Date Thesis Awarded
Honors Thesis -- Access Restricted On-Campus Only
Bachelors of Science (BS)
Understanding residence time and flow rate of water is essential to monitoring and protection of water resources. Young fresh waters in particular are a vital resource that humans depend on today. Previous research has explored the viability of using cosmogenic sodium-22 (22Na) to date young fresh waters. 22Na is naturally produced in the atmosphere, scavenged by storms, and precipitated into water systems on the earth. 22Na has a relatively short half-life (2.605 years), a currently stable atmospheric concentration, and conservative behavior in water, all of which are ideal for dating young water. An age for water can be derived by testing 22Na in groundwater, stream water, and precipitation samples.
This study develops and tests three models for 22Na-derived water age: the decay model, the ratio model, and the flux model. These models were tested in three different watersheds on the east coast of the United States: Hubbard Brook (Woodstock, New Hampshire), Jones Run (Shenandoah National Park, Virginia) and Pogonia Stream (Williamsburg, Virginia). Stream water collection methodology was significantly improved via an in-situ cation resin bag placed directly in the stream. The resin bag consistently collected samples that represented large volumes of stream water. Labor-intensive physical collection of stream water samples was thus unnecessary. This stream water resin was eluted with acid. Groundwater was analyzed for sodium concentrations. Precipitation and stream water was analyzed for sodium and 22Na concentrations and fluxes.
Sodium concentrations in precipitation ranged from 0.02 mg/L to 0.14 mg/L. Stream water sodium concentrations ranged from 0.795 mg/L to 2.54 mg/L. When analyzed for 22Na, Hubbard Brook had a concentration of 0.162 mBq/L (± 0.01 mBq/L). Jones Run was found to have a 22Na concentration of 0.063 mBq /L (± 0.007 mBq/L). Pogonia Stream had a 22Na concentration of 0.04 mBq/L (± 0.01 mBq/L).
Stream water age, defined as the amount of time since the stream water was precipitation, was derived using the three 22Na age models. The decay model provided problematic ages due to evapotranspiration artificially increasing concentrations of 22Na. The ratio model age provided error due to sodium present in underlying stream geology, as well as sodium in throughfall rain. As the flux model is only affected by changes in 22Na flux, it can be concluded the flux model provides the most accurate water age as compared to independently derived ages.
Goydan, Claire, "Development of cosmogenic 22Na as a tool to measure young water age in multiple watersheds" (2016). Undergraduate Honors Theses. William & Mary. Paper 880.
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