Date of Award

1999

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Geology

First Advisor

S.F. Korom

Abstract

Groundwater serves more than half of North Dakota's population as a drinking water source. Nitrate (NO₃¯) is a common groundwater contaminant associated with fertilizers, septic systems and livestock waste in shallow unconfined aquifers. As a suspected carcinogen and cause of potentially lethal methemoglobinemia in infants, the natural attenuation of NO₃¯ by denitrification can be an important process in keeping groundwater as a reliable source of clean drinking water for North Dakota. Organic carbon is often considered the most important electron donor for the chemical reduction of NO₃¯ into harmless nitrogen gas. However, several studies have shown that inorganic electron donors in the form of FeS₂ and Fe₂⁺ participate in denitrification as electron donors. The intent of this study was to provide evidence for sulfide mineral participation as an electron donor in denitrification within the Elk Valley aquifer of eastern North Dakota.

To test the ability of sulfides to serve as an electron donor in denitrification, two in-situ mesocosms (ISMs) were built and placed within the EVA about 1.5-m below the water table. The ISMs were stainless steel chambers 1.5-m long with a 39.37-cm interior diameter, open on the bottom, and capped with a 40-mesh/cm screen and sample tube assembly. One ISM served as the Research ISM into which NO₃¯ and bromide (Br¯) amended groundwater was pumped. The second or Control ISM received only Br¯ amended groundwater. Monthly sampling of the ISMs over a 272-day study period resulted in nitrate attenuation of 4.18 mmol/L of nitrate-N (NO₃¯-N), not attributable to dilution as measured by the Br¯ tracer, within the Research ISM.

A progressive increase of δ¹⁵N in the remaining NO₃¯-N from 2.4 to 43.5‰ and an isotopic enrichment factor of -20.4‰ also provide evidence of denitrification. An increase of 1.70 mmol/L in sulfate identifies pyrite as the electron donor for 61% of the observed denitrification with the remaining 39% attributed to heterotrophic processes and possibly non-pyritic ferrous iron in the sediments. The observed change in δ¹⁵N during the study also suggests that the bioavailability of electron donors controls the rate of denitrification.

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