Date of Award

1992

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Geology

First Advisor

P.J. Gerla

Abstract

Biological denitrification controls nitrate distribution in the Elk Valley aquifer, which underlies approximately 725 square kilometres of northeastern North Dakota and provides drinking water for farms and rural communities in the area. Increased irrigation has raised concerns over potential contamination from agricultural practices. The purpose of this study was to establish the role of biological denitrification in minimizing concentration and distribution of nitrate and to determine the effect irrigation practices have on groundwater flow and nitrate distribution in the saturated zone.

The aquifer is unconfined and consists of up to 20 metres of glacial outwash and deltaic shale-rich sand deposits. The water table ranges from two to five metres below the surface. Groundwater flows to the southeast at an average velocity ranging from 4. 2 x 10-7 to 1. 6 x 10-6 metres per second.

Nitrate (as N03-N) was detected only in wells completed at depths less than three metres below the water table and ranged from 5 to 21 mg/Lat the Oto 1.5 metre level and from 2 to 6 mg/Lat the 1.5 to 3 metre level. Dissolved oxygen was near saturation in the upper 1.5 metres of the saturated zone and less than 1 mg/Lin the deeper wells. Dissolved organic carbon ranged from 18.7 to 91.4 mg/L. Biological oxygen demand, which can be used to indicate biodegradable organic carbon, was not detected in the upper 1.5 metres of the saturated zone and ranged from 4 to 6 mg/L at greater depths. Ammonia was not detected in the upper three metres of the saturated zone and was present at concentrations below 1 mg/Lin the deeper wells. Denitrifying bacteria were found to be present throughout the aquifer.

Intermittent pumping of an irrigation well was found to affect groundwater flow and the vertical displacement of contaminants. Numerical modeling of particle movement indicated a sine-wave path with upward movement during periods of irrigation pumping and downward movement during periods of recovery in the upper part of the aquifer. In addition, overall upward movement of particles occurred at distances greater than 30 metres from the irrigation well in the upper part of the aquifer. Near the irrigation well, there was a strong downward movement of particles even during periods of recovery because of the vertical contrast in hydraulic heads that result from pumping.

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