ABSTRACT
The location and magnitude of ground-water seepage can be determined by measuring the activity of 222Rn gas in streams. Radon in ground water may be 2 to 4 orders of magnitude greater activity than in surface water. Thus, grouna-water seepage to a stream usually increases 222Rn in the streamflow. Downstream of ground-water seepage, 222Rn decreases in tne stream as radon escapes to the atmosphere, particularly in turbulent reaches of the stream. The relation between ground-water and surface-water flows can be determined by mass balance assuming no other 222Rn sources and no significant gas loss from the mix of surface water and ground water at tne sampling point.
Measurements of 222Rn in water from a 0.75-mile reach of a small bedrock-channel stream in Middle Tennessee ranged from 32 to 196 disintegrations per minute per liter. A sample of ground water from an adjacent spring contained 222Rn activity of 489 disintegrations per minute per liter. Based on 222Rn activities down the sampled reach of the stream, 36 percent of flow leaving the reach was ground-water seepage at a point 0.5 mile downstream from the upstream sampling boundary. Measurements of temperature in the water and bed of the stream verified the point location of ground-water seepage.
