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Norvell, Audrey S ¹ ; Ryan, Joseph Nolte ² ; McKnight, Diane M ³ ; Ren, Jianhong ⁴

¹ Civil, Environmental, and Architectural Engineering, University of Colorado, Boulder, Boulder, CO, USA. 
² Civil, Environmental, and Architectural Engineering, University of Colorado, Boulder, Boulder, CO, USA. 
³ Civil, Environmental, and Architectural Engineering, University of Colorado, Boulder, Boulder, CO, USA. 
⁴ Environmental Engineering, Texas A&M University-Kingsville, Kingsville, TX, USA

A primary component of human and ecological risk assessments of acid mine drainage is predicting the fate and transport of metals in receiving streams. In order to learn more about the processes that control metal removal in the stream, we studied the role of colloids and the exchange of stream water with the subsurface in Lefthand Creek, a stream contaminated by acid mine drainage in northwestern Boulder County, Colorado. We installed a set of mini-piezometers in the streambed and sampled the hyporheic pore waters along a 90 m reach of the creek for metals, colloids, and other geochemical parameters. We conducted tracer dilution tests to determine the extent and time scale of hyporheic exchange. The results of these investigations showed that hyporheic exchange is a significant process in the attenuation of metals. The conservative tracer concentration reached plateaus of up to 80 % of the surface concentration at depths of 40 cm. Hydraulic residence times in the upper 40 cm range from 20 minutes to under 3 hours. Up to 98% and 96% of lead and copper, respectively, were associated with colloids composed primarily of iron and aluminum, while zinc was largely found in the dissolved form. Sequential extractions of the sediments indicate that the colloid-associated trace metals were removed from the stream through hyporheic exchange. Large quantities of these metals are incorporated in amorphous iron and manganese oxide coatings found on the streambed sediments.