Document Type
Thesis
Abstract
The ability to determine the residence time of young surface and subsurface freshwaters is vital for understanding how water moves through the hydrosphere. Most critically, reliable measurements of stream water age are needed to assess contaminant transport rates and storm hydrograph separation in watersheds. Cosmogenically produced sodium-22 (t1/2= 2.605 years) has great potential as a tracer of solute transport in watersheds because the element generally behaves conservatively. Here we develop the use of 22Na as a clock for stream water age for a coastal plains stream by measuring 22Na concentrations in precipitation and stream water export from a small watershed in Williamsburg, Virginia (37.26° North latitude). We use large polypropylene bins to collect precipitation samples on the campus of The College of William and Mary. The **Na was extracted from the samples with cation exchange resin, and the resin was counted directly on ultra-low background Intrinsic Gamma Detectors. From February 1, 2012 to January 31, 2013, the average volumeweighted 22Na concentration was 0.107±0.024 mBq/L, and the annual 22Na flux was 109±24.3 mBq/m*. By using the mean **Na concentration and the annual flux and assuming steady-state behavior of sodium, we calculated stream water age of the Pagonia stream, a forested coastal plain river that drains non-calcareous sandy sediments. Here, stream water age is defined as the amount of time since the Pagonia stream water was precipitation. The Na concentration in this small watershed (16 hectares) was measured in October 2012 at 0.056±0.020 mBq/L, suggesting a minimum water age of 2.4 years. When correcting for evapotranspiration processes using annual 22Na flux in and out of the watershed, we determine the Pagonia stream may actually be up to 4.7 years old. Unlike 22Nla concentration, "Na flux in and out of the watershed is only affected by radioactive decay. Therefore, we conclude that 22Na flux measures the truest Pagonia age and is the most useful means for future stream water age calculations.
Date Awarded
2013
Department
Geology
Advisor 1
James M. Kaste
Recommended Citation
Lauer, Nancy E., "Development of cosmogenic 22Na as a clock for stream water age in Virginia's Costal Plain" (2013). Geology Senior Theses. William & Mary. Paper 294.
https://scholarworks.wm.edu/geologyseniors/294
