DETERMINING NUTRIENT SOURCES AFFECTING THE ALLEGHENY RESERVOIR USING STABLE ISOTOPE BIOGEOCHEMISTRY

Algae blooms cause detrimental economic and health effects across the United States. Although studied extensively in large waterbodies, algae blooms are poorly understood in freshwater systems, especially reservoirs. Gathering insight into the nutrient dynamics of reservoirs would allow for the implementation of well-informed monitoring and reduction strategies by a monitoring government body. The Allegheny Reservoir (AR), located on the border of New York and Pennsylvania, has been the subject to algae blooms as early as 1972, but have now been observed yearly starting in 2012 at approximately the same time each year. Although poorly understood, the AR represents a unique case from the perspective of reservoir nutrient dynamics as there are no clear nutrient indicators when compared to other dam operated reservoir systems. The AR watershed is primarily forested cover (80.19%) with little agriculture (8.72%) and developed land (<0.01%). In this study, we use nitrate isotope values (δ¹⁵N, δ¹⁸O, and Δ¹⁷O) and nitrate concentration data to explore variations in isotopic composition that may be helpful in elucidating nitrate sources to the AR. Samples were collected along five sites once per month from March – October in 2021 and biweekly in two locations from April – October for 2022 and 2023. This project is a unique collaboration between the Seneca Nation of Indians (SNI), the United State Army Corps of Engineers (USACE), and the Pittsburgh Water Collaboratory at the University of Pittsburgh. Analyses of samples taken from March – September 2021 by the US Army Corps of Engineers Pittsburgh District, has shown average δ¹⁵N isotope values between +4.1‰ to +8.7‰ with δ¹⁸O values ranging from -1.4‰ to 4.17‰ which are indicative of soil N sources (δ¹⁵N: +2.5‰ to +7.5‰, δ¹⁸O: -5‰ to +15‰). Nitrate concentrations ranged from 0.1 mg/l – 0.6 mg/l with higher concentrations observed in early March and lower concentrations observed in September. While preliminary isotopic results suggest soil N as the predominant source of nutrient pollution for 2021, further isotope and nutrient samples are required to determine if these findings are continuous in 2022 and 2023. Results from this study will be used by SNI and USACE to inform reservoir management decision making to reduce algae bloom occurrence by way of limiting soil erosion.