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PNNL-13501 - Pacific Northwest National Laboratory

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Approach<br />

The adaptation of partitioning interwell tracer tests<br />

technology to tank leak detection requires the<br />

characterization of tracer and tank-waste partitioning<br />

behavior under nonequilibrium mass-transfer conditions<br />

and an analysis of flow-field design requirements. The<br />

partitioning interwell tracer tests concept is illustrated in<br />

Figure 1. A sample data analysis is shown in Figure 2.<br />

The proposed flow velocity for the partitioning interwell<br />

tracer tests would sweep one pore volume (the area under<br />

the tanks) every 4 to 12 hours. While tracer behavior can<br />

give real-time indications of a leak after only a short time,<br />

Tracer<br />

Injection<br />

Well<br />

Inject Tracer Extract Tracer Tracer<br />

Single Shell Tank<br />

Leak<br />

Tank Waste<br />

Tank Waste Vapor<br />

Conservative Tracer Partitioning Tracer<br />

Tracer<br />

Extraction<br />

Well<br />

Figure 1. Partitioning interwell tracer test for tank leak<br />

detection<br />

Experimental Results of Partitioning Tracer Test<br />

in a Soil Column at 3.83 cc/min<br />

Methane Difluoromethane<br />

Mean Residence Volume (cc) 37.6 61.7<br />

% Tracer Recovered 93% 95%<br />

Retardation Factor 1.00 1.64<br />

Water Saturation - 0.274<br />

Mean Residence Time (hrs) 0.16 0.27<br />

Tracer Co ncentration (pp m w<br />

1000<br />

100<br />

10<br />

1<br />

0.1<br />

M ethane (954 ppm inj.)<br />

Difluoromethane<br />

(1 132 ppm in j.)<br />

0 50 100 150 200 250<br />

Volum e (ml)<br />

Figure 2. Data from a fast-flow soil-column experiment,<br />

using methane as the conservative tracer and<br />

difluoromethane as the partitioning tracer. Temporal<br />

moment analysis measures the difference between the<br />

centroids under the elution curves to provide water<br />

saturation and mean residence volume (or pore volume).<br />

Note the higher concentration peak of the conservative<br />

tracer, and the lower peak concentration and higher tail<br />

concentration of the partitioning tracer; this is characteristic<br />

of partitioning tracer behavior.<br />

accurate quantification of the leak might require two or<br />

three swept pore volumes. One method of implementing<br />

the leak-detection technology is to use tank-waste<br />

components as leak detectors, and use the partitioning<br />

interwell tracer tests as a quantification method after a<br />

leak has been detected through the presence of volatile<br />

waste components or radioactive gases in the advection<br />

exhaust stream.<br />

Since tracer behavior governs the shape of the elution<br />

curve required for soil moisture quantification, the<br />

partitioning behavior of the tracer and its elution under<br />

conditions of exposure to actual tank-waste saline<br />

solutions must be fully understood before tank leaks may<br />

be accurately quantified. As this project continues, tracer<br />

behavior will be investigated both by theoretical studies<br />

that predict tracer behavior using mathematical modeling<br />

and thermodynamic approximations of partitioning<br />

behavior. Soil-column experiments using representative<br />

soil samples with varying moisture contents will be<br />

flushed at high flow rates with tracers (see Figure 3) and<br />

the elution curves will be studied for their adherence to<br />

theoretical models. The alterations of water-partitioning<br />

behavior in the presence of the concentrated metallic<br />

salts, ammonia, and organics present in the tank wastes<br />

are being investigated by thermodynamic modeling<br />

methods, and may soon be the subject of soil-column<br />

experiments using simulated tank wastes to<br />

experimentally quantify the non-ideal partitioning<br />

behavior of potential tracers.<br />

Non-Equilibrium Partitioning Experiments<br />

Chromatographic Column Apparatus<br />

N 2 Tracer Gas<br />

Flow Controller<br />

Sample<br />

Loop<br />

Chromatographic<br />

Column<br />

Gas<br />

Chromatograph<br />

(packed with soil)<br />

•Measure partitioning and elution<br />

in fast-flow regime<br />

•Values measured in experiments:<br />

–Gas flow rate<br />

–Water saturation of soil<br />

in chromatographic column<br />

–Swept pore volume<br />

–Tracer elution<br />

–Tracer recovery percentage<br />

Figure 3. Apparatus for nonequilibrium tracer<br />

characterization experiments using soil-column<br />

chromatography<br />

The soil moisture in the area under the tanks swept by the<br />

partitioning interwell tracer tests will affect the elution<br />

behavior and peak concentration of gaseous tank-waste<br />

components used as adjunct leak detection gases.<br />

Potential leak-detection gases will be screened for<br />

Earth System Science 249

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