Treatment of nitric acid-, U(VI)-, and Tc(VII)-contaminated groundwater in intermediate-scale physical models of an in situ biobarrier.
Metal and hydrogen ion acidity and extreme nitrate concentrations at Department of Energy legacywaste sites pose challenges for successful in situ U and Tc bioimmobilization. In this study, we investigated a potential in situ biobarrier configuration designed to neutralize pH and remove nitrate and radionuclides from nitric acid-, U-, and Tc-contaminated groundwater for over 21 months. Ethanol additions to groundwater flowing through native sediment and crushed limestone effectively increased pH (from 4.7 to 6.9), promoted removal of 116 mM nitrate, increased sediment biomass, and immobilized 94% of total U. Increased groundwater pH and significant U removal was also observed in a control column that received no added ethanol. Sequential extraction and XANES analyses showed U in this sediment to be solid-associated U(VI), and EXAFS analysis results were consistent with uranyl orthophosphate (UO2)3(PO4)2.4H2O(s), which may control U solubility in this system. Ratios of respiratory ubiquinones to menaquinones and copies of dissimilatory nitrite reductase genes, nirS and nirK, were at least 1 order of magnitude greater in the ethanol-stimulated system compared to the control, indicating that ethanol addition promoted growth of a largely denitrifying microbial community. Sediment 16S rRNA gene clone libraries showed that Betaproteobacteria were dominant (89%) near the source of influent acidic groundwater, whereas members of Gamma- and Alphaproteobacteria and Bacteroidetes increased along the flow path as pH increased and nitrate concentrations decreased, indicating spatial shifts in community composition as a function of pH and nitrate concentrations. Results of this study support the utility of biobarriers for treating acidic radionuclide- and nitrate-contaminated groundwater.
Michalsen
Mandy M
MM
Environmental Engineering & Technology Section, U.S. Army Corps of Engineers, Seattle, Washington 98134, USA. mandy.m.michalsen@usace.army.mil
Peacock
Aaron D
AD
Smithgal
Amanda N
AN
White
David C
DC
Spain
Anne M
AM
Sanchez-Rosario
Yamil
Y
Krumholz
Lee R
LR
Kelly
Shelly D
SD
Kemner
Kenneth M
KM
McKinley
James
J
Heald
Steve M
SM
Bogle
Mary Anna
MA
Watson
David B
DB
Istok
Jonathan D
JD
D019015:Geologic Sediments; D008956:Models, Chemical; D008958:Models, Molecular; D015394:Molecular Structure; D017942:Nitric Acid / Q000737:chemistry*; D013667:Technetium / Q000737:chemistry*; D014501:Uranium / Q000737:chemistry*; D014871:Water Microbiology; D014874:Water Pollutants, Chemical / Q000737:chemistry; D014875:Water Pollutants, Radioactive / Q000737:chemistry; D014881:Water Supply / Q000032:analysis*
D014874:Water Pollutants, Chemical; D014875:Water Pollutants, Radioactive; D017942:Nitric Acid; D014501:Uranium; D013667:Technetium