence on the day to day variation of their concentrations. The emission flux of terpenes from flowering rape plants was determined to be 16–32 μg h⁻¹ m⁻² (30–60 ng h⁻¹ per g dry plant––540–1080 ng h⁻¹ per plant), in total. Limonene, α-thujene and sabinene were the most important compounds (about 60% of total terpenes). For limonene and sabinene reference emission rates (M_S) and temperature coefficients were determined: β_limonene=0.108 K⁻¹ and M_S=14.57 μg h⁻¹ m⁻²; β_sabinene=0.095 K⁻¹ and M_S=5.39 μg h⁻¹ m⁻².The detected carbonyl compound concentrations were unexpectedly high (maximum formaldehyde concentration was 18.1 ppbv and 3.4 ppbv for butyraldehyde) for an open field. Possible reasons for these concentrations are the combination of primary emission from the plants induced by high temperature and high ozone stress, the secondary formation from biogenically and advected anthropogenically emitted VOC at high radiation intensities and furthered by the low wind speeds at this time.     

Emissions and demonstration of an emission control technology for small two-stroke utility engines

Small utility engines represent an important contribution to the emissions inventory and have been subjected to increasingly stringent regulations in recent years. For this project, a Tanaka two-stroke engine was tested in its original condition and with a modified fuel/oil injection system. The modified fuel/oil injection system applied to the Tanaka two-stroke engine resulted in significant emissions reductions of approximately 52% for carbon monoxide (CO), 70% for total hydrocarbons (THC), 70% for particulate matter (PM), and 67% for the regulated THC + nitrogen oxides metric. This technology met the California Air Resources Board's 2000 model-year regulations for all pollutants, with the exception of slightly higher PM emissions. Two additional two-stroke engines were tested under a new condition and after at least 100 hr of use to examine the effects of deterioration on in-use, two-stroke engines. For one engine, CO and PM emissions more than tripled after 162 hr of operation in the field, with smaller increases also observed for THC (20%). For the second engine, significant repairs were required throughout the 100 operating hours, which counteracted the effects of the emissions deterioration and resulted in lower CO and THC emissions.     

A net loss: policy instruments for commercial cod fishing in Sweden

This paper describes the overfishing of the seas, in particular for cod in Sweden. It discusses policy instruments such as individual quotas, labelling and marine reserves. A tentative conclusion is that something needs to be done to the very structure of the policy instruments used. Information on stock depletion is available but goes unheeded and is counteracted by fishermen who want to fish more and who are routinely supported by "the political establishment" who appear to be most concerned about (short-term) employment. The whole debate is taking place before the eyes of a general public that does not care sufficiently and difficult international bureaucracy. In this situation, the most important changes may be to strengthen both the rights and the duties of the fishermen. The integrity of fish stocks must be given absolute priority, but it is also important to motivate the fishermen with a greater stake and interest in the stock by giving them quotas that have, as far as possible, the characteristics of property.     

Partitioning of polychlorinated biphenyls in octanol/water, triolein/water, and membrane/water systems

The use of the octanol/water partition constant (Kow) as a surrogate parameter for lipid/water partitioning of persistent organic pollutants (POPs) was reassessed by comparing the measured Kow of 12 selected polychlorinated biphenyl congeners (PCBs) with partition constants in triolein/water (Ktw) and membrane/water (Kmw) systems. Kow and Ktw were measured by the slow-stirring method. Kmw was measured by an adaptation of the slow-stirring method using suspensions of phosphatidylcholine and phosphatidylserine liposomes. Partitioning of POPs to octanol, triolein, and liposomes is similar but not equal. The log-log correlation for Kow and Ktw is excellent (r2 = 0.982) and that for Kow and Kmw is somewhat weaker (r2 = 0.856). Ktw values are greater than Kow by a factor of 1.6. Kmw of some PCB congeners exceed both Kow and Ktw by an order of magnitude. The differences are attributed to different PCB activity coefficients in the different lipid phases. The results imply that Kow can be used as a reasonable conservative estimate of lipid/water partitioning. But the observed differences between Kow and Kmw also indicate that using Kow to predict accumulation of POPs, particularly highly hydrophobic ones, in the polar lipids of organisms will underestimate their concentrations at equilibrium.     

Degradation of 2,4,6-trinitrotoluene by selected helophytes

Four emergent plants (helophytes, synonyms emersion macrophytes, marsh plants, etc.) Phragmites australis, Juncus glaucus, Carex gracillis and Typha latifolia were successfully used for degradation of TNT (2,4,6-trinitrotoluene) under in vitro conditions. The plants took up and transformed more than 90% of TNT from the medium within ten days of cultivation. The most efficient species was Ph. australis which took up 98% of TNT within ten days. The first stable degradation products 4-amino-2,6-dinitrotoluene (4-ADNT) and 2-amino-4,6-dinitrotoluene (2-ADNT) were identified and analysed during the cultivation period. [14C] TNT was used for the detection of TNT degradation products and their compartmentalization in plant tissues after two weeks of cultivation. Forty one percent of 14C was detected as insoluble or bound in cell structures: 34% in roots and 8% in the aerial parts. These results open the perspective of using the above-mentioned plants for the remediation of TNT contaminated waters.     

Major challenges of integrating agriculture into climate change mitigation policy frameworks

Taking the European Union (EU) as a case study, we simulate the application of non-uniform national mitigation targets to achieve a sectoral reduction in agricultural non-carbon dioxide (CO₂) greenhouse gas (GHG) emissions. Scenario results show substantial impacts on EU agricultural production, in particular, the livestock sector. Significant increases in imports and decreases in exports result in rather moderate domestic consumption impacts but induce production increases in non-EU countries that are associated with considerable emission leakage effects. The results underline four major challenges for the general integration of agriculture into national and global climate change mitigation policy frameworks and strategies, as they strengthen requests for (1) a targeted but flexible implementation of mitigation obligations at national and global level and (2) the need for a wider consideration of technological mitigation options. The results also indicate that a globally effective reduction in agricultural emissions requires (3) multilateral commitments for agriculture to limit emission leakage and may have to (4) consider options that tackle the reduction in GHG emissions from the consumption side.