Measuring instantaneous solute flux and loading with time domain reflectometry

Time domain reflectometry (TDR) can be used to measure water content and conservative tracer concentration in soils. An important contribution of TDR for assessing or monitoring water-soluble contaminant loading in real time under transient now may be possible because of the ability to acquire rapid, multiplexed measurements of variables necessary for calculating loading - the water flux and resident Contaminant concentration. An empirically derived equation relating resident soil Br- concentration (C-Br, mg L-1) to apparent soil bulk electrical conductivity (sigma(a), dS m(-1)) and soil water content (theta, m(3) m(-3)) was established for Sparta sand (mesic, uncoated Typic Quartzipsamment). This equation has the form C-Br = alpha sigma(a)(gamma)theta(delta), where alpha,gamma, and delta are empirical constants. Using TDR, we conducted realtime monitoring of a Br- tracer transported through a repacked Sparta sand soil column at four depths. Under steady-state and transient-flow experimental conditions and assuming a unit gradient and uniform vertical now, instantaneous water flux at each monitored depth was estimated from TDR-acquired theta data and soil hydraulic conductivity. For steady-state conditions, recovery of Br- ranged from 72% at 15 cm to 133% at 45 cm, while water recovery averaged 92% for all depths. Under transient conditions, Br- recovery ranged from 65% at 15 cm to 121% at 60 cm. Percentages of Br- recovery were congruent across depths between steady- and transient-now experiments, leading us to conjecture that nonhomogeneous now patterns and pockets of immobile water had developed within the column.