TECHNETIUM-99 AS AN IN-SITU TRACER OF GROUND-WATER/STREAM INTERACTIONS AND ORGANIC CO-CONTAMINANT FATE

If sources and timing of releases can be constrained, contaminants can be useful as in-situ tracers of ground-water flow and biogeochemical processes. Contaminants are commonly exotic and detectable at low concentrations by techniques such as mass spectrometry. Chemically conservative species can serve as baselines for assessing the attenuation of co-contaminants by reactions such as volatilization, sorption, degradation, or precipitation. We used the radionuclide technetium-99 (⁹⁹Tc) to delineate seepage to a stream and infer attenuation of trichloroethene (TCE) in the vicinity of a U.S. Department of Energy facility in western Kentucky. ⁹⁹Tc, which was released as a result of nuclear fuel reprocessing at the Paducah Gaseous Diffusion Plant, is long-lived (half life 215 ka) and has a detection limit of < 10 pCi/L. Under aerobic conditions, ⁹⁹Tc occurs as the relatively conservative pertechnetate anion, whereas TCE is volatile and can be sorbed. From June 1999 through May 2001, we monitored water quality in contaminated springs discharging to Little Bayou Creek as well as seasonal changes in contaminant concentrations and stream flow along a reach of the creek bracketing the springs. Contaminant concentrations in the springs tended to vary with distance from the axis of the plume but not with time. Concentrations of ⁹⁹Tc for the springs as a whole varied linearly with those of TCE (r²=0.86), as previously observed for upgradient wells. However, the ⁹⁹Tc/TCE ratio was greater for springs than for wells, which suggests attenuation of TCE as a result of volatilization or sorption during discharge. Dissolved oxygen concentrations in springs and wells indicated aerobic conditions. Given the ⁹⁹Tc/TCE ratio for springs and the concentrations in stream water immediately upstream and downstream of the springs, the mass of TCE discharged to the stream can be estimated. Farther downstream, seasonal decreases in ⁹⁹Tc concentrations suggest dilution by diffuse, uncontaminated seepage, which is qualitatively consistent with gains in stream flow. The ⁹⁹Tc/TCE ratio increased with distance downstream, which also suggests volatilization of TCE. Pending analyses of (a) ³⁷Cl of TCE for springs and (b) in-stream tracer tests with injected propane and bromide should help to evaluate inferences about volatilization.