Tracer-Injection Results

A mass-loading profile for a stream can be developed using synoptic streamflow and water-quality data for many sites along a stream. Such a profile can reveal notable spatial differences in loads, identify the loca­tion of significant sources of constituents, and indicate where geochemical reactions affect instream concentrations and loads. Because load is the product of streamflow and concentration, good streamflow mea­surements are necessary for meaningful results. The tracer-injection method utilizes conservation-of-mass theory and represents an alternative to traditional current-meter methods for determining streamflow in high-gradient mountain streams, particularly where traditional methods are hampered by turbulent flow and irregular channel cross sections (Bencala and others, 1990). The tracer-injection method also accounts for the portion of streamflow, referred to as underflow or hyporheic flow, that moves through the gravels and cobbles comprising the bed of the stream. In mountain streams, hyporheic flow can represent a substantial portion of the total streamflow (Kimball, 1997). Hyporheic flow cannot be measured by current-meter methods; therefore, loads may be underestimated with streamflow values obtained by current-meter measurements. Another advantage of the tracer-injection method is that streamflow data can be obtained at many sites more quickly than with current-meter measurements, thus allowing a long study reach to be characterized in less time and minimizing the potential effect of temporal changes in streamflow that could complicate a synoptic profile of load.

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