Date of Award

1-1-2015

Document Type

Thesis

Degree Name

Master of Science (MS)

First Advisor

Scott F. Korom

Abstract

Lake Taupo, a lake in New Zealand valued for its clear blue waters, is increasingly under the threat of degraded water clarity. Understanding the fate of nitrate entering the lake from surrounding pastoral lands is critical to maintaining the water quality.

Electron donor contributions to denitrification in the Oruanui ignimbrite were estimated using an in situ mesocosm, water quality data, and the geochemical modeling software PHREEQC. The geochemical results were modeled using denitrification reactions for electron donors and compared to measured water quality data. A successful model was defined as the best match of measured and modeled values of major water quality parameters, especially those involved in denitrification reactions. The evolved water quality was best modeled by a combination of OC and Fe(II) as both amorphous volcanic glass and a hypothetical pyroxene. OC and total Fe(II) contributed an average of 29.4% and 70.2% to denitrification, respectively. Amorphous Fe(II) made up an average of 60% of total Fe(II) and pyroxene Fe(II) made up about 40%. Ion exchange was incorporated into the model to explain cation behavior, but it failed to elucidate losses of potassium (K+) and increases in sodium (Na+). Na+ was best modeled by incorporating it into the hypothetical pyroxene Na0.11Fe+2Fe+30.63Si2O6. The behavior of K+ could not be fully explained by dilution and ion exchange, and it was possibly taken up in the interlayer space of halloysite or other clay minerals.

The apparent rate of denitrification by Fe(II) at the Spydia site was the fastest estimated rate compared to those reported in literature. Fe(II) acted as a major electron donor at the Spydia site, and should be considered in future research in the Oruanui ignimbrite and volcanic regions worldwide.

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