Date of Award
Doctor of Philosophy (PhD)
Nitrate is one of the most common groundwater contaminants, and ingesting it leads to potential health risks. Denitrification, the only effective process to eliminate nitrate, is limited by the abundance of biologically available electron donors. Thus, understanding the natural denitrification capacity of aquifers, through the analysis of all the major electron donors, is essential.
A better way to estimate groundwater denitrification reactions is to compute the mass balance of the redox sensitive species. The University of North Dakota (UND) denitrification team installed mesocosms (ISMs) to understand the fate of nitrate in field conditions. Accordingly, the team has shown the significant role of sulfides (dominantly pyrite) and organic carbon in the denitrification processes of the regional aquifers. However, the role of Fe(II) has largely been overlooked in regional studies mainly because of two reasons: 1) the geochemical evidence for ferrous iron is more difficult to decipher due to the precipitation of Fe(III)-oxyhydroxides from the aqueous solution. 2) in the event when denitrification by both Fe(II) and organic carbon gave rise to precipitating reaction products, the role of Fe(II) is deceivably masked by that of the organic carbon. Thus far, little is known about the significance of solid phase biologically available ferrous iron in our region. We hypothesized that Fe(II)-supported denitrification, owing to the abundance of iron in aquifer sediments, has regional environmental significance.
Three techniques, wet chemical extraction, x-ray diffraction and Mössbauer spectroscopic measurements, were combined to determine ferrous iron contents and Fe(II)-bearing minerals of aquifer sediments. Geochemical modeling (PHREEQC) was employed to get an insight into the in situ denitrification processes that take place via all the common electron donors. Emphasis was given to Fe(II)-supported denitrification reactions because it has been overlooked in our region.
All aqueous analytical data, mineralogy and chemistry of sediments and geochemical modeling work support the research hypothesis. As a result, all the major electron donors are found to be important and Fe(II)-supported denitrification appears to have a significant role as a natural remediation process in the aquifers of our region.
Tesfay, Tedros, "Modeling Groundwater Denitrification by Ferrous Iron Using PHREEQC" (2006). Theses and Dissertations. 294.