NITROGEN ISOTOPE ANALYSIS TO EVALUATE LAND USE SIGNATURE ON STREAM WATER QUALITY

Located in Ulster County, New York, the Lower Esopus Creek begins at the outflow of the Ashokan Reservoir and encompasses different land uses along its course as it flows into the Hudson River. The water is impacted by nitrate and other nutrient runoff from the surrounding agricultural, residential, and urban areas. Various sources of nitrates to have differential fractionation of the two isotopes ¹⁵N and ¹⁴N. The δ¹⁵N values of surface water nitrate could provide a clue as to whether the nitrate originated from organic nitrogen, animal waste, or from artificial fertilizer. Twenty water samples collected along the Lower Esopus Basin (LEB) and within the Hudson Valley Farm Hub (HVFH) area were analyzed for δ¹⁵N. Sample sites were grouped based on similar location relationship or similar δ¹⁵N values. Sample sites 12 and 18 showed an anthropogenic wastewater signal of δ¹⁵N_. Sample sites 8 and 19 were farthest upstream in the LEB; site 19 had a signal comparable to ambient air. Site 8, downstream of 19, had a δ¹⁵N increase with a nitrate concentration increase signifying assimilation and denitrification of N¹⁴ preferentially. Sample sites 1, 2, 3, 4, 6, 7, 13, 14, 15, 16, and 17 were along the HVFH and showed a similar and non-discernable mixing of nitrate sources. Sample sites 5, 9, 10 and 11 were from Lower Esopus tributaries that had similar sources of nitrate mixing. Sample 20 was from a lysimeter taken directly from HVFH soil having a mixed δ¹⁵N signal of organic fertilizer and nitrate fixation from microorganisms. These findings showed that HVFH may be contributing additional nitrates from their land usage, but it is not discernible as the primary δ¹⁵N signature. Mixing of different nitrogen isotopes from the nitrogen cycle complicates determining the primary source of nitrates particularly along the HVFH. Additional nitrate isotope analysis would help provide more conclusive results on the effect that the HVFH, Ashokan Reservoir water release, seasonal changes, and precipitation events have on the LEB.