Detection of inorganic chemicals in a sandy soil using TDR: Effect of probe geometry and water content

Monitoring of soil contamination becomes an important issue for effective remediation of soil and groundwater. Recently time domain reflectometry (TDR) has emerged as a promising tool for monitoring soils contaminated with inorganic chemicals since concentration of these chemicals can significantly contribute to the electrical property of a porous medium such as conductance or inverse of resistance (1/R) which can be measured by TDR. In this study, we investigated a possibility of detecting two inorganic chemicals (zinc and nitrate) in a sandy soil using TDR by conducting a number of batch experiments on TDR-measured resistance for various probe geometries (length (L) = 10, 15, 25 cm; ratio of rod spacing to diameter (s/d) = 5, 6) and saturation degrees (Se = 0.1-1.0). Column experiment was also performed to investigate the extent of input concentration which allows the detection of nitrate by TDR for various travel distances under saturated flow condition. The results of batch experiments showed that the inverse of TDR-measured resistance was sensitive to both probe geometry and saturation degree. The short probe (L = 10 cm) could not detect properly the electrical property in the low concentration range (0~0.5 g/L) due to the higher s/d ratio and short length. This phenomenon was more pronounced for zinc than nitrate. Saturation degree also affected significantly the measurement of resistance especially for the short probe when low saturation degree (Se < 0.5) was imposed, showing incapability of detection due to the resistance higher than the upper limit (1 kΩ) of TDR measurement. Column experiment revealed that the short probe could not detect nitrate at a travel distance of 5 cm when input concentration of nitrate lower than 500 mg/L was imposed. This indicates that the long probe (L = 25 cm) is mostly recommended to use for detection of zinc and nitrate contamination whereas use of short probe is strictly limited to the condition of high water content (Se > 0.5) and input concentration higher than 2000 mg/L in the sandy soil.