Storm-Water Services Division Using Stable Isotopes of Nitrogen and Oxygen to Identify Sources of Nitrate in Three Creeks, Durham County, North Carolina, 2011– 12.

摘要

A preliminary assessment of nitrate sources was conducted in three creeks that feed nutrient impaired Falls and Jordan Lakes in the vicinity of Durham County, North Carolina, from July 2011 to June 2012. Cabin Branch, Ellerbe Creek, and Third Fork Creek were sampled monthly to determine if sources of nitrate in surface water could be identified on the basis of their stable isotopic compositions. Land use differs in the drainage basins of the investigated creeks—the predominant land use in Cabin Branch Basin is forest, and the Ellerbe and Third Fork Creek Basins are predominantly developed urban areas. Total nutrient concentrations were below 1 milligram per liter (mg/L). All measured nitrate plus nitrite concentrations were below the North Carolina standard of 10 mg/L as nitrogen with the highest concentration of 0.363 mg/L measured in Third Fork Creek. Concentrations of ammonia were generally less than 0.1 mg/L as nitrogen in all creek samples. More than 50 percent of the total nitrogen measured in the creeks was in the form of organic nitrogen. Total phosphorus and or thophosphate concentrations in all samples were generally less than 0.2 mg/L as phosphorus. The isotopic composition of surface water (? 2HH2O and ?18OH2O ) is similar to that of modern-day precipitation.