Use of radon-222 and calcium as tracers in a three-end-member mixing model for streamflow generation on the West Fork of Walker Branch Watershed

Abstract

Measurements of naturally occurring radon-222 (222Rn) and calcium (Ca) in surface and subsurface waters on the West Fork of Walker Branch suggest that a simple three-end-member mixing model provides a realistic and useful framework for streamflow generation over a wide range of flow conditions. The three end members are vadose zone water, soil groundwater, and bedrock groundwater. Bedrock groundwater was distinguished from the soil end members on the basis of its high Ca content; 222Rn concentration was the basis for the distinction between vadose zone water (low 222Rn) and soil groundwater (high 222Rn). The behavior of the end members with changing flow was consistent with a wide variety of environmental observations, including temperature and flow variations at springs, water table responses, the general lack of saturated zones on hillslopes and even near the stream in some places, and the importance of water movement through bedrock. Variability in the chemistry of the end members precluded using other solutes (Na, K, and SO4) to test the mixing fractions derived from 222Rn and Ca data; during those times of year when the soil temperature is most different from that of the underlying bedrock (late summer and late winter), temperature may be a useful tracer for distinguishing between water from the soil end members and that from bedrock. The mixing model provides a simple framework for analyzing the essential features of streamflow generation in this highly heterogeneous terrain.