The Application of time domain reflectometry in solute transport experiments

• Main Author: Yu, Chunming,1957-
• Other Authors: Conklin, Martha H.
• Language: en
• Published: The University of Arizona. 1998
• Subjects: Hydrology.; Soils > Solute movement.; Time-domain reflectometry.
• Online Access: http://hdl.handle.net/10150/191226

Contaminants can enter groundwater through the unsaturated zone as dissolved solutes. To predict the location and extent of these contaminants, transport parameters such as pore water velocity y and dispersion coefficient D are required. These parameters are often obtained through transport experiments. The goal of this study is to determine y and D using time domain reflectometry (TDR) technique. Using TDR for transport experiments under unsaturated conditions, we investigated the effects of volumetric water content θᵥ, distance of flow path, and draining-wetting history on D. TDR was used to measure θᵥ, and salt concentration in twenty-one unsaturated column experiments. The 105 cm-long column was homogeneously packed with silica sand (particle size: 53 to 425 pm). Ten TDR probes at ten depths were used to obtain in situ breakthrough curves and a chloride electrode was used to measure effluent breakthrough curves at the bottom of the column. A 35 mM NaC1 (sodium chloride) was used as the tracer with 20 mM NaC1 as background solution. We developed a three-parameter expression relating θᵥ, to measured dielectric constant Kₐ: θᵥ =aKₐᵅ + b. This calibration expression fits as closely or better than the "universal polynomial" and is also consistent with the well-known mixing model. For an isotropic soil with homogeneous water distribution, this expression is further simplified to two parameters by taking α = 0.5. The effects of temperature, porosity, soil solid and bound water can be taken into account by varying a and b of the two-parameter expression. TDR measurements have been shown to be sensitive to bound water and not particular sensitive to the other factors. To calculate y and D from breakthrough curves of step-input experiments, a new moment analysis method has been developed. The transport parameters obtained from this new method show a little difference from the parameters determined from the convection-dispersion equation using the CXTFIT model (a published computer program for estimating solute transport parameters from observed breakthrough curves). Our results demonstrated that D is dependent on measurement methods and concentrations of experimental solutions.