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Paul
A. Skubinna Paul is from Stillwater MN. He completed a B.S. in Geological Engineering at UND, with emphasis in hydrogeology, in May 1998, and is writing his M.S. thesis in Geology. He would like to work with a state agency or the EPA where he could be involved in compliance enforcement, site investigations and remediation. His long-range goal is to establish an environmental engineering and consulting firm. |
Modeling
the Hydrogeochemistry of Denitrification in In-Situ Microcosms in the Elk Valley
Aquifer
Fellow:
Paul A. Skubinna, Department of Geology and Geological
Engineering, UND
Advisor:
Scott F. Korom, Assistant Professor of Geology and Geological Engineering, UND
Matching
Support: North Dakota
Department of Health, Bismarck; North Dakota State Water Commission, Bismarck
Degree Progress: M.S. completion anticipated in August 2001.
Nitrate
is one of the most common groundwater contaminants observed in aquifers. In
areas where agricultural activity is prominent, nitrate from fertilization may
infiltrate and contaminate aquifers like the Elk Valley Aquifer (EVA) in Eastern
North Dakota. The objective of this study is to gain a better understanding of
the geochemical processes associated with the denitrification observed in the
EVA. The research is focused on reaction paths, rates, products, denitrification
capacities, and prediction of potential undesirable secondary products
associated with the denitrification.
The
methodology of this study has been to replicate the experiment done by Schlag
and Korom using in-situ mesocosms (ISMs) located near Larimore, ND. The approach
incorporates fieldwork and analytical lab work, with the addendum of computer
modeling. The fieldwork consisted of amending groundwater contained within the
ISMs with potassium nitrate and potassium bromide on October 8, 1998, followed
by monthly sampling of the amended water using the protocol outlined by Standard
Methods. Sample analyses for
general anions, cations and other basic water parameters were performed by North
Dakota Department of Health. Bromide analysis, along with duplicate analysis of
dissolved carbon, nitrate-nitrogen, and sulfate, were performed at the UND Water
Quality Lab.
Since
October 8, 1998 the ISMs have been sampled fifteen times on an approximate
monthly schedule. Sample analysis indicated a decrease of 5-10 mg/L NO3-–
N per month. As of February 19, 2000 nitrate-nitrogen concentration in the R-ISM
had been totally degraded to an effective concentration of 0 mg/L. The initial
nitrate value was approximately 105mg/L. Bromide concentration in the R-ISM
remained approximately 45 mg/L at that time. In the ISM amended with KBr only
(referred to as the C-ISM), the Br- concentration, which began the
study at approximately 720mg/L, was approximately 600mg/L. Bromide observations
in both ISMs indicate its effectiveness as a relatively conservative tracer.
Sulfate
concentrations in the R-ISM increased to approximately seven times the initial
concentration at the end of denitrification while the sulfate concentration in
the C-ISM remained roughly constant. Comparatively, dissolved inorganic carbon (DIC)
concentrations displayed little fluctuation in either the C-ISM or the R-ISM.
This suggests that denitrification by sulfide minerals is the prominent process
in this groundwater system. However, dilution accounted for, sulfate production
in the R-ISM does not stoichiometrically account for all missing nitrate. Though
sulfide minerals may be thermodynamically favorable electron donor, it appears
organic carbon (OC) is also participating in the denitrification process. Possibly,
denitrification sourced DIC is being offset by a secondary DIC sink reaction
resulting in no net observable increase.
Comparing
the data set of the second tracer test to that of the first tracer test
performed by Schlag, the following conclusions are apparent: (1) Denitrification
rates in the second tracer test are about half of those observed in the first
tracer test. (2) Percent of
denitrification by sulfide minerals, accounted for by sulfate production,
decreased in the second tracer test to 48 – 73% vs. 61-89% in the first tracer
test. (3) Nitrogen isotope
fractionation analyses suggests a more electron donor deficient environment in
the second tracer test than first tracer test.
The latest modeling methodology has substantiated pyrite
as a major electron donor to denitrification, suggested the formation of iron
oxy-hydroxides as a product of denitrification by pyrite, and provided a more
substantiated line of evidence that OC is simultaneously active as a second
major electron donor to denitrification.
Presentations
Korom, S. F., A. E. Kammer, A. J.
Schlag, and P. A. Skubinna, In-situ study of denitrification in the Elk Valley
Aquifer, North Dakota, EOS, Transactions, American Geophysical Union/Supplement,
82(20), S168, 2001 (Abstract)
Skubinna, P. A., and S. F. Korom,
In-situ study of denitrification in the Elk Valley Aquifer:
Second tracer test, EOS, Transactions, American Geophysical
Union/Supplement, 82(20), S168, 2001 (Abstract).
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Paul A. Skubinna performing total organic carbon analyses |
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Advisor:
Dr.
Scott F. Korom Assistant
Professor of Geology and Geological Engineering University of North Dakota |