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IN-SITU REMEDIATION OF CHROMIUM-CONTAMINATED SOILS AND SEDIMENTS
Citation:
Paul*, C J., R W. Puls*, AND F A. Khan*. IN-SITU REMEDIATION OF CHROMIUM-CONTAMINATED SOILS AND SEDIMENTS. AGU 1999 Spring Meeting, Boston, MA, May 31 - June 4, 1999.
Description:
The objectives of this study were to characterize the extent of total chromium and Cr(VI)contamination in the underlying soils and sediments of a chrome-plating shop at the USCG Support Center near Elizabeth City, NC and to evaluate the use of liquid reductants for in situ treatment of the soils at the site. A chrome-plating shop had been the source of discharges of acidic chromium wastes into the soils and groundwater beneath the shop. An initial characterization of the site was conducted in 1990 and 1991 to determine the extent of soil and aquifer contamination by chromium and Cr(VI). Laboratory studies and a field-scale pilot study using zero-valent iron led to the installation of an in situ permeable reactive barrier (PRB) in 1996 to remediate the diffuse chromate plume at the site. While PRB treatment has proven effective for remediating the contaminant plume, its long-term effectiveness is unknown, therefore, the chromium source must be treated in order to ultimately meet remedial performance objectives. Reduction of Cr(VI) to Cr(III)and subsequent precipitation as (Fe, Cr)(OH)3 is the proposed primary reduction mechanism in the soils immediately beneath the shop. Because PRB technology has limited effectiveness in the vadose zone, in situ treatment utilizing a liquid reductant has been proposed for treating the soils and sediments beneath the shop.
A water-main break in the immediate vicinity of the chrome-plating shop in 1994 and the subsequent release of large amounts of water into the soils beneath the shop resulted in the desorption of soluble Cr(VI) and its re-distribution throughout the underlying soils. In an effort to re-characterize the distribution and extent of the total chromium and Cr(VI), soil cores were obtained from over 20 locations in 1997 and 1998, resulting in more than 300 discrete samples to provide data for vertical and horizontal delineation of the chromium. Soils were collected for total metal analysis and selective extractions were used to assess the chemical speciation and distribution of the chromium on the soils and its mobilization potential.
Phosphate was used in laboratory extractions to selectively remove Cr(VI) from contaminated soils in order to delineate the extent of the soluble, easily mobilized Cr(VI) beneath the shop. Total chromium was determined using x-ray fluorescence and Inductively Coupled Plasma (ICP) Spectrometry. Results indicate the highest concentrations of total chromium and Cr(VI) are found in the shallow soils immediately beneath that area of the shop where chromic acid tanks had been located. The highest concentrations of total chromium and Cr(VI) were found at concentrations of 10,436 mg/Kg (1.0-1.5 ft) and 625 mg/Kg (1.5-2.0 ft) respectively. The calculated mass of the total chromium and Cr(VI) to be treated within the shop soils and sediments is 145 kg and 7.4 kg, respectively, or a total soil volume of 2,737 ft3.
Historical concentrations of Cr(VI) in the groundwater beneath the shop have ranged from 1.60 mg/L in February, 1991 to 28.0 mg/L in June, 1994 prior to discovery of the water main break. Concentrations in June, 1998 were down to 3.00 mg/L.
After delineating the extent of the contamination in the soils, laboratory studies were conducted using the most contaminated soils to evaluate the feasibility of using liquid reductants at the site to treat the chromium in situ. Three different liquid reductants were used in laboratory studies to assess their effectiveness in reducing and immobilizing Cr(VI) on soils and sediments. Reductants evaluated were sodium dithionite, 1-ascorbic acid, and free hydroxylamine. Cr(VI) reduction studies showed all three chemicals to effectively reduce Cr(VI); however, l-ascorbic acid only reduced Cr(VI)to intermediary Cr(V)and/or Cr(IV), but not to Cr(III). All three reductants also reduced redox sensitive metals including Fe and Mn. Results of this study show sodium dithionite to be the most effective liquid reductant evaluated.