Using Nitrate δ15N and δ18O Values to Identify Potential Sources of Nutrient Inputs to a Highly Impacted Inlet in South Puget Sound, Washington

Abstract:

Quartermaster Harbor (QMH) is a restricted inlet on Vashon Island in Puget Sound. Environmental issues in QMH include poor water quality, limited flux, and annual toxic algal blooms related to high nutrient loading. Natural and anthropogenic nitrogen compounds may enter QMH through a number of sources. Previous work found seasonally elevated groundwater and surface marine water concentration of nitrate ([NO₃^-]). We aimed to identify potential NO₃^- source(s) using N and O stable isotope values of NO₃^- in both freshwater and marine water samples. In October, 2014 we collected freshwater samples from three tributary streams to QMH, and a residential well adjacent to QMH. We also collected paired surface and deep ( ~1m above sediment) marine samples at 5 sites along an 8000m transect from the mouth of the inlet to the inner harbor. Each sample was analyzed for [NO₃^-], and δ¹⁵N_NO3 and δ¹⁸O_NO3 values. Possible NO₃^- sources that may have discreet δ¹⁵N and δ¹⁸O values include septic seepage, marine NO₃^-, and natural (terrestrial) NO₃^-.

All deep marine sample δ¹⁵N_NO3 values were similarly low (~2 ‰), but the surface values increased by ~8 ‰ from the inlet mouth to inner harbor. Marine sample δ¹⁸O_NO3 values were invariant (~1 ‰ range), except for one high surface value. The deep [NO₃^-] decreased slightly towards the inner harbor, and were consistently higher than paired surface concentration, which decreased dramatically. The uniformity of [NO₃^-] and isotopic values in deep marine samples may be explained by groundwater recharge. The isotopic values of NO₃^- in the creeks were not characteristic of a specific anthropogenic source, but one creek had a very high [NO₃^-]. The only isotopic evidence for an anthropogenic NO₃^- source (septic) was found in the two surface marine samples from the inner harbor, and the well water sample. Seasonal sampling and additional sites will yield a more complete understanding of the relative contributions of NO₃^- sources to QMH.