Monitoring surface and sub-surface nitrogen contamination: Utilizing kriging to plan monitoring well installations

Background/Question/Methods

Environmental monitoring is often constrained by labor and expense.  Management of surface water and groundwater resources would benefit from more cost-effective, user-friendly tools to evaluate areas of concern.  Use of stable nitrogen isotopic analyses (d¹⁵N) of tree leaves to monitor nitrate contamination in groundwater and surface water could serve as an inexpensive alternative to costly methods.  The ease of field collection and inexpensive cost of d¹⁵N analyses make this approach a promising, cost-effective tool to monitor surface and subsurface areas for nutrient contamination.

North Carolina State University recently acquired a 127 acre tract of land adjacent to a defunct hog farm.  This land is currently being utilized as an educational plot by the North Carolina State University Forestry and Environmental Resources Department.  The hog farm operated with waste lagoons from 1977-2002 and therefore nitrate contamination in groundwater is a concern. Although the hog facilities are no longer in use for hog production, residual effects of its operation may still be seen in current foliar, soil, and groundwater samples.  Organic nitrogen and ammonia can be converted to nitrates in natural water sources.  Nitrogen from hog waste is enriched with δ¹⁵N relative to other sources such as tree leaves (see below).  Trees that take up groundwater contaminated with hog waste nitrogen should reflect this enriched nitrogen source.

Leaf, soil, and groundwater samples were taken by an environmental forensics class during the fall 2007 semester.  Stable nitrogen isotopic analyses were performed on the dendrology samples. All isotopic data were recorded as δ¹⁵N (‰) values.  Linear regression was used to asses the relationship between δ¹⁵N levels and proximity to the lagoons. 

Results/Conclusions

Results from this preliminary analysis indicate a possible correlation between the hog lagoons and the δ¹⁵N levels in trees proximate to the lagoons.  Kriging was utilized to plan future monitoring well installations.  The wells will be installed and maintained by students at North Carolina State University as a component of the environmental forensics program.  Data collected will be utilized to assess the relationship between the d¹⁵N in tree leaves and the d¹⁵N levels in groundwater.