Portland State University. Department of Geology
Andrew G. Fountain
Date of Publication
Master of Science (M.S.) in Geology
Water quality -- Columbia River, Stream measurements -- Columbia River
1 online resource (127p.)
Several chemical and radiological contaminants are present in an unconfined aquifer underlying the U.S. Department of Energy's Hanford Site in southeast Washington State. Hexavalent chromium [Cr(VI)], previously used as an anticorrosive in plutonium production reactors on the shoreline of the Columbia River, is of particular concern because of its conservative nature, toxicity to humans and aquatic life, and proximity to protected salmon spawning habitat. Hydrogeologic data are abundant from the unconfined aquifer, but are lacking from the hyporheic zone through which Cr(VI) is transported into the river. A hydrogeologic study was conducted near one known Cr(VI) plume to determine the concentration and extent of Cr(VI), effect of anisotropy on transport of Cr(VI) into the river, and the relationship between changing river stage and water quality within the hyporheic zone. Forty-one piezometers were installed along a 4,000 m reach of the river at depths from 0.2 m to 2.0 m below the riverbed. The piezometers were used to sample Cr(VI), specific conductance, dissolved oxygen, and temperature. Cr(VI) concentrations ranged from undetectable to >0.7 mg/L, with 75% of the samples above the federal standards for protection of aquatic life and 25% above federal drinking water standards. Vertical hydraulic gradients ranged from -0.01 to +0.30. Hydraulic conductivity values, estimated using slug tests, ranged from 2.8 x 10–5 cm/s to 4.3 x 10-2 cm/s and were largest in areas with high Cr(VI) concentrations. Specific discharge values were estimated using borehole dilution and ranged from 4.0 x 10-3 cm/s to 1.5 x 10-1 cm/s. In general, an inverse correlation was determined for river stage versus Cr(VI) and other water quality parameters; further investigation determined the relationship to be hysteretic. Results showed that 0.044 kg/d Cr(VI) entered the river, and the concentration of Cr(VI) in the Columbia River downstream of the 100D Area was 1.8 x 10-7 mg/L . This study produced results which will allow managers to make better risk-based decisions on Cr(VI) impacts to the biota living in the hyporheic zone.
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Arntzen, Evan V., "In Situ Hydraulic Testing and Water Quality Sampling in the Hyporheic Zone of the Columbia River, Hanford Reach, Washington" (2002). Dissertations and Theses. Paper 5441.