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Measuring Instantaneous Solute Flux and Loading with Time Domain Reflectometry

Department of Soil Science, Univ. of Wisconsin, 1525 Observatory Drive, Madison, WI 53706-1299

*Corresponding author (blowery{at}facstaff.wisc.edu)

ABSTRACT

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 (C_Br, mg L^–1) to apparent soil bulk electrical conductivity (_a, dS m^–1) and soil water content (, m³ m^–3) was established for Sparta sand (mesic, uncoated Typic Quartzipsamment). This equation has the form C_Br = _a, where , , and 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 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.

Received for publication May 27, 1996.

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