Evaluating the Reliability of the Stream Tracer Approach to Characterize Stream‐Subsurface Water Exchange

Abstract

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-reach-scale measurement of hydraulic heads and hydraulic conductivity to compute streambed fluxes (hydrometric approach) and reachscale modeling of in-stream solute tracer injections to determine characteristic length and timescales of exchange with storage zones (stream tracer approach). Subsurface data were the standard of comparison used to evaluate the reliability of the stream tracer approach to characterize hyporheic exchange. The reach-averaged hyporheic exchange flux (1.5 mL s -• -•_)•, determined by hydrometric methods, was largest when stream base flow was low (10 • s ); 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 assessed using first-order uncertainty analysis that considered model parameter sensitivity. The stream tracer approach did not reliably characterize 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 characterized exchange between the stream and gravel streambed (timescale of hours) but was relatively insensitive to slower exchange with deeper alluvium (timescale of tens of hours) that was detected by subsurface measurements. The stream tracer approach was therefore not equally 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 routinely 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 constants to consider both fast exchange with streambed gravel and slower exchange with deeper alluvium appear to be warranted. This paper is not subject to U.S. copyright. Published in 1996 by the American Geophysical Union. Paper number 96WR01268. face flow systems can be large or small in extent. Individual flow paths of exchange range in scale from hundreds of meters, in which transport occurs on a timescale of years, to centimeter-long flow paths, in which transport occurs on a timescale of minutes. Interactions are driven at small scales by steady flow of surface water over roughness features such as sand waves or pools and riffles. The resulting uneven pressure distributions on the channel bed cause surface water to flow into and out of the bed [Thibodeaux and Boyle, 1987; Harvey and Bencala, 1993]. We refer to small-scale (centimeter to meter) exchanges of water between channels and the subsurface as "hyporheic exchange" (Figure la) in order to emphasize the ...

Spatio‐temporal variations of hyporheic flow in a riffle‐step‐pool sequence

“…We consider hyporheic exchange flow (HEF) as a specific process by which stream water infiltrates the subsurface and returns to the stream over relatively small distances (Harvey et al, 1996). HEFs may exhibit some variability that affects the ecological functions of groundwater and lotic ecosystems.…”

A physicochemical model for colloid exchange between a stream and a sand streambed with bed forms

“…All of the available models for hyporheic exchange depend on the empirical evaluation of effective exchange coefficients using data from solute transport experiments conducted at the site being modeled. A comparison of transient storage model parameters (obtained from a tracer injection) and observed subsurface flows indicated that the model does not always adequately represent actual stream-subsurface exchange [Harvey et al, 1996]. While existing models employ a single exchange coefficient, in reality, exchange is driven by multiple processes that operate at several spatial scales [Harvey and Bencala, 1993;Harvey and Fuller, 1998;Wroblicky et al, 1998].…”

Transient storage and hyporheic flow along the Willamette River, Oregon: Field measurements and model estimates

“…Dye tracer experiments on streams in conjunction with onedimensional solute transport modeling provide estimates of the transient storage zone cross-sectional area As and the transient storage exchange coefficient a s [Stream Solute Workshop, 1990; Runkel, 1998]. With As and as, estimates can be made of residence time of main channel water within the transient storage zone [Harvey et al, 1996].…”