Measuring Instantaneous Solute Flux and Loading with Time Domain Reflectometry
1998
Hart, G. L. | Lowery, B.
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 flow 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 (CBᵣ, mg L⁻¹) to apparent soil bulk electrical conductivity (σₐ, dS m⁻¹) and soil water content (θ, m³ m⁻³) was established for Sparta sand (mesic, uncoated Typic Quartzipsamment). This equation has the form CBᵣ = ασₐᵞθᵟ, where α, γ, and δ are empirical constants. Using TDR, we conducted real-time monitoring of a Br⁻ tracer transported through a repacked Sparta sand soil column at four depths. Under steady-state and transient-flow experiemntal conditions and assuming a unit gradient and uniform vertical flow, instantaneous water flux at each monitored depth was estimated from TDR-acquired θ 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-flow experiments, leading us to conjecture that nonhomogeneous flow patterns and pockets of immobile water had developed within the column. Research supported by USDA-ARS Management Systems and Evaluation Area, Univ. of Wisconsin-Madison College of Agriculture and Life Sciences, and Univ. of Wisconsin-Madison Dep. of Soil Science Nonpoint Pollution and Demonstration Project.
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