APPLICATION OF STABLE ISOTOPE TECHNIQUES TO IDENTIFY SOURCES OF NITROGEN IN A SHALLOW HYDROLOGIC SYSTEM

Twenty (20) private wells and 10 stream locations were sampled to assess the source and fate of dissolved nitrate in the Cedar River watershed of Iowa, USA. The average levels of nitrate in ground water decreased from 39.5 ppm in May, to 38 ppm in July, to 30 ppm in September. Although several surface water samples exceeded MCL in May, most values dropped to below 20 ppm by July and September. The decreasing N levels were attributed to the gradual uptake of nitrate by growing crops as well as the cyanobacterial growth in the aquatic systems.

The d¹⁵N values of dissolved nitrate in ground water ranged from +0.45 ‰ to + 5.35 ‰, whereas those in surface water ranged from +1.48 ‰ to + 5.16 ‰. The results suggested that commercial fertilizers and soil organic nitrogen were probably mixed up in their transport pathways. A fertilizer-only source would provide much lower delta values whereas soil nitrogen would provide higher than observed delta values. Denitrification was considered unlikely because of the low d¹⁵N values, high nitrate concentrations, and moderately high DO content in ground water. Animal wastes were not found as a possible source of nitrate in the water. This is supported by the low chloride concentrations and the lower than 10 ‰ delta values in the water samples. The study demonstrates that nitrogen isotope data in coordination with the dissolved nitrate levels and land use can be effectively used in nitrogen source identification and its transformation studies.