Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/11/2000
Publication Date: N/A
Citation: Interpretive Summary: Research on the impact of agriculture on water quality requires methods for measuring or estimating the concentration of chemicals in soil water. Since dissolved chemicals affect the electrical conductivity of water (ECw), measurements of soil electrical conductivity (ECb) offer potential for estimating solution concentration. This is attractive because such measurements can be easily made on an automated, continuous basis using time domain reflectometry. However, there is much uncertainty regarding the relationship between ECb and ECw; i.e. - how do soil water content and soil physical properties affect the measurement? Many conceptual relationships have been proposed, but there has been little comparative testing. We evaluated a number of potential approaches using carefully collected data in which ECw was known and ECb was measured at a variety of temperatures and water contents. We found that there is no benefit to using the more complicated models that require detailed information on soil hydraulic properties. We obtained the best results with simple models that consider electrical conduction in soils to be analogous to gas diffusion in soils. These models require only soil water content in order to calculate ECw from ECb measurements. The results will be valuable in monitoring chemical transport in variably saturated soils, enabling scientists and regulatory agencies to more accurately estimate the movement of agricultural chemicals to groundwater.
Technical Abstract: Studies of solute transport through soil and it's environmental impacts are hampered by the lack of methods for continuous monitoring of solute concentration. Measurement of bulk soil electrical conductivity (ECb)using time domain reflectometry (TDR) is a promising technique, but it is indirect, and estimation of solute concentration from such measurements first requires a model relating soil solution electrical conductivity (ECw to ECb. Several models of varying complexity exist, but further testing is needed to determine their relative merits and applicability. All models assume that the ratio of ECb/ECv is proportional to soil water content, theta, with the coefficient of proportionality referred to as a geometric or tortuosity factor, Fg. In this study, two types of models were compared: those in which Fg is obtained from soil hydraulic properties and those in which Fg is estimated as it is in gas diffusion models, except with theta rather than porosity as the independent variable. In addition, the commonl used empirical model of Rhoades et at. (1976) was also included in the comparison. Measurements were conducted in a sandy soil, over a range of ECw from 0.1 to 0.56 dS/m. The models in which Fg is obtained from soil hydraulic properties performed poorly. The results with the gas diffusion analog models were variable; the most successful was based on the model of Marshall (1959) in which Fg is a power function of theta. The empirical model of Rhoades et al. (1976) also fit the data well, though not as closely as the Marshall-based equation. We conclude that there is no benefit to the use of soil hydraulic properties in estimating ECw from ECb measurements, at least for sandy soils, where simpler relationships appear to provide superior results.