Modeling the hydrogeochemistry of denitrification in the Elk Valley Aquifer

Nitrate (NO₃¯) in groundwater has become an important tissue in the last few decades. It is known to cause methemoglobinemia, commonly known as “blue baby syndrome,” and is suspected of being a carcinogen. The most common sources for dissolved NO₃¯ in shallow groundwater include excessive application of nitrogenous fertilizers, misuse of septic systems, improper disposal of domestic wastewater or sludge, and livestock waste. Denitrification is the most effective sink for NO₃¯. The Elk Valley aquifer (EVA) of eastern North Dakota is known to support denitrification. The objective of this study was to gain a better understanding of the geochemical processes associated with the denitrification observed in the EVA. The hypothesis tested was that geochemical conditions are proper for organic carbon to be a primary electron donor responsible for denitrification in the Elk Valley Aquifer at the Larimore Field Site (LFS). This thesis presents data from a second tracer test performed at the LFS. The methodology of this study was to replicate the first tracer test using the same in-situ mesocosms (ISMs) located near Larimore, ND. The approach incorporated fieldwork, analytical lab work and geochemical 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 and analysis of the amended water. 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 University of North Dakota, Department Geology and Geological Engineering, Water Quality Lab. The geochemical modeling code PHREEQC-2 was used to simulate the observed groundwater quality to gain insights regarding a possible a second primary electron donor in addition to pyrite documented during the first tracer test. Additionally, the modeling work was completed in to gain insights into the secondary geochemical reactions resulting from denitrification and reactions caused by the study methodology. The model simulations employ mineral equilibria, cation exchange, dilution and oxidation-reduction calculations to describe the major water quality parameters observed during the study. Ultimately, the model output supports my hypothesis that geochemical conditions are proper for organic carbon to be participating as a primary electron donor to the observed denitrification