Date of Award

January 2013

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Geology

First Advisor

Scott Korom

Abstract

Denitrification, the microbial reduction of nitrate (NO3-) in groundwater, has three requirements: limited oxygen, bacteria capable of mediating the reaction, and electron donors for the bacteria to use in the redox reactions. The critical factor for aquifer denitrification is the concentrations of electron donors. Without an adequate supply of electron donors, bacteria cannot reduce concentrations of either oxygen or NO3-.

Artificial drainage within agricultural areas may allow contaminants to bypass, or increase conveyance through, reduced areas in which denitrification is likely to occur. This contributes to elevated NO3- yields as it is discharged directly into surface water. Depending on site geology, however, bacterial biofilms capable of NO3- reduction may cultivate within the tile drainage. This study aims to quantify and compare the potential for denitrification, as well as denitrification rates, within tile drainage and aquifer sediments at the Best Management Practices (BMP) site, within the Oakes Irrigation Test Area (OITA) near Oakes, North Dakota.

For the aquifer sediment study, sediment samples (n=43) were collected from 10 locations at the BMP site. All samples were analyzed for the following potential electron donors: organic carbon, ferrous iron, manganese, and inorganic sulfide. A subset of samples was analyzed for organic sulfur, but all were below detection limits (<0.01%). Samples were also analyzed for texture and color. For the subsurface

drainage study, a nutrient tracer test was conducted within a drain tile transect at the BMP site. Sediment samples (n=6) from the gravel pack surrounding the tile drains were analyzed in the same manner as the aquifer sediments.

The major finding was that the biofilm accumulation within the gravel pack surrounding the tile drains contains adequate electron donor concentrations. However, when compared with denitrification rates occurring in the aquifer sediments, the tracer test in the drain tile produced no observable denitrification. Secondary findings indicated that electron donors are correlated with one another and electron donor concentrations are inversely correlated with grain size in the aquifer sediments.

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