Date of Award


Document Type


Degree Name

Master of Science (MS)



First Advisor

A.E. Kehew


Upon completion of an oil-and-gas well in North Dakota the drilling fluid is buried in the reserve pit at the site. Reclamation of the drill site is expedited by digging a series of trenches which radiate out from the reserve pit, The majority of buried drilling fluid is ultimately contained within these 5-7 metre deep trenches. These fluids are commonly salt-based, i.e. they contain a con centration of 300,000±20,000 ppm NaCl. In addition, these drilling fluids also contain additives including toxic trace-metal compounds.

Four reclaimed oil~and-gas well sites were chosen for study in western North Dakota. The ages of these sites ranged from 2 to 23 years. These sites were chosen in an effort to encompass as many as possible of the geologic and geohydrologic variables that exist in this area. A total of 31 piezometers and 22 soil water samplers were installed in and around the drill sites and quarterly ground water samples were obtained from these instruments, The local groundwater flow conditions were also determined at these sites.

Results of both the water analyses and earth resistivity surveys indicate that leachate is being generated at all of the study sites, Water obtained from the unsaturated zone beneath the buried drilling fluid at all of the four study sites exceeds some of the Recommended and Maximum Permissible Concentration Limits for trace elements and major ions (As, Cl-, Pb, Se and N03-). These values are greatly reduced in the unsaturated zone as the depth from the buried drilling fluid increases. This reduction is assumed to be the result of attenuation of these ions by cation exchange on Na montmorillonitic clays.

Two of these study sites represent the typical geohydrologic setting for the majority of oil-and-gas well sites in this area. At these sites the saturated zone was not monitored. The reduction in ion concentration in the unsaturated zone suggests that there would be very little impact on the groundwater from this buried drilling fluid at these two sites.

The two other study sites were situated in geohydrologic settings that offered a great potential for leachate migration in the saturated zone. The chloride ion was chosen as an indicator of maximum leachate migration because of its high mobility and lack of attenuation other than by dispersion. The chloride concentrations returned to background levels within the saturated zone 60 to 90 metres downgradient of the buried drilling fluid at these two sites.

The consumption of shallow groundwater beneath one of the study sites constitutes a danger to human health. The Maximum Permissible Concentration Limits for Cd, Pb, and Se were exceeded in the shallow groundwater beneath the buried drilling fluid at this site. These limits were exceeded in an area approximately 60 by 110 m. The concentration level of N03- ranged from 1310 to 12.2 mg/1 (as N) throughout the groundwater at this study site. It is not known conclusively whether the buried drilling fluid at this site is the source of the N03- contamination.

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