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Item Groundwater nitrate transport and residence time in a vulnerable aquifer under dryland cereal production(Montana State University - Bozeman, College of Agriculture, 2013) Miller, Christine Ross; Chairperson, Graduate Committee: Stephanie A. Ewing; Stephanie A. Ewing, W. Adam Sigler, E. N. J. Brookshire, Clain A. Jones, Douglas Jackson-Smith and Gary S. Weissmann were co-authors of the article, 'Groundwater nitrate transport and residence time in a vulnerable aquifer under dryland cereal production' submitted to the journal 'Journal of geophysical research - biogeosciences' which is contained within this thesis.Selection of agricultural management practices to reduce nitrate leaching from soils can only be successful if both nitrate loading rates from soils to shallow aquifers and groundwater residence times are quantified. Elevated nitrate concentrations in shallow unconfined aquifers are commonly observed in agricultural areas as a result of increased N inputs. In the Judith River Watershed (JRW) in central Montana, USA, notably high nitrate concentrations in groundwater and stream water have exceeded the U.S. EPA drinking water standard of 10 mg L -1 for at least two decades. This large (24,400 ha) watershed drains immediately into the Missouri River, a tributary of the Mississippi River. Over an eleven month period in 2012, we measured groundwater and surface water nitrate concentrations across a hydrologically isolated strath terrace. We use the resulting data to constrain nitrate accumulation dynamics in the shallow aquifer. Nitrate is relatively conservative in this location, as it is high in groundwater (17.57 +/- 4.29 mg L -1; all groundwater samples pooled together), and remains high in streams and springs that drain the landform (15.67 +/- 9.45 mg L -1; all surface water and spring samples pooled together). We use a numerical model to simulate the character of nitrate accumulation in the aquifer as a whole, in order to evaluate how the entire period of cultivation has contributed to current nitrate concentrations, and begin to predict response times for effects of land use change. We consider the effect of groundwater residence time and travel time on nitrate loading using particle tracking in a three dimensional model aquifer. We find no correlation with nitrate concentrations in groundwater and emerging surface waters, and suggest approaches for improving both the geometry of the model and the selection of sites in future work. Overall, our results imply that groundwater residence times are several decades at most, suggesting that similar timeframes will be needed to reduce overall nitrate concentrations in groundwater and emergent streams to below drinking water standards. Preliminary evaluation of several management scenarios suggests that both increased fertilizer use efficiency and rotational strategies may be needed to prevent the loss of soil N to groundwater.Item Chromatographic, spectroscopic and microscopic analyses reveal the impact of iron oxides and electron shuttles on the degradation pathway of 2,4,6- trinitrotoluene (TNT) by a fermenting bacterium(Montana State University - Bozeman, College of Agriculture, 2003) Borch, Thomas; Chairperson, Graduate Committee: William P. Inskeep and Robin Gerlach (co-chair)Contamination of surface and subsurface environments with explosives such as 2,4,6-trinitrotoluene (TNT) is a worldwide problem. The fate and analysis of TNT were investigated in numerous artificially contaminated model systems. We developed a unique high performance liquid chromatography gradient elution method for the analysis of commonly observed TNT metabolites and EPA explosives. Column temperature was identified as the key parameter for optimal separation. Iron (hydr)oxides play an important role in the reduction, sorption and fate of TNT in soil and sediment. Consequently, characterization of the nature and properties of natural and synthetic Fe (hydr)oxides is important for determining reaction mechanisms and surface-associated chemical processes. This work thus summarizes the potential applicability of imaging and spectroscopic techniques for eliciting chemical and physical properties of iron (hydr)oxides. TNT is persistent in soils due to its low redox potential and sorption. Batch and column studies revealed some of the first results on TNT desorption behavior in two well-defined model soil systems. Biosurfactants were found to be the most promising technique for enhanced TNT desorption. Batch studies with a Cellulomonas sp. in the presence of ferrihydrite and the electron shuttle anthraquinone-2,6-disulfonate (AQDS) were conducted to reveal biotic and abiotic mechanisms contributing to the degradation of TNT. Strain ES6 was found to reduce TNT and ferrihydrite with enhanced reduction in the presence of AQDS. Ferrihydrite stimulated the formation of more reduced TNT metabolites such as 2,4-diamino-6-nitrotoluene. Interestingly, a completely different degradation pathway was observed in AQDS-amended iron-free cell suspensions, showing a rapid transformation of TNT to 2,4-dihydroxylamino-6-nitrotoluene, which transformed into unidentified polar products. The influence of iron phases (i.e. hematite, magnetite, and ferrihydrite) and secondary Fe mineral formation on the degradation of TNT was also evaluated. The initial reduction of TNT was fastest in the presence of hematite; however, the further reduction of hydroxylamino-dinitrotoluenes was fastest, in the presence of magnetite and ferrihydrite (no AQDS). The impact of AQDS was predominant in the presence of hematite resulting in the formation of 2,4,6-triaminotoluene. Ferrihydrite underwent reductive dissolution with the formation of secondary hematite. The enhanced TNT reduction in ferfihydrite-amended systems was therefore most likely due to redox-active Fe(II) rather than secondary Fe phases.