Characterization of preferential flow in undisturbed, structured soil columns using a vertical TDR probe

Abstract

Rapid movement of agricultural chemicals through soil to groundwater via preferential flow pathways is one cause of water contamination. Previous studies have shown that time domain reflectometry (TDR) could be used to characterize solute transport in soil. However, previous studies have only scarcely addressed preferential flow. This study presents an extended application of TDR for determining preferential flow properties. A TDR method was tested in carefully controlled laboratory experiments using 20-cm long and 12-cm diameter undisturbed, structured soil columns. The method used a vertically installed TDR probe and a short pulse of tracer application to obtain residual mass (RM) breakthrough curves (BTC). The RM BTC obtained from TDR were used to estimate mobile/immobile model (MIM) parameters that were compared to the parameter estimates from effluent data. A conventional inverse curve fitting method (CXTFIT) was used to estimate parameters. The TDR-determined parameters were then used to generate calculated effluent BTC for comparison with observed effluent BTC for the same soil columns. Time moments of the calculated and observed BTC were calculated to quantitatively evaluate the calculated BTC. Overall, the RM BTC obtained from TDR were similar to the RM BTC obtained from effluent data. The TDR-determined parameters corresponded well to the parameters obtained from the effluent data, although they were not within the 95% confidence intervals. Correlation coefficients between the parameters obtained from TDR and from effluent data for the immobile water fraction (θ im /θ), mass exchange coefficient (α), and dispersion coefficient (D m ) were 0.95, 0.95, and 0.99, respectively. For three of the four soil cores, θ im /θ ranged from 0.42 to 0.82, indicating considerable preferential flow. The TDR-calculated effluent BTC also were similar to the observed effluent BTC having an average coefficient of determination of 0.94. Time moments obtained from calculated BTC were representative of those obtained from observed BTC. The vertical TDR probe method was simple and minimally destructive and provided representative preferential flow properties that enabled the characterization of solute transport in soil. RightsWorks produced by employees of the U.S. Government as part of their official duties are not copyrighted within the U.S. The content of this document is not copyrighted. AbstractRapid movement of agricultural chemicals through soil to groundwater via preferential flow pathways is one cause of water contamination. Previous studies have shown that time domain Ž . reflectometry TDR could be used to characterize solute transport in soil. However, previous studies have only scarcely addressed preferential flow. This study presents an extended application of TDR for determining preferential flow properties. A TDR method was tested in carefully controlled laboratory experiments using 20-cm long and 12-cm diameter undisturbed, structured soil columns. The method used a vertically installed TDR probe and ...

Deslocamento miscível de nitrato e fosfato proveniente de água residuária da suinocultura em colunas de solo

“…Os resultados obtidos podem ser explicados pela alta correlação entre as superfícies específica de óxidos de ferro e adsorção de fósforo em solos argilosos, conforme Fontes & Weed (1996); outro fator que pode ter influenciado na obtenção desses dados pode estar relacionado ao fato da possibilidade de ter ocorrido fluxo preferencial, ainda que o valor da umidade imóvel em relação à umidade total (θ im /θ) tenha o valor de 0,2881, abaixo da faixa de 0,48 a 0,82 definido por Lee et al (2001) como indicativo de considerável fluxo preferencial.…”

A review of model applications for structured soils: a) Water flow and tracer transport

“…For example, the immobile water content, the solute transfer coefficient, and the dispersion coefficient were estimated using a vertical TDR method that was used to analyze resident tracer breakthrough (Lee et al, 2000(Lee et al, , 2001(Lee et al, , 2002. A flow interruption technique for intermittent solute leaching through soil columns was applied to estimate the mobile and immobile water contents, and the mass transfer coefficient (Ilsemann et al, 2002).…”

“…( Lee et al, 2000Lee et al, , 2001Lee et al, , 2002 UC θ im , α (MIM) Intermittent solute leaching and flow interruption for soil columns Ilsemann et al (2002) UC θ(h) and K(h) parameters (bimodal, multimodal, or DPM)…”

The Longitudinal Dispersion Coefficient of Soils as Related to the Variability of Local Permeability

“…Measured solute concentrations were fitted with the CXTFIT equilibrium model to estimate the pore-water velocity and dispersion coefficient. A TDR method was tested in laboratory experiments using undisturbed, structured soil columns (Lee et al 2001). In this technique, a TDR probe is installed vertically and a short pulse of tracer is applied to obtain residual mass (RM) breakthrough curves (BTC).The vertical TDR probe method is simple and rapid with minimum disturbance of soil, and it is a promising technique for characterizing preferential solute transport (Lee et al 2001).…”