Description of the Ciritcal Water Problem

Denitrification is a natural process exhibited in some aquifers by which bacteria reduce NO₃ - to N₂ through the oxidation of organic or inorganic electron donors. One very distinct trait of denitrification is the resulting increase of the heavier isotopes of nitrogen and oxygen in the undenitrified fraction of the nitrate. During denitrification bacteria prefer to attack the bonds of the lighter isotopes because they are easier to break. Thus, as denitrification proceeds, corresponding increases in the ratios of ¹⁵N to ¹⁴N and ¹⁸O to ¹⁶O in the remaining nitrate result. Other processes that decrease nitrate concentrations, such as dilution, do not change ¹⁵N/¹⁴N and ¹⁸O/¹⁶O ratios in the nitrate. Thus, noting increases in ¹⁵N/¹⁴N and ¹⁸O/¹⁶O groundwater nitrate samples verify that denitrification is occurring. The nitrogen ratio is based on the following:

G¹⁵N= (^Rsample-^Rstandard) / ^Rstandard

Where G¹⁵N is the variation in the isotope expressed in parts per thousand (typically denoted “permil”) at a specific time compared to the original amount, ^Rsample is the isotopic ratio ( ¹⁵N/¹⁴N) some point into the denitrification process, and ^Rstandard is the isotopic ratio of atmospheric nitrogen (N₂). A similar relationship exists for G¹⁸O.

Under some idealized conditions, such as in situ mesocosms (ISMs), denitrification results in a linear relationship with G¹⁵N, such as shown on Figure 1 (Schlag, 1999; Korom et al., in review). This is because a known amount of nitrate is put into the ISMs at the beginning of a tracer test and observed in isolation from other sources of nitrate for the duration of the test. Nitrate entering an aquifer, however, may vary in its source, concentration, and time of introduction into the aquifer. With such variability, plots with G¹⁵N and concentration may not be helpful. Consider Figure 2 from a site in the Assiniboine Delta aquifer in Manitoba (Phipps and Betcher, 2003). It is difficult to see any trend between nitrate-N concentrations and G¹⁵N with sample depth. Under these conditions, also including G¹⁸O analyses provides considerable improvement. Consider Figure 3 using the same data for G15N as in Figure 2, plus some additional G¹⁸O data. The resulting enrichment trend was attributed to denitrification (Phipps and Betcher, 2003).

Methods, Procedures, and Facilities 

The North Dakota State Water Commission (NDSWC) provided us with well data on nitrate concentrations from the Karlsruhe aquifer. These include nitrate (NO₃ ^- ) concentrations from the fall of 2002 and the spring and summer of 2003 from twelve wells owned and operated by the NDSWC. Data from each well consists of concentrations at various depths ranging from five to forty feet below the ground surface. It is from these wells that we will be drawing multiple samples from various levels. Currently it is planned that sampling trips will be conducted this fall (2003), winter (2004), and spring (2004). This schedule is subject to change depending on weather conditions (time of spring thaw) and results from the first two sampling events.

Water from each sample will be tested for nitrate, chloride, and isotope concentrations. The University of North Dakota Environmental Analytical Research Laboratory (EARL) will be used to measure nitrate and chloride levels while isotope analysis will be conducted by the University of Waterloo Stable Isotope Laboratory. Our denitrification research team has used this lab for over six years and has been satisfied with the quality of service and results. Chloride (Cl^- ) will be used as a natural tracer to be compared with nitrate levels. This anion is relatively inert in groundwater and does not participate in denitrification reactions. Simple dilution by foreign groundwater should be the only means for chloride levels to fluctuate. Thus if nitrate levels decrease at an accelerated rate compared to chloride, it is likely that nitrate has been eliminated by some process other than dilution, such as denitrification.

Isotopic analysis of G¹⁵N and G¹⁸O will be conducted once the sampling portion of the project is complete. Evidence of denitrification will be directly dependent on the relationship between nitrate and isotope fractionation as shown in Figure 3. This coupled with chloride levels will be the basis for determining the process leading to reduction of nitrate in the Karlsruhe Aquifer.

Progress to Date

Our first sampling trip was conducted on October 21, 2003. A total of thirty-one samples were taken from the five different wells with multi-level sampling ports and transported back to the UND EARL for immediate analysis. Once the results were produced and approved by Dr. Korom, the remaining groundwater samples were treated with mercuric chloride to kill bacteria and prevent further denitrification. Samples were then refrigerated and will remain there until isotopic analysis will be conducted at a later date in the project. Up to two additional sampling trips will be conducted before the end of the project in the summer of 2004. Below is a table showing results from one of the wells in the sample set. It is apparent at the deepest level that the nitrate concentration has decreased much more than the chloride concentration. This is evidence that the groundwater at this site has been denitrified. Isotope data should help us confirm this hypothesis.

Anticipated Results and Deliverables

It is believed by those associated with this project, that the G¹⁸O and G¹⁵N data will verify that denitrification is occurring in the Karlsruhe aquifer. Also it is felt that nitrate concentrations will decrease over the winter months at an accelerated rate compared to chloride. Ultimately these factors will provide supporting evidence to the opinion that the lowering of NO₃^- concentrations within the Karlsruhe aquifer is the direct result of natural denitrification.

Literature Cited

     Korom, S.F., A.J. Schlag, W.M. Schuh, and A.K. Schlag. In situ mesocosms: Denitrification in the Elk Valley aquifer. In review with Ground Water Monitoring and Remediation.

     Phipps, G. C., and Betcher, R.N., 2003. The source and fate of groundwater nitrate beneath a field fertilized with hog manure: Assiniboine Delta aquifer, Manitoba, Canada. 4th Joint IAH-CNC/CGS Conference, Winnipeg.

     Schlag, A.J., 1999. In-situ measurement of denitrification in the Elk Valley aquifer, Masters Thesis, Department of Geology and Geological Engineering, University of North Dakota.