QUANTIFICATION OF THE ROLE OF INSTREAM IRON AND ALUMINUM COLLOIDS IN THE TRANSPORT OF COPPER

The fate and transport of metals in natural streams is affected by instream chemical reactions, and also by the physical processes that occur in the stream setting. Understanding chemical reactions in the context of their physical setting requires a sequence of detailed spatial sampling, identification of plausible reactions that might account for changes, and quantification of mass transfer for the reactions. This sequence is accomplished by a mass-loading approach that combines synoptic sampling for detailed chemical profiles and tracer dilution for quantification of discharge. The approach provides data for calculating mass transfer in an hydrologic context. This is illustrated by in-stream experiments in Mineral Creek, Colorado. Along a 15-kilometer study reach, instream pH varied above and below pH 5 four times, ranging from 2.84 to 6.89, in response to acidic and basic inflows. With each change in pH, dynamic transformations between dissolved (10K Dalton filtration) and colloidal (total recoverable less filtered) concentrations of aluminum, iron, and copper occurred. With a change to higher pH, colloidal iron and aluminum formed, with the subsequent sorption of copper. With a change to lower pH, all of the aluminum and part of the iron colloidal material dissolved, and copper was released as dissolved copper. Thus, the plausible reactions included precipitation and dissolution of iron and aluminum colloids and the sorption and desorption (upon dissolution) of copper. Mass-transfer calculations quantified the extent of chemical reaction, but also the physical mixing in mixing zones downstream from the inflows. Results indicated that variation in the data mostly were a result of mixing downstream from the Middle Fork and the result of chemical reaction downstream from the other principal inflows. A mass-loading approach that includes detailed spatial sampling, identification of plausible reactions, and mass-transfer calculations, provides a tool for the detailed evaluation and quantification of reactions in the stream setting.