Solute movement through intact columns of cryoturbated Upper Chalk

Abstract

Abstract:Cryoturbated Upper Chalk is a dichotomous porous medium wherein the intra-fragment porosity provides water storage and the inter-fragment porosity provides potential pathways for relatively rapid flow near saturation. Chloride tracer movement through 43 cm long and 45 cm diameter undisturbed chalk columns was studied at water application rates of 0Ð3, 1Ð0, and 1Ð5 cm h 1 . Microscale heterogeneity in effluent was recorded using a grid collection system consisting of 98 funnel-shaped cells each 3Ð5 cm in diameter. The total porosity of the columns was 0Ð47 š 0Ð02 m 3 m 3 , approximately 13% of pores were ½15 µm diameter, and the saturated hydraulic conductivity was 12Ð66 š 1Ð31 m day 1 . Although the column remained unsaturated during the leaching even at all application rates, proportionate flow through macropores increased as the application rate decreased. The number of dry cells (with 0 ml of effluent) increased as application rate decreased. Half of the leachate was collected from 15, 19 and 22 cells at 0Ð3, 1Ð0, 1Ð5 cm h 1 application rates respectively. Similar breakthrough curves (BTCs) were obtained at all three application rates when plotted as a function of cumulative drainage, but they were distinctly different when plotted as a function of time. The BTCs indicate that the columns have similar drainage requirement irrespective of application rates, as the rise to the maxima (C/C o ) is almost similar. However, the time required to achieve that leaching requirement varies with application rates, and residence time was less in the case of a higher application rate. A two-region convection-dispersion model was used to describe the BTCs and fitted well (r 2 D 0Ð97-0Ð99). There was a linear relationship between dispersion coefficient and pore water velocity (correlation coefficient r D 0Ð95). The results demonstrate the microscale heterogeneity of hydrodynamic properties in the Upper Chalk.