No abstract

[1] Long-term elution tailing of organic contaminants, often observed when water or air is used to flush contaminated porous media, has been attributed to several factors. Characterization of this tailing behavior through the quantitative analysis of multiple coupled factors is necessary to enhance our understanding of contaminant transport. The objective of this study was to investigate the transport and elution behavior of trichloroethene in a naturally heterogeneous (poorly sorted) aquifer material, with a specific focus on characterizing and quantifying the relative contributions of rate-limited immiscible-liquid dissolution and nonlinear, rate-limited sorption/desorption to lowconcentration elution tailing. A comparison of trichloroethene elution behavior for systems with and without immiscible-liquid phase present suggests that the lowconcentration elution tailing observed in the former experiments is associated primarily with nonlinear, rate-limited sorption/desorption. The transport and elution of trichloroethene was successfully simulated using a mathematical model that combines independent, coupled descriptions of rate-limited dissolution and nonlinear, rate-limited sorption/desorption. Specifically, immiscible-liquid dissolution was described using a first-order mass transfer approach with a temporally variable dissolution rate coefficient, and sorption/desorption was described using an approach incorporating a continuous distribution of rate-limited domains. The results of this study indicate that multiple processes contributed to trichloroethene elution behavior when immiscible-liquid phase was present and that a multiprocess model was required to accurately simulate the measured data.

SUMMARYAgeing, leukaemia and acquired immune deficiency syndrome (AIDS ) are conditions with dysregulated cytokine production. As dehydroepiandrosterone sulphate (DHEAS ) restored normal cytokine production in old mice its effects on retrovirally infected old mice were investigated. Retrovirus infection and ageing-induced immune dysfunction. Murine retrovirus-infected old C57BL/6 female mice consumed 0·22 or 0·44 mg of DHEAS/mouse/day beginning 2 weeks postinfection for 10 weeks. DHEAS largely prevented the retrovirus-induced reduction in T-cell and B-cell mitogenesis. DHEAS supplement prevented loss of cytokines [interleukin-2 (IL-2) and interferon-c] secretion by mitogen-stimulated splenocytes representing T helper 1 (Th1) cell phenotypes. It also suppressed the retrovirus-induced, excessive production of cytokines (IL-6 and IL-10) by Th2 cells. The highest dose of DHEAS reduced IL-6 production by splenocytes from uninfected old mice by 75% while increasing their IL-2 secretion by nearly 50%. Thus immune dysfunction induced by ageing, even when exacerbated by murine retrovirus infection, was largely prevented by DHEAS.

The use of a lumped-process mathematical model to simulate the complete dissolution of immiscible liquid non-uniformly distributed in physically heterogeneous porous-media systems was investigated. The study focused specifically on systems wherein immiscible liquid was poorly accessible to flowing water. Two representative, idealized scenarios were examined, one wherein immiscible liquid at residual saturation exists within a lower-permeability unit residing in a higher-permeability matrix, and one wherein immiscible liquid at higher saturation (a pool) exists within a higher-permeability unit adjacent to a lower-permeability unit. As expected, effluent concentrations were significantly less than aqueous solubility due to dilution and by-pass flow effects. The measured data were simulated with two mathematical models, one based on a simple description of the system and one based on a more complex description. The permeability field and the distribution of the immiscible-liquid zones were represented explicitly in the more complex, distributed-process model. The dissolution rate coefficient in this case represents only the impact of local-scale (and smaller) processes on dissolution, and the parameter values were accordingly obtained from the results of experiments conducted with one-dimensional, homogeneously-packed columns. In contrast, the system was conceptualized as a pseudo-homogeneous medium with immiscible liquid uniformly distributed throughout the system for the simpler, lumped-process model. With this approach, all factors that influence immiscible-liquid dissolution are incorporated into the calibrated dissolution rate coefficient, which in such cases serves as a composite or lumped term. The calibrated dissolution rate coefficients obtained from the simulations conducted with the lumped-process model were approximately two to three orders-of-magnitude smaller than the independently-determined values used for the simulations conducted with the distributed-process model. This disparity reflects the difference in implicit versus explicit consideration of the larger-scale factors influencing immiscible-liquid dissolution in the systems.