Time domain reflectometry (TDR) was used to monitor the transport of
conservative tracers in the field under transient water flow in a controlled
experiment under a kiwifruit vine. A mixed pulse of chloride and bromide was
applied to the soil surface of a 16 m2 plot that had
been isolated from the surrounding orchard soil. The movement of this solute
pulse was monitored by TDR. A total of 63 TDR probes were installed into the
plot for daily measurements of both the volumetric water content (θ) and
the bulk soil electrical conductivity (σa). These
TDR-measured σa were converted into pore water
electrical conductivities (σw) and solute
concentrations using various
θ–σa–σw
relationships that were established in the laboratory on repacked soil. The
depth-wise field TDR measurements were compared with destructive measurement
of the solute concentrations at the end of the experiment. These results were
also compared with predictions using a deterministic model of water and solute
transport based on Richards’ equation, and the
convection–dispersion equation. TDR was found to give a good indication
of the shape of the solute profile with depth, but the concentration of solute
was under- or over-estimated by up to 50%, depending on the
θ–σa–σw
relationships used. Thus TDR can be used to monitor
in situ transport of contaminants. However, only rough
estimates of the electrical conductivity of the soil solution can so far be
obtained by TDR.
