Date of Award

Spring 1-1-2011

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

First Advisor

Joseph N. Ryan

Second Advisor

Diane M. McKnight

Third Advisor

Harihar Rajaram

Fourth Advisor

Robert L. Runkel

Fifth Advisor

Alexis Templeton

Abstract

A key component of human and ecological risk assessments of acid mine drainage is predicting the fate and transport of metals in receiving streams. Some contaminant metals are associated with colloids; therefore, better understanding of the transport and removal of colloids in stream systems is needed. The hyporheic exchange process transports water from the stream channel into the hyporheic zone and may play an important role in the removal of both metals and colloids.

In order to learn more about the processes that control metal removal in the stream, we studied the exchange of water, metals, and colloids with the hyporheic zone of Left Hand 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 in the water and sediments. Though a transient storage model describing solute transport indicated that subsurface exchange was limited, piezometer measurements indicated that the rapid subsurface interactions occur over the upper 5 cm of the streambed and that the hyporheic zone may extend to depths of 40 cm in some locations. Large fractions of lead and copper were associated with colloids composed primarily of iron, manganese, and aluminum, while zinc was not significantly associated with colloids. Comparison of the colloidal concentrations with dissolved concentrations in the subsurface relative to surface concentrations indicated that colloids facilitated transport of lead, copper, and zinc. Sequential extractions of the sediments showed that trace metals were incorporated in iron and manganese oxide coatings found on the streambed sediments.

We injected synthesized ferric (oxy)hydroxide colloids into a stream contaminated by acid mine drainage and monitored their transport in the stream and subsurface pore waters. A one dimensional transient storage model (OTIS) was used to quantify parameters describing the transport of the ferric (oxy)hydroxide colloids and bromide, which was injected to serve as a conservative tracer. Based on integration of the area under the tracer dilution plot, 12.3% of the colloids were lost over the 61 m reach. Of the colloids entering the hyporheic zone, greater than 98% attenuation occurred within the upper 40 cm of the streambed at four locations downstream of the injection. Transient storage and first-order removal in the storage zone 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 indicates irreversible removal and indicates that modeling using first-order removal is acceptable.

The transport of metals strongly associated with colloids, lead and copper, was compared with the transport of zinc, a metal which tends to remain in the dissolved phase in stream systems. A continuous step injection of dissolved lead, copper, zinc, and a conservative tracer (bromide) was performed on a short reach of a low-order subalpine stream contaminated with acid mine drainage metals. Approximately 17.6% of lead and 4.6% of copper were lost from the main channel of the stream over the reach, while zinc was transported conservatively through the reach (0.061% removal). Zinc was found to be almost entirely dissolved, while colloids played a role in the transport of lead and copper in the reach.

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