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In polluted areas organisms are generally exposed to mixtures of toxic chemicals rather than a single toxicant only. Since the number of mixture toxicity studies with regard to soil systems is limited, the research in this thesis was focused on investigating ecotoxicological consequences of combined exposure for soil invertebrates. Two topics were studied: i) population-level consequences of chronic combined exposure of individual organisms, and ii) the influence of interactions between the toxic compounds in the soil matrix on their joint toxicity. The soil dwelling, free living nematode Caenorhabditis elegans was chosen as test species.A quantitative description procedure was proposed for interpreting the joint toxicity of chemical mixtures, compared to the toxicity of the individual components. It enabled the identification of four biologically relevant deviation patterns from either the additive or the independent reference model, by means of likelihood analysis: i) no deviation, ii) synergism/antagonism, iii) toxicant ratio- and iiii) effect level dependent deviation. Using these models, sublethal effects of chronic stress of binary mixtures of copper and cadmium, and copper and carbendazim on life cycle events of the nematode C. elegans were studied. The cadmium-copper effect on reproduction was transient: it changed from synergistic, to a toxicant ratio dependent deviation from additivity. The effect of copper-carbendazim was synergistic at low effect levels and antagonistic at high effect levels. The juvenile period was a relatively non-sensitive parameter, whereas the length of the reproductive period was relatively more sensitive. In conclusion, it should be realised that mixture toxicity may be transient and that interactions may differ among life history traits.It was discussed that the effect translation of combined effects to the population level depended on three factors: i) the sensitivity of each life history trait to each of the toxicants, ii) the combination effect of the toxicants on each life history trait, iii) the sensitivity of to changes in each life history trait. A detailed analysis of mixture effects on the life history of nematodeCaenorhabditis elegans showed that synergistic effects on reproduction were transferred to the population level, despite the low sensitivity of λ to changes in this trait.Subsequently single and combined toxicity of copper-zinc, copper-cadmium, cadmium-lead, copper-carbendazim and copper-carbendazim-iprodione to the nematode Caenorhabditis elegans were studied in LUFA 2.2 soil , and the one-week population increase was estimated as toxicity endpoint. Metals with the highest partition coefficient affected the sorption of metals with the lowest partition coefficient when both were combined. However, comparing soil sorption characteristics with joint toxicity patterns did not give general results. Nevertheless, it was discussed that for identifying fundamental principles of joint toxicity in ecotoxicology, and for developing predictive models, ecological "mechanisms" should be investigated.

In polluted areas organisms are generally exposed to mixtures of toxic chemicals rather than a single toxicant only. Since the number of mixture toxicity studies with regard to soil systems is limited, the research in this thesis was focused on investigating ecotoxicological consequences of combined exposure for soil invertebrates. Two topics were studied: i) population-level consequences of chronic combined exposure of individual organisms, and ii) the influence of interactions between the toxic compounds in the soil matrix on their joint toxicity. The soil dwelling, free living nematode Caenorhabditis elegans was chosen as test species.A quantitative description procedure was proposed for interpreting the joint toxicity of chemical mixtures, compared to the toxicity of the individual components. It enabled the identification of four biologically relevant deviation patterns from either the additive or the independent reference model, by means of likelihood analysis: i) no deviation, ii) synergism/antagonism, iii) toxicant ratio- and iiii) effect level dependent deviation. Using these models, sublethal effects of chronic stress of binary mixtures of copper and cadmium, and copper and carbendazim on life cycle events of the nematode C. elegans were studied. The cadmium-copper effect on reproduction was transient: it changed from synergistic, to a toxicant ratio dependent deviation from additivity. The effect of copper-carbendazim was synergistic at low effect levels and antagonistic at high effect levels. The juvenile period was a relatively non-sensitive parameter, whereas the length of the reproductive period was relatively more sensitive. In conclusion, it should be realised that mixture toxicity may be transient and that interactions may differ among life history traits.It was discussed that the effect translation of combined effects to the population level depended on three factors: i) the sensitivity of each life history trait to each of the toxicants, ii) the combination effect of the toxicants on each life history trait, iii) the sensitivity of to changes in each life history trait. A detailed analysis of mixture effects on the life history of nematodeCaenorhabditis elegans showed that synergistic effects on reproduction were transferred to the population level, despite the low sensitivity of λ to changes in this trait.Subsequently single and combined toxicity of copper-zinc, copper-cadmium, cadmium-lead, copper-carbendazim and copper-carbendazim-iprodione to the nematode Caenorhabditis elegans were studied in LUFA 2.2 soil , and the one-week population increase was estimated as toxicity endpoint. Metals with the highest partition coefficient affected the sorption of metals with the lowest partition coefficient when both were combined. However, comparing soil sorption characteristics with joint toxicity patterns did not give general results. Nevertheless, it was discussed that for identifying fundamental principles of joint toxicity in ecotoxicology, and for developing predictive models, ecological "mechanisms" should be investigated.

In this thesis multi-phase flow models are used to study the flow behavior of liquid contaminants in aquifers and of gases that are injected below the groundwater table for remediation purposes. Considered problems are redistribution of a lens of light nonaqueous phase liquid(LNAPL)on a horizontal water table with emphasis on the effect ofNAPLentrapment by water and its removal through a well with appropriate multi-phase seepage conditions at the well boundary. In addition, air injection into groundwater (air sparging) in a homogeneous soil and in a layered soil are modeled. Accurate but very time-consuming numerical simulations for the various problems are performed. For further analysis appropriate reductions are made. Assuming vertical equilibrium and vertical averaging of the flow equations reduce the geometrical dimensionality of theLNAPLlens problems and of the problem of air sparging in a layered soil. The air sparging problems are analyzed at steady state, which eliminates the time dependence. Sparging in a homogeneous soil admits further reduction by neglecting capillary forces in vertical direction. For all problems the resulting equations are of nonlinear diffusion type, that can be cast into the porous medium equation. For the various problems similarity solutions to the porous medium equation exist, that show good agreement with numerical results. These analytical solutions give the main features of the spreading velocity of anLNAPLlens and the amount ofNAPLthat becomes trapped, of theLNAPLremoval rate and the extension of the remainingNAPLin case of extraction, and of the air sparging radius of influence in a homogeneous and in a layered soil.

In this thesis multi-phase flow models are used to study the flow behavior of liquid contaminants in aquifers and of gases that are injected below the groundwater table for remediation purposes. Considered problems are redistribution of a lens of light nonaqueous phase liquid(LNAPL)on a horizontal water table with emphasis on the effect ofNAPLentrapment by water and its removal through a well with appropriate multi-phase seepage conditions at the well boundary. In addition, air injection into groundwater (air sparging) in a homogeneous soil and in a layered soil are modeled. Accurate but very time-consuming numerical simulations for the various problems are performed. For further analysis appropriate reductions are made. Assuming vertical equilibrium and vertical averaging of the flow equations reduce the geometrical dimensionality of theLNAPLlens problems and of the problem of air sparging in a layered soil. The air sparging problems are analyzed at steady state, which eliminates the time dependence. Sparging in a homogeneous soil admits further reduction by neglecting capillary forces in vertical direction. For all problems the resulting equations are of nonlinear diffusion type, that can be cast into the porous medium equation. For the various problems similarity solutions to the porous medium equation exist, that show good agreement with numerical results. These analytical solutions give the main features of the spreading velocity of anLNAPLlens and the amount ofNAPLthat becomes trapped, of theLNAPLremoval rate and the extension of the remainingNAPLin case of extraction, and of the air sparging radius of influence in a homogeneous and in a layered soil.

The author has investigated the occurrence of mixtures of two or more species of a genus at the same place for six genera of plant-parasitic nematodes (Heterodera, Pratylenchus, Paratylenchus, Tylenchorhynchus, Rotylenchus and Meloidogyne), and shows how one species may become dominant as a result of a particular cropping or soil type, and may be subsequently suppressed by a change in environment. J.J.H.

The author has investigated the occurrence of mixtures of two or more species of a genus at the same place for six genera of plant-parasitic nematodes (Heterodera, Pratylenchus, Paratylenchus, Tylenchorhynchus, Rotylenchus and Meloidogyne), and shows how one species may become dominant as a result of a particular cropping or soil type, and may be subsequently suppressed by a change in environment. J.J.H.

The author has investigated the occurrence of mixtures of two or more species of a genus at the same place for six genera of plant-parasitic nematodes (Heterodera, Pratylenchus, Paratylenchus, Tylenchorhynchus, Rotylenchus and Meloidogyne), and shows how one species may become dominant as a result of a particular cropping or soil type, and may be subsequently suppressed by a change in environment. J.J.H.