Advection‐Dispersion Analysis of Solute Transport in Undisturbed Soil Monoliths

Abstract

The results of an extended analysis of the chloride breakthrough curves (BTCs) obtained from transport experiments on undisturbed soil monoliths previously reported by Sassner et al. (1994) are discussed. Parameter values for different forms of the advection‐dispersion equation (ADE) were obtained by curve‐fitting using the nonlinear least‐squares optimization code CXTFIT developed by Parker and van Genuchten (1984). Good fits to the experimental BTCs from the individual soil monoliths were obtained for three different forms of the ADE, allowing for anion exclusion, immobile water, and both of these processes, respectively. Although the more complex forms of the ADE possess more flexibility to provide a slightly more refined fit to the individual breakthrough curves, larger uncertainty is associated with the fitted parameter values. Due to the limited amount of information manifested in the BTCs it is not possible on the basis of this information to distinguish which of the models is physically more meaningful. Furthermore, when more parameters are included in the optimization procedure, the resulting values are more uncertain. The same ADE models were also fitted with equally good results to a hypothetical large‐scale BTC derived by flux‐averaging the responses from the individual monoliths. However, regardless of the form of the ADE used, the resulting parameter values for the large‐scale transport (i.e., not only the dispersivity value that is expected to increase) were inconsistent with the corresponding parameter values obtained for the individual BTCs. This supports previous indications that ADE models may not be accurate for predicting large‐scale transport in heterogeneous soil systems corresponding to e.g. the scale of an agricultural field or a grid element in a numerical catchment model because model parameters are not determinable from independent measurements on a scale corresponding to a practical manageable core size or to standard intrumentation.

Applied tracers for the observation of subsurface stormflow at the hillslope scale

“…Besides dyes, inorgani c hal ogen comp ounds, especially bromide and chloride spec ies, have been established to investigate flow and leaching processes in the vado se zone in soil column experiments (e.g . Binl ey et al, 1996;Jensen et al, 1996;Hend erson et al, 1996), at the plot or lysim eter scale (e.g. Hornb erger et al, 1990 ;Tsubo yama et al, 1994 ;Deeks et al, 2008) and at the field or hillslope scale (e.g.…”

“…The assumptions underlying the one-dimensional CDE are obviou sly strong simplifications, in particular for the hillslope scale. If it is employed as a functional description of solute transport notwithstandin g, the resulting parameters do not necessarily have observable physical meaning (Jensen et al, 1996).…”

STANMOD: Model Use, Calibration, and Validation

“…Hundreds of other examples can be retrieved readily from the peerreviewed literature, with applications involving the transport of broad range of chemicals, including nonadsorbing tracers, pesticides, heavy metals, radionuclides, and other organic and inorganic chemicals. We list here rather arbitrarily a few publications in which the CXTFIT codes have been used successfully to estimate transport parameters from observed concentration distributions versus time and/or distance: Gamerdinger et al (1990), Jacobsen et al (1992aJacobsen et al ( , 1992b, Pivetz and Steenhuis (1995), Spurlock et al (1995), Huang et al (1995), Jensen et al (1996), Seuntjens et al (2001), Kay and Conklin (2001), Kamra et al (2001), Antoniadis and McKinley (2003), Pace et al (2003), Gaur et al (2003), Tilahun et al (2005) and Koestel et al (2011). Similar applications can be found for the older CFITM and CFITIM codes but are not further listed here.…”