Using 22Na to Measure Effects of Impervious Surfaces on the Age of Groundwater


My name is Kira Holmes, and as a junior Geology major I’ll be working with Greg Hancock and Jim Kaste over the summer on research for my senior thesis.

Development of urban areas has been rapidly increasing over recent decades. Cities are using every square foot of space, while suburban areas are building strip malls and town centers with increasingly large parking lots. With each of these additions, the natural environment is further altered. When the Earth’s surface is covered by impervious tar or concrete, precipitation that would normally infiltrate the soil and enter the groundwater system must be diverted elsewhere, through runoff or stormwater drainage systems. During dry periods, when there is little runoff entering a stream, streams are mostly replenished via groundwater. The lack of infiltration, however, will cause a lowering of the water table, and thus a decrease in base flow.  This base flow is imperative in preventing a stream from drying up during droughts. The decrease in infiltration will also have an effect on the age of groundwater as it leaves the groundwater system. We hypothesize that, as impervious surface cover increases, the age of groundwater also increases. Over the summer, I will be collecting data from three watersheds within the Williamsburg area. We will use cation exchange resin to capture 22Na, a radionuclide, in streams at base flow. Comparing the concentration of Sodium-22 in the water to the atmospheric concentration will allow us to date the groundwater and confirm or deny our hypothesis.