Field Bromide Transport under Transient-State

The contamination of groundwater by excess fertilizer and pesticides is a problem associated with modern agricultural practices. Estimating fluxes of these contaminants to groundwater requires frequent soil water sampling. Time domain reflectometry (TDR) techniques appear well-suited to this purpose because of their potential for automation and the limited calibration work required, despite potential constraints associated with probe geometry, temperature, and the nonspecificity of the probe. The objective of this study was to compare the performance of the TDR technique with porous cup samplers for estimating solute masses and concentrations in a soil at the lysimeter scale. Potassium bromide was applied at the soil surface in a 0.91 by 0.52 m area and solute was sampled with TDR and porous cops. Recovered Br masses were calculated based on measured water contents and Br - concentrations. When the solute was concentrated in the top 0 to 45 cm of soil, the biases for solute mass reached 2.6 and 5.3 times the applied mass for TDR and the porous cup samplers, respectively. As the solute spread out below this depth, the bias with the TDR technique decreased to between 1.17 and 1.27 times the applied mass, whereas for the porous cup samplers, it varied between 0.97 and 1.83 times the applied Br mass. Differences in soil structure appeared the most likely explanation for the bias. The study also indicates that field calibration of TDR from porous cup samplers may be difficult to achieve under transient state conditions.