Proceedings of the U.S. Geological Survey (USGS) Sediment Workshop, February 4-7, 1997

EFFECTS OF COLLOIDAL IRON AND ALUMINUM ON THE TRANSPORT AND TRANSFORMATION OF METALS IN RIVERS AFFECTED BY MINE DRAINAGE

Acidic, metal-rich drainage from inactive and abandoned mines often enters upland watersheds and eventually flows into non-acidic streams. As physical and chemical conditions change, metals from mine drainage are transformed through chemical and biological processes. For example, some metals that are dissolved in acidic waters become colloidal solids in non-acidic streams (Kimball and others, 1995; Buffle and Leppard, 1995a, 1995b). Colloids from mine drainage typically are hydrous oxides of iron (Fe) and aluminum (Al) that have extensive surface area and sorb toxic metals (Morel and Gschwend, 1987). As colloids aggregate, they settle to the streambed where they mix with algae and are available to organisms. Thus the fate of toxic metals is influenced by the chemistry of Fe and Al. To effectively remediate adverse effects of toxic metals in streams, the processes that affect metals and colloids must be understood. The purpose of this discussion is to outline the methods and results of a recent study of colloids in the Animas River basin and to review the geochemical and biological implications of metal transport by colloids.

The study in the Animas River basin involved the measurement of selected metals in water, colloid, and bed sediment samples from the mainstem of the river and from the major tributaries. The sampling included sites near Silverton, Colorado, and downstream to Aztec, New Mexico. This discussion, however, will focus only on samples from the area near Silverton (fig. 1) and the effects of Fe and Al colloids on zinc (Zn). Discharge was measured at the time of sample collection, and samples were collected by equal-width integration across the channel (Ward and Harr, 1990).

Figure 1. Sampling sites in the Silverton, Colorado area.

Legally and in many standard methods, a dissolved-metal concentration is defined as the concentration that passes through a 0.45-micrometer (µm) filter (Horowitz and others, 1996). Hydrous Fe oxides can be on the order of 0.001 µm when they initially form in stream water. However, such small particles rapidly aggregate, forming a continuous size range from 0.001 µm to greater than 1 µm (Stumm and Morgan, 1996). Thus, for Fe-rich streams affected by mine drainage, 0.45 µm is neither an effective nor a natural break for the distinction of dissolved and particulate concentrations. In this study tangential-flow ultrafiltration through a membrane with an effective pore size less than 0.001 µm was used to differentiate between dissolved and particulate metal fractions (Hernandez and Stallard, 1988; Moran and Moore, 1989; Kimball and others, 1995).

The procedure to distinguish concentrations of metals in water from concentrations in colloidal particles is summarized in fig. 2. The integrated sample was screened to remove sand, gravel, and debris. A split of the screened sample allowed the determination of a "total recoverable metal concentration." The rest of the sample was used for ultrafiltration to determine two metal concentrations, the "dissolved" concentration in the filtrate less than 0.001 µm, and the "colloidal concentration from the concentrated colloids in water. Tangential-flow filtration keeps solid material in suspension rather than forcing it against a filter membrane. This allows water to be removed by osmotic pressure across a filter membrane without "packing" the suspended solids onto a filter membrane, which would change the membrane pore size.

Figure 2. Diagram showing sample processing procedures

The Animas River near Silverton had three main sources of metals: the upper Animas River, Cement Creek, and Mineral Creek (figs. 3 and 4). Downstream from Silverton, there are no substantial sources of metals.. These sources near Silverton differed in the loads of metals they contributed and in the forms of the metals, primarily because of the different pH among the sources. For example, at a pH of about 3.9 in Cement Creek Al mostly was dissolved and at a pH of about 6.4 in Mineral Creek, Al mostly was colloidal. Dissolved Al from Cement Creek was transformed to colloidal Al after it was discharged to the Animas River, which had a pH near 7. The sum of the dissolved-Al loads (load is the product of metal concentration and stream discharge) from the upper Animas River and Cement Creek sites was 293 kilograms per day (kg/day), but only 88 kg/day was measured downstream from Cement Creek (fig. 3). The 205 kg/day that was lost from the dissolved load almost equaled the 240 kg/day of colloidal Al that was formed (fig. 3).

Figure 3. Dissolved and colloidal aluminum loads in the Animas River and tributaries near Silverton, Colorado.

Figure 4. Dissolved and colloidal zinc loads in the Animas River and tributaries near Silverton, Colorado.

In the Animas River, downstream from Mineral Creek, there was a net loss of 89 kg/day dissolved Al and 264 kg/day colloidal Al. This net loss suggests that colloidal Al continued to form in the stream, and that it was lost from the water as it aggregated into larger particles and settled to the streambed.

Similar calculations showed the transformation of Fe in the Animas River downstream from Cement Creek and Mineral Creek. Like Al, the gain in colloidal Fe downstream from Cement Creek was comparable to the loss of dissolved Fe; the total mass of colloidal Fe removed downstream from Mineral Creek was greater than the loss of dissolved Fe.

Unlike Al and Fe, only a small amount of Zn was transformed from dissolved to colloidal forms. In the Animas River downstream from Cement Creek, there was 184 kg/day dissolved Zn, which was greater than the sum of the individual sources of the upper Animas River and Cement Creek (fig. 4). This increased load likely indicates a contribution of Zn from some non-point sources, perhaps from dispersed tailings in the alluvium. In the Animas River downstream from Mineral Creek, the load of dissolved Zn was 99 kg/day (fig. 4), which represented a net loss of 131 kg/day of dissolved Zn. This net loss coincided with a gain of 6 kg/day of colloidal Zn, so most of the dissolved Zn was not transformed to colloidal Zn. Instead, a substantial loss of dissolved Zn to the streambed occurred through sorption to the abundant Al and Fe colloids that had settled to the streambed.

Loss of dissolved and colloidal loads indicated the streambed sediments were gaining metals. If the colloids had a substantial influence on the streambed-sediment chemistry, then metal ratios in the colloids and the sediments should have been similar. Upstream from Silverton, the ratios did not correspond; but downstream from Silverton, after colloids had formed downstream from Cement and Mineral Creeks, the ratios agreed closely (fig. 5). In fact, the ratios had the same variations for 100 kilometers of the Animas River. The close agreement in these ratios likely results from the process of colloidal formation, aggregation, and settling.

Figure 5. Iron-to-zinc ratio in colloids and streambed sediment in the Animas River, near Silverton, Colorado.

The source of dissolved and colloidal metals was clearly the Silverton area, and in particular the inflows of Cement and Mineral Creeks. There was a clear distinction between metals like Al and Fe that were transported mostly in the colloidal load and Zn that was transported in the dissolved load. Results from this study not only demonstrate the masses involved in the active geochemical processes, but also indicate the large mass of colloidal material that becomes stored in the streambed material. As the aggregated Al and Fe colloids settle to the streambed, they can affect the habitat of the stream by filling pore space among cobbles and promoting the hardening of the streambed into "ferricrete." The aggregated colloids also can become part of the metal-rich sediment coatings on streambed material. Any of the colloidal material that is not part of coatings or filling streambed ferricrete could be remobilized during runoff from storms or snowmelt.

REFERENCES CITED

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