TRANSPORT AND HYPORHEIC EXCHANGE OF IRON OXIDE COLLOIDS IN A STREAM AFFECTED BY ACID MINE DRAINAGE

The fate and transport of some metals in natural waters is influenced by association with colloids. To learn more about the processes that control metal removal in a stream affected by acid mine drainage, we studied the exchange of colloids between surface and subsurface waters over a 61 m reach of Left Hand Creek, a stream contaminated by acid mine drainage in northwestern Boulder County, Colorado. We injected bromide as a conservative tracer and ferric (oxy)hydroxide colloids synthesized in the presence of yttrium and monitored their transport in the stream and in the hyporheic zone with multilevel samplers at four locations downstream of the injection. Over the stream reach, 13% of the colloids were removed. Of the colloids entering the hyporheic zone, greater than 97% attenuation occurred within the upper 40 cm of the streambed. A one-dimensional transient storage model (OTIS-P) was used to quantify parameters describing the transport of the bromide and the colloids. The transient storage parameters indicate significant exchange and first order colloid removal coefficients for the main channel and storage zone area accounted for the loss of the ferrihydrite colloids from the stream. The rapid decline in the concentrations of colloids in the tail of the breakthrough curves indicated irreversible removal and made modeling using first order removal acceptable. The colloid removal rate in the subsurface determined from the model was smaller than that determined based on application of classical filtration theory for colloid transport in porous media to a colloids in a sediment-packed column experiment.