Naturally Occurring Radon 222 as a Tracer for Streamflow Generation: Steady State Methodology and Field Example


  • David P. Genereux,

  • Harold F. Hemond


This paper presents a quantitative framework for the use of naturally occurring 222Rn as a hydrologic flow path tracer under conditions of steady streamflow. The methodology consists of two distinct parts, the first of which is the determination of inline image, the average 222Rn content of the water feeding a given stream reach. inline image is determined by measuring the concentrations of 222Rn and two injected tracers (one conservative, the other volatile) in the stream water and solving a mass balance equation for 222Rn around the reach of interest. The second part of the methodology involves using inline image values to determine the sources of stream inflow (and, implicitly, the flow paths important in streamflow generation). One means of accomplishing this, simple “geographic source” separations, is presented here. Both parts of the methodology were illustrated with a field experiment at the Bickford watershed in central Massachusetts. The injected tracers used were NaCl (conservative) and propane (volatile). The value of inline image (700 disintegrations per minute (dpm)/L) was found to be closer to the 222Rn content of vadose zone water (500 dpm/L) than to that of saturated zone water (2000 dpm/L), suggesting that lateral unsaturated flow was important in supplying base flow at the study site.