DEVELOPMENT OF 22NA AS A TRACER OF SOLUTE TRANSPORT IN EASTERN COASTAL PLAIN STREAMS

The ability to determine the residence time of surface and subsurface freshwater is vital for understanding how humans might impact 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 (t_1/2 = 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 tracer of solute transport in a coastal plain watershed by measuring concentrations in monthly precipitation and streamwater export from a small watershed on the coastal plain in Virginia, U.S.A..

We use large polypropylene bins to collect precipitation samples in Williamsburg, Virginia (37.26 degrees North latitude). Precipitation samples were filtered, and the ²²Na was extracted with cation exchange resin. The resin was counted directly on ultra-low background Intrinsic Gamma Detectors. ²²Na concentrations were measured in rainfall during February, March and April combined, May, and June of 2012. Average volume-weighted ²²Na concentrations were 0.180 ± 0.041, 0.150 ± 0.025, 0.177 ± 0.031, and 0.200 ± 0.040 mBq/L respectively (with +/-95% confidence intervals). These findings indicate that the average monthly dissolved ²²Na in precipitation is relatively consistent. By using mean ²²Na concentrations in precipitation and assuming steady-state behavior of sodium, we can assess stream water age in Pagonia stream, a forested coastal plain river that drains non-calcareous sandy sediments. Sodium-22 concentrations in this small watershed (16 hectares) measured in June 2012 were less than the detection limit (0.072 mBq/L), suggesting a minimum age of 3.38 years. However, in order to determine the water age of the stream water, future research needs to be done on the variables that affect ²²Na concentrations in stream water, most prominently evapotranspiration and the travel time of a water molecule through the watershed. To assess the reliability of our ²²Na stream water dating model, we will be comparing our calculated ages with a traditional flow model that utilizes a digital elevation map, flow routing, and hydraulic conductivity measurements.