Model of Nitrogen Source Allocations and Transformations Using Stable Nitrogen Isotopes

Stream restoration for water quality and nutrient management relies on effective assessment methods that measure the before-and-after improvement of water quality, or lack thereof, from implementation. We present a water quality and nutrient modeling tool for potential use in such applications. The model is applicable to low-gradient streams and watersheds with agricultural and urban land use such as those found in the Ohio River Basin, which tend to have relatively high DIN inputs to the stream channel and a need for water quality restoration. The model is unique in that it makes use of nitrogen stable isotopes for constraining the nutrient budget; and we model rates of nutrient removal by the streambed associated with restorative measures. A mass balance nutrient model that simulates the nitrogen cycle, e.g. uptake rates, nitrification, denitrification, and ammonification, is coupled with a nitrogen isotope mass balance model, and associated Rayleigh fractionations, for transported sediments and streamwater DIN. We formulate and then apply the model for the South Elkhorn Creek, a stream in Kentucky, USA, affected by both urban and agricultural land uses, which has undergone restoration in recent years. The stable nitrogen isotopic signature of sediment (δ15NSed) was measured to verify the model. Preliminary results of the study highlight that the fate of the benthic sediments dominate the isotopic signature with δ15NSed being highly dependent on the streamwater DIN isotopic signature (δ15NDIN) and fractionation associated with uptake and denitrification processes. Preliminary sensitivity analysis shows that the isotopic signature is extremely sensitive to uptake and denitrification rates highlighting its effectiveness to assess if nitrogen is being fully removed from the stream system. Model estimates show that the system efficiently removed ammonium from the streamwater during all seasons. The model advancement of coupling the isotope fate model with the unmixing model provides a potentially powerful tool to constrain the nitrogen budget for watersheds with comparable characteristics. However, testing is needed in restoration projects, which are part of our ongoing research.