Submitted to: Advances in Water Resources
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/28/2004
Publication Date: 2/1/2006
Citation: Al-Jabri, S.A., Lee, J., Gaur, A., Horton, R., Jaynes, D.B. 2006. A dripper-TDR method for insitu determination of hydraulic conductivity and chemical transport properties of surface soils. Advances in Water Resources. 29(2):239-249. Interpretive Summary: Chemicals that leach through soil pose contamination threats to surface and groundwater quality and can ultimately impact human health. It is difficult and expensive to monitor chemical movement and fate below the surface of soils. Computer models can be used instead of direct measurements to determine chemical fate and movement, but the soil properties needed to run the models are also difficult to measure. In this research, we developed and tested a new technique for measuring the soil properties that are needed for modeling water and chemical movement in soil. The simple method uses irrigation drip emitters and a chloride tracer and can give multiple soil measurements in a limited amount of time. This technique and the soil parameters it measures will be of use to scientists and engineers interested in determining the movement of water and chemicals in soils. Ultimately, the technique developed here will be of use to decision makers and others for regulating chemical use in the environment.
Technical Abstract: Field determined hydraulic and chemical transport properties can be useful for the protection of groundwater resources from land-applied chemicals. Most field methods to determine flow and transport parameters are either time or energy consuming and/or they provide a single measurement for a given time period. In this study, we present a dripper-TDR field method that allows measurement of hydraulic conductivity and chemical transport parameters at multiple field locations within a short time period. Specifically, the dripper-TDR determines saturated hydraulic conductivity, macroscopic capillary length, immobile water fraction, mass exchange coefficient, and dispersion coefficient. Multiple dripper lines were positioned over five crop rows in a field. Background and step solutions were applied through drippers to determine surface hydraulic conductivity parameters at 44 locations and surface transport properties at 38 locations. The hydraulic conductivity parameters were determined by application of three discharge rates from the drippers and measurements of the resultant steady-state flux densities at the soil surface beneath each dripper. Time domain reflectometry (TDR) was used to measure the bulk electrical conductivity of the soil during steady infiltration of a salt solution. Breakthrough curves (BTCs) for all sites were determined from the TDR measurements. The saturated hydraulic conductivity values and macroscopic capillary length were found to be lognormally distributed with average values of 31.4 cm h**-1 and 6.0 cm, respectively. BTC analysis produced chemical properties, immobile water fraction, mass exchange coefficient, and dispersion coefficient with average values of 0.23, 0.0036 h**-1 and 1220 cm**2 h**-1, respectively. The estimated values of the flow and transport parameters were found to be within the ranges of values reported by previous studies conducted at nearby field locations. The dripper TDR method is a rapid and useful technique for in-situ measurements of hydraulic conductivity and solute transport properties. The measurements reported in this study give clear evidence to the occurrence of nonequilibrium water and chemical movement in surface soil. The method allows for quantification of nonequilibrium model parameters and preferential flow. Quantifying the parameters is a necessary step toward determining the influences of surface properties on infiltration, runoff, and vadose zone transport.