Date of Award

2008

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Geology

First Advisor

S.F. Korom

Abstract

Research involving a network of in situ mesocosms (ISM) in North Dakota, Minnesota, and Iowa has revealed significant variation in the denitrification capacity of aquifers in different regions of these states. One aquifer in particular, the Elk Valley Aquifer (EVA) of east-central North Dakota, has the highest concentrations of electron (e) donors (organic carbon -0.4%, pyrite as S -0.4%, and ferrous iron -0.3%) within its sediments and the highest rates of denitrification. The sediments at all of the ISM sites were deposited as outwash during the last Wisconsinan glaciation (-12ka), yet their e· donor concentrations vary. I suggest that the e· donor concentrations in the EV A are high because the local bedrock units that would have contributed sediments to it as glacial outwash also have high concentrations of e- donors. The purpose of this study is to evaluate the amount of e- donors available in the local Late Cretaceous bedrock units.

Bedrock samples (n = 38) from 20 sites in eastern North Dakota were obtained by drilling during the summer of 2006 in conjunction with the North Dakota State Water Commission. Samples were frozen before analysis for mineralogical content by X-ray diffraction, organic carbon, pyrite as inorganic sulfide, organic sulfide, and ferrous iron concentrations. It was hypothesized that the Pembina member of the Pierre Shale would have the highest donor concentrations, but the other formations tested, the Niobrara, Carlile, and Greenhorn, contain higher concentrations of e- donors.

Organic carbon concentrations in the Pierre (0.3% to 0.7%) are relatively low while concentrations in the Niobrara (-2.9%) and Greenhorn (-3.7%) are significantly higher. Pyrite as inorganic as concentrations in the Pierre (0.01 % to 0.5%), Niobrara (-0.2 to 0.9%), and Greenhorn (-1.1 %) are similarly distributed. Ferrous iron was generally present in concentrations of 1.0% to 2.0% over the entire study area with some high concentrations measured in Greenhorn (2.2%) and Niobrara (3.5%) samples. Organic sulfide was generally insignificant, but was present in measureable amounts (0.1 % to 0.8%) in samples that were also rich in organic carbon. The Pierre showed the least variation of the formations tested, while the Niobrara showed the greatest. This is likely the expression of two distinct members, a calcareous shale and a chalky shale.

Regional bedrock maps of shale formations and ice movements of the late Wisconsinan glaciation are used to show that there are likely other aquifers in North Dakota and indeed other regions in the Upper Midwest that contain aquifers rich in e- donors just as the EV A is. These potentially reactive aquifers play an important role in water quality as the redox conditions within these aquifers are likely very different from that of others. It is suggested that future research focus on identifying other aquifers composed, at least in part, of glaciated shale.

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