Transect study on solute transport in a macroporous soil

Abstract

Solute transport experiments using a non-reactive tracer were conducted on short, undisturbed, saturated columns of a sandy loam soil. All columns, 20 cm in diameter and 20 cm long, were collected along a transect of 35 m. Most of the soil columns had pre-existing macropores. The columns were leached at a steady flow-rate under ponding conditions. The resulting breakthrough curves (BTCs) showed a large heterogeneity. Several of the BTCs displayed early breakthrough and long tailing. All the data were interpreted in terms of dimensional time moments, the classical convection-dispersion equation (CDE) and the mobile-immobile transport model (MIM). Experimental time moments were found to vary significantly among the different BTCs. Analysis of the time moments also revealed that the variance of the field-scale BTC was several times larger than the average of the local-scale variance. The pore water velocity w and dispersion coefficient D were obtained by fitting the CDE to the local-scale BTCs, resulting in an average dispersivity of 7.4cm. Frequency distributions for the CDE parameters w and D were equally well described by a normal or lognormal probability density function (pdf). When a log-normal pdf for D is considered, the variance of the log, transformed D values (a:nD) was found to be 2.1. For the MIM model, two additional parameters were fitted: the fraction of mobile water, O,/O, and the first-order mass transfer coefficient, a. The MIM was more successful in describing the data than the CDE transport model. For the MIM model, the average dispersivity was about 2 cm. The MIM parameters w, D and O, / O were best described by a log-normal pdf rather than a normal pdf. Only the parameter cr was better described by a normal pdf. Mobile water fractions, Om/@ ranged from 0.01 to 0-98, with a mean of 0.43 (based on a log-normal pdf). When the CDE and MIM were applied to the data, the fitted pore water velocities, w, compared favourably with the effective pore water velocities, weff, obtained from moment analysis.

Simultaneous Inverse Estimation of Soil Hydraulic and Solute Transport Parameters From Transient Field Experiments: Homogeneous Soil

“…Values for q im for the various plots varied between 0.010 and 0.057 cm 3 cm -3 . Unlike q im , the first-order exchange coefficient (w) values (table 2) were found to vary highly between the different experiments (CV = 86.4%), with the estimated values being about one to two orders of magnitude higher than those reported in the literature for both laboratory (e.g., Jacobsen et al, 1992;Mallants et al, 1996) and field studies (Jaynes et al, 1995;Jacques et al, 2002). The high values reflect rapid solute exchange between the mobile and immobile regions.…”

“…The highly transient nature of the 2-D infiltration processes may well have contributed to the relatively standard convective-dispersive nature of the transport processes in our study. Other studies in which mobile-immobile transport processes were investigated and/or MIM parameters were derived usually involved 1-D studies, mostly during flow conditions at or close to steady-state (NkediKizza et al, 1983;Seyfried and Rao, 1987;Jaynes et al, 1995;Mallants et al, 1996; among others). Additional 2-D water flow and transport studies under transient infiltration conditions may be needed to confirm our results; we are unaware of any other studies of this type in the literature.…”

“…Immobile water contents were more or less the same for the different experiments, giving an average value of only 0.025 cm 3 cm -3 . The first-order mass transfer coefficient was higher than those reported for most 1-D analyses on soil columns (Seyfried and Rao, 1987;Mallants et al, 1996) and field studies (Jaynes et al, 1995;Jacques, 2000). This reflects rapid exchange of solute with any immobile water that is present, and hence indicates that the effects of immobile water were relatively minimal in our study.…”

Criteria for selecting fluorescent dye tracers for soil hydrological applications using Uranine as an example

“…Field investigations and modeling of water and tracer movement are important in studying numerous hydrological problems, including water resource contamination (Gödeke et al, 2006;Sudicky and Illman, 2011;Zheng et al, 2011), flood mitigation (Baker, 1987;Gubareva and Gartsman, 2010;Wood et al, 1990), and landslide initiation due to excess pore pressure generation (Iverson, 2000;Wilson et al, 2012). Usually the influence of PFP on mass transfer is determined experimentally by measuring water or tracer outflow from soil columns, lysimeters or in natural catchments (Everts and Kanwar, 1989;Ghodrati and Jury, 1992;Kung et al, 2000;Mallants et al, 1996;Tsuboyama et al, 1994;Vanderborght et al, 2000). Experimental data collected this way simplifies often tortuous flow pathways to a quasi-1-D process.…”

On the distribution of scaling hydraulic parameters in a spatially anisotropic banana field

“…If the former are not satisfactorily described nor will the latter. In most occasions normal and log-normal density functions are the only distributions checked, with very few exceptions (Mallants et al, 1996). In either case this is not usually carried out on a systematic fashion.…”