Quantification of the heterogeneity of the unsaturated zone based on environmental deuterium observed in lysimeter experiments

Abstract

An approach is presented which enables the quantification, in integrative form, of the preferential flow and the heterogeneity in water transport through the unsaturated zone under natural atmospheric conditions. For these purposes, deuterium transport was estimated in seven lysimeters filled with different sediment materials by applying a conceptual model, which consists of preferential and matrix flow paths for each using the lumped parameter approach. The piston flow and dispersion transit time distribution functions were assumed, for preferential and matrix flow paths, respectively. Weekly 2H data measured in precipitation (1984–1991) were taken as the input function. Combining modelling of isotopes and hydrological data enabled the quantification of preferential flow rates. The fraction of preferential flow directly appearing in the outflow within one week varied between 17 and 30%. The fraction of preferential flow was practically independent of the soil texture and flow rates. The crucial parameter influencing the fraction of direct flow was found to be the saturated hydraulic conductivity (Ks ). In the matrix flow, it was found that the apparent dispersion parameter (PD )* is indirectly proportional to the mean water content (). This relationship shows that the heterogeneity of the water flux in the matrix is higher for lower water contents. Finally, the transit time distribution functions, determined for both flow paths and the fractions of preferential flow, were used to construct vulnerability diagrams. Such a diagram gives the amounts of tracer mass (conservative pollutant), which appear weekly in the outflow between 0 and 60 weeks. The vulnerability diagrams showed different patterns for different soils, depending on the saturated conductivity and mean water content. Coarser material with low and high Ks showed a short mean transit time for the matrix flow (about 10 weeks) and mean preferential flow equal to or higher than 20%. Finer sand, with lower Ks and higher , resulted in mean transit times of approximately 30 weeks and preferential flow of 17–21%. These diagrams can be used to estimate the vulnerability of groundwater to pollution for different soil materials of the unsaturated zone.