EVALUATING THE RELIABILITY OF THE STREAM TRACER APPROACH TO CHARACTERIZE STREAM-SUBSURFACE WATER EXCHANGE

Stream water was locally recharged into shallow groundwater flow paths that returned to the stream (hyporheic exchange) in St. Kevin Gulch, a Rocky Mountain stream in Colorado contaminated by acid mine drainage . Two approaches were used to characterize hyporheic exchange: sub-rea ch-scale measurement of hydraulic heads and hydraulic conductivity to compute streambed fluxes (hydrometric approach) and reach-scale modeli ng of in-stream solute tracer injections to determine characteristic l ength and timescales of exchange with storage zones (stream tracer app roach). Subsurface data were the standard of comparison used to evalua te the reliability of the stream tracer approach to characterize hypor heic exchange. The reach-averaged hyporheic exchange flux (1.5 mL s(-1 ) m(-1)), determined by hydrometric methods, was largest when stream b ase flow was low (10 L s(-1)); hyporheic exchange persisted when base flow was 10-fold higher, decreasing by approximately 30%. Reliability of the stream tracer approach to detect hyporheic exchange was assesse d using first-order uncertainty analysis that considered model paramet er sensitivity. The stream tracer approach did not reliably characteri ze hyporheic exchange at high base flow: the model was apparently more sensitive to exchange with surface water storage zones than with the hyporheic zone. At low base flow the stream tracer approach reliably c haracterized exchange between the stream and gravel streambed (timesca le of hours) but was relatively insensitive to slower exchange with de eper alluvium (timescale of tens of hours) that was detected by subsur face measurements. The stream tracer approach was therefore not equall y sensitive to all timescales of hyporheic exchange. We conclude that while the stream tracer approach is an efficient means to characterize surface-subsurface exchange, future studies will need to more routine ly consider decreasing sensitivities of tracer methods at higher base flow and a potential bias toward characterizing only a fast component of hyporheic exchange. Stream tracer models with multiple rate constan ts to consider both fast exchange with streambed gravel and slower exc hange with deeper alluvium appear to be warranted.