Using Tracer Tests to Assess Natural Attenuation of Contaminants along a Channelized Coastal Plain Stream

Abstract

Tracer tests have been widely used in studies of solute transport, gas exchange, and nutrient cycling in streams. However, the use of tracer tests to assess natural attenuation of ground-water-derived contaminants in streams, particularly from point sources, has been limited. We used tracer tests in conjunction with stream gauging and contaminant analyses to study the fate of trichloroethene (TCE) and technetium-99 (99Tc), which seep from industrial contaminant plumes into a channelized, first-order stream in the Coastal Plain of western Kentucky. Six tests were conducted over a 20-month period along a 300-m reach downstream of contaminated springs. Bromide, rhodamine WT, and nitrate were introduced as slug tracers to assess dilution, sorption, and reduction, respectively. Propane was added as a continuous, volatile tracer. Tracer transport was modeled as one-dimensional, with transient storage and first-order mass loss. Results indicate that (1) TCE is attenuated by volatilization; (2) TCE sorption, TCE reduction, and 99Tc reduction are negligible or absent; and (3) dilution is negligible along the study reach.