Date of Award

2005

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Geology

First Advisor

S.F. Korom

Abstract

The Karlsruhe Aquifer of north-central North Dakota has experienced considerable nitrate contamination sparking an investigation by the North Dakota State Water Commission. Certain portions of the aquifer have experienced nitrate-N concentrations of up to seven times the drinking water standard. Although contamination is evident it is possible the NO₃¯ is naturally being eliminated by denitrification, whereby bacteria reduce NO₃¯ to nitrogen gas. Evidence of denitrification may be illustrated by accelerated reduction of NO₃¯ relative to the conservative tracer Cl¯ coupled with an increase in the heavy isotopes of 15N and 18O within the NO₃¯ ion. Such an increase is not evident in other nitrate attenuation processes. This dual isotope method may even be extended to identify the source of the contamination based upon isotopic signatures of specific nitrate sources.

Periodic sampling was conducted from the fall of 2003 to the spring of 2004 to monitor the relationship between NO₃¯ concentrations and isotopic fractionation. All samples were tested for NO₃¯ and Cl¯ while select samples were tested for nitrate-δ15N, nitrate-δ18O, H2O-δ2H, and H2O-δ18O. An inverse relationship between nitrate concentrations and isotopic fractionation and a direct linear trend between 15N and 18O enrichment shows that denitrification did occur at some locations. Additionally the fractionation trends of 15N and 18O indicate that NO₃¯ in the aquifer is predominantly derived from the oxidation of ammonia fertilizer.

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