Determining Nitrate Sources Using Dual Nitrate Isotopes in Grand Lake St. Marys Watershed, Ohio

Grand Lake St. Marys, one of the largest lakes in Ohio, regularly experiences harmful algae blooms that disrupt access to drinking water and recreation. The Grand Lake St. Marys watershed, designated as a distressed watershed since 2011, is one of the most nutrient impacted lakes in Ohio with pollutants potentially originating from anthropogenic sources including organic and inorganic fertilizers, human wastes, and urban runoff. The eutrophication of the lake is in part caused by an excess of nitrate, which can come from these sources. The aim of this study is to identify and quantify the relative contributions of these anthropogenic sources of nitrate in the Grand Lake St. Marys watershed to better inform nutrient management decisions. We collected field environmental data and 43 water samples from the lake and all streams in the six sub-basins of the watershed. Samples were analyzed for nitrate concentration and isotope ratios of nitrogen and oxygen in nitrates. Dual isotope tracing of nitrate (δ15N and δ18O) allows us to determine nitrate sources based on isotope end-member values from those sources. But as δ15N of human and animal waste overlaps, further analysis of boron isotopes (δ11B) is used to distinguish anthropogenic sources from natural sources. Our results indicate that the main source of nitrate in this watershed is manure, which contributed 77-100% of all nitrate sources in the watershed. Although denitrification could increase the δ15N and δ18O values of nitrate and overlap with manure, based on high level of measured dissolved oxygen (DO) in the field and slope of δ15N and δ18O values, we were able to rule out this process. This research provides an initial nitrate contaminant source tracing data and helps to better inform state and local water quality and nutrient management planning.https://ecommons.udayton.edu/stander_posters/3515/thumbnail.jp