Using N15 and O18 Isotopes To Determine Groundwater Contamination Sources In Colorado.

Groundwater wells along the South Platte River alluvial aquifer in Colorado have medium nitrate-nitrogen levels often exceeding the EPA drinking water quality standard of 10 milligrams per liter (mg L⁻¹).  This area offers an interesting problem in identifying the source of this groundwater contamination because it is highly diverse.  The area has large areas of irrigated cropland, large animal feeding operations and dairies, as well as an urban influence from the Denver metropolitan area.  While these wells have been carefully monitored and documented since 1993, analysis has not been done to determine the sources of these high nitrate levels. 

Groundwater samples were taken 3 times from 22 wells along the South Platte in Weld County and analyzed for δ¹⁵N and δ¹⁸Oisotopic composition in order to attempt to better understand the sources of the high nitrate levels in the groundwater. Data from the first two sampling sets will only be available at this time.  δ¹⁵N and δ¹⁸O ratios were determined using the “bacterial denitrification method” which uses cultured bacteria that convert NO₃⁻ to N₂O gas which is then analyzed by an Isotope Ratio Mass Spectrometer (IRMS). δ¹⁵N can be used to trace natural and anthropogenic sources, identify transformation processes, and asses the N budget in water samples.  δ¹⁸O is helpful in distinguishing atmospheric nitrate deposition from microbially-produced soil nitrate.  It can also help to distinguish between synthetic nitrate fertilizers from other sources. 

Determining the source of groundwater contamination will be useful in enacting policies and education in order to mitigate future contamination and begin the process of lowering levels below the EPA safe drinking standard of 10 mg L⁻¹.