Date Thesis Awarded

4-2016

Access Type

Honors Thesis -- Access Restricted On-Campus Only

Degree Name

Bachelors of Science (BS)

Department

Geology

Advisor

Jim Kaste

Committee Members

Greg Hancock

Randy Chambers

Abstract

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.

Recommended Citation

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.
https://scholarworks.wm.edu/honorstheses/880