Field bromide transport under transient-state: Monitoring with time domainreflectometry and porous cup

The contamination of groundwater by excess fertilizer and pesticides is a p roblem associated with modern agricultural practices. Estimating fluxes of these contaminants to groundwater requires frequent soil water sampling, Ti me 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, te mperature, and the nonspecificity of the probe. The objective of this study was to compare the performance of the TDR technique with porous cup sample rs for estimating solute masses and concentrations in a soil at the lysimet er 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 cups. Recovered Br- ma sses were calculated based on measured water contents and Br- concentration s. When the solute was concentrated in the top 0 to 45 cm of soil, the bias es for solute mass reached 2.6 and 5.3 times the applied mass for TDR and t he porous cup samplers, respectively, As the solute spread out below this d epth, the bias with the TDR technique decreased to between 1.17 and 1.27 ti mes the applied mass, whereas for the porous cup samplers, it varied betwee n 0.97 and 1.83 times the applied Br- mess. Differences in soil structure a ppeared the most likely explanation for the bias, The study also indicates that field calibration of TDR from porous cup samplers mag be difficult to achieve under transient state conditions.