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FRY CANYON

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Fry Canyon Initial Barrier Monitoring Results

Iron and Phosphate Concentrations

The concentration of phosphate is being monitored in the PO4 barrier and the concentration of iron is being monitored in both the ZVI and AFO barriers. These chemical constituents are being monitored to evaluate possible impacts of the barrier materials to offsite ground-water quality.

Elevated concentrations of phosphate are observed within the PO4 barrier and in the pea gravel upgradient of the barrier. The migration of phosphate into the pea gravel is in the opposite direction of the preinstallation ground-water flow. Heavy rainfall since barrier installation has probably resulted in temporary reversals in ground-water flow directions. Since the PO4 barrier is closest to Fry Creek, this barrier is likely to be the most sensitive to flow reversals. The persistence of high phosphate concentrations in the pea gravel is probably the result of the limited amount of metal oxides contained on the pea gravel. Metal oxides, especially iron and manganese oxy-hydroxides usually limit phosphate concentrations to significantly less than 1 mg/L (Hem, 1989). The low phosphate concentrations below the PO4 barrier are probably due to the large amounts of naturally occurring iron oxy-hydroxides in the colluvial sediments.

Contour map of dissolved phosphate in the PO<sub>4</sub> barrier.

In contrast to the AFO barrier, the iron concentrations within the ZVI barrier are high, with most of the samples in the first 0.5 m of the barrier exceeding 6 mg/L. Redox conditions within the ZVI barrier are extremely reducing with no measurable dissolved oxygen and extremely negative oxidation reduction potential values in the range of -500 to -600 millivolts. Dissolved iron concentrations decrease substantially in the last 0.2 meter of the ZVI barrier as well as downgradient of the barrier.

Contour map of dissolved iron in the ZVI barrier

Dissolved iron concentrations are generally less than 0.3 mg/L within and upgradient from the AFO barrier. The oxidizing conditions present in the AFO barrier (measurable dissolved oxygen) are consistent with the low concentrations of dissolved iron.

Contour map of dissolved iron in the AFO barrier

pH Values

Native ground water entering the reactive chemical barriers is near neutral in pH. After contacting the ZVI barrier material, the pH of the water exceeds 9.0 units. After exiting the ZVI barrier, the pH value of the water is near neutral. The pH values of water from within AFO and PO4 barriers are near neutral, ranging between 6.4 to 7.6 pH units.

Contour map of pH in the PO<sub>4</sub> barrier.

Contour map of pH in the ZVI barrier.

Uranium Removal

Three weeks after installation, the reactive chemical barriers are removing the majority of uranium from the contaminated ground water. Prior to entering the PO4 barrier, the contaminated ground water contains between 3,050 and 3,920 mg/L dissolved uranium. After entering the barrier, the uranium concentration is reduced substantially, containing dissolved uranium of concentrations between 2 and 580 mg/L.

The observed increase in uranium concentration after the ground water exits each of the barriers is from uranium that is desorbed from the previously contaminated colluvial sediments. In an actual ground-water remediation application, the uranium would not increase after exiting the barriers because the reactive chemical barrier would be positioned in uncontaminated sediments downgradient from the maximum extent of the plume.

Plot of uranium concentration in PO<sub>4</sub> barrier.

Prior to entering the ZVI barrier, the contaminated ground water contains between 1,510 and 8,550 mg/L dissolved uranium. After traveling 0.15 m into the reactive barrier, the uranium concentration is reduced to below the analytical reporting limit of 0.06 mg/L.

Prior to entering the AFO barrier, the contaminated ground water contains between 14,870 and 17,590 mg/L dissolved uranium. After traveling 0.61 m into the reactive barrier, the uranium concentration is reduced to less than 500 mg/L.

The observed increase in uranium concentration after the ground water exits each of the barriers is from uranium that is desorbed from the previously contaminated colluvial sediments. In an actual ground-water remediation application, the uranium would not increase after exiting the barriers because the reactive chemical barrier would be positioned in uncontaminated sediments downgradient from the maximum extent of the plume.

Plot or uranium concentration in AFO barrier.

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Page Last Modified: Thursday, 10-Jan-2013 12:05:34 EST