Streambed Sediment

Streambed sediment can be used to identify metal sources on the basis of downstream patterns in concentration and locations of abrupt increases. Particulate-bound metals commonly occur where enriched hydrous amorphous iron- and manganese-oxide mineral coatings and colloidal particles have been deposited on the streambed. Although streambed-sediment samples were analyzed for a full suite of metals (table 3), this discussion focuses on four metals (cadmium, copper, lead, and zinc) that are commonly found to have elevated concentration in some surface water and streambed sediment in the mining district. Concentrations of these metals in streambed sediments typically are several orders of magnitude greater than in stream water. When these sediments are flushed from the streambed during high flow, they can contribute to the total- recoverable concentration. When these metals are weakly bound to sediments, they can become a source of waterborne-metal contamination that can be detrimental to aquatic biota. The partial-digestion method used to analyze sediment samples for this study extracts the weakly bound (leachable) metals from the sediment and provides an estimate of metal concentrations that could be solubilized under reducing conditions or if ingested and thus potentially affect aquatic organisms.

Leachable-cadmium concentrations in streambed sediment samples generally were 3 ppm or less (table 3). One notable exception (17 ppm) occurred at the Black Warrior Mine inflow (site 25), which was over 5 times higher than the cadmium concentration at site 0. In addition, copper, lead, and zinc concentrations were substantially higher at site 25 than site 0.

Leachable-concentration profiles of copper, lead, and zinc determined from streambed-sediment samples are shown in figure 7. Lead and zinc profiles were similar, with the greatest mainstem concentrations just downstream from the Black Warrior Mine inflow (site 190). Leachable-metal concentrations in the streambed-sediment sample collected directly from the Black Warrior Mine inflow (site 25) were more than 5 times greater for cadmium, 20 times greater for lead, and 11 times greater for zinc than in the streambed-sediment samples collected upstream at site 0. Although leachable-metal concentrations were elevated at site 25, lead and zinc concentrations at site 190, just downstream from the Black Warrior Mine inflow, were only slightly higher than at site 0 (fig. 7). Downstream from site 190, both leachable lead and zinc concentrations in the streambed sediment decreased to concentrations less than site 0. The streambed-sediment sample collected at site 3,205 (East Fork Miller Creek) had about the same leachable lead concentration as nearby mainstem site 3,225 and slightly lower zinc concentration. Leachable lead and zinc concentrations showed little change from site 1,170 to site 2,225 where Miller Creek dumps 1 and 2 are located (figs. 2 and 7).

Figure 7.  Concentration profiles of leachable copper, lead, and zinc in streambed-sediment samples collected in the Miller Creek watershed, Montana, August 28-31, 2000.  Canadian interim sediment quality guidelines (ISQG) and probable effect levels (PEL) typically are used to compare concentrations of elements in bulk sediment (Canadian Council of Ministers of the Environment, 1998) and are shown here for general comparison only; samples in this study were sieved to 2 millimeters and the fine fraction analyzed. (click here for pdf file)

The leachable-concentration profile for copper was substantially different from that for lead and zinc (fig. 7). Leachable-copper concentrations in streambed sediment steadily increased from site 190 to site 7,120 (SW-2). Leachable-copper concentrations peaked at site 7,120 (about 500 ppm) and remained elevated at a similar concentration throughout the rest of the study reach. This increase in leachable-copper concentration occurred in the reach downgradient from an area in the vicinity and northwest of adit M-25 (fig. 2), where anomalies of as much as 400 ppm of copper in soils overlying intrusive rocks of Henderson Mountain Stock were identified (Allan R. Kirk, Maxim Technologies, Inc., written commun., 2001). The sample collected at the Black Warrior Mine inflow (site 25) had moderately elevated leachable copper concentration (330 ppm), but no apparent effect on the mainstem site immediately downstream. The East Fork Miller Creek (site 3,205) had a similarly elevated leachable-copper concentration (320 ppm) in streambed sediment but this value was only slightly higher than the concentrations at adjacent mainstem sites.

At the neutral pH of Miller Creek, the metals that are associated with the iron- and manganese-oxide coatings on the sediments do not tend to dissolve; however, when ingested by an aquatic organism and subject to the acidic digestive tract, the weakly bound metals associated with these coatings can be liberated and become incorporated into the tissue of the organism. Bioaccumulation of metal, over time, can detrimentally affect the biota in a stream system, either directly or through the food chain. Currently (2001), the State of Montana has no guidelines to assess the potential toxicity to aquatic biota from the leachable-metal concentrations in streambed sediment. Therefore, leachable concentrations measured during this study were compared to two guidelines established by the Canadian Council of Ministers of the Environment (1998) for the protection of aquatic life. The lower guideline is the interim sediment quality guideline (ISQG). Leachable concentrations less than the ISQG are not expected to have adverse effects on the aquatic biota. Leachable concentrations exceeding the upper guideline, the probable effect level (PEL), are expected to adversely affect aquatic biota. Values between the two levels are associated with occasional adverse effects to aquatic biota. These guidelines typically are used as references for concentrations of elements in bulk sediment, whereas the results from this study are associated with the sieved fraction finer than 2 mm. Higher metal concentrations typically are associated with the smaller size fraction analyzed in this study, and comparison to bulk sediment guidelines is intended only to provide a general reference to the exposure risk from the streambed sediment in Miller Creek.

Both ISQG and PEL guidelines are shown on figure 7 for copper, lead, and zinc. Upstream from East Fork Miller Creek (site 3,205), leachable-copper concentrations in the streambed sediment at all sites were between the ISQG (35.7 ppm) and PEL (197 ppm) guidelines. Downstream from East Fork Miller Creek, all leachable-copper concentrations exceeded the PEL. Leachable concentrations of lead in all streambed-sediment samples collected during this study exceeded the ISQG (35.0 ppm) and the PEL (91.3 ppm). Upstream from East Fork Miller Creek (site 3,205), leachable concentrations of zinc in streambed sediment either exceeded or were near the PEL (315 ppm). Downstream from East Fork, leachable-zinc concentrations were between the ISQG (123 ppm) and the PEL. Leachable-cadmium concentrations in Miller Creek (table 3) ranged from 2 to 3 ppm, which were between the ISQG (0.6 ppm) and the PEL (3.5 ppm) guidelines. Based on these comparisons to the available bulk sediment guidelines, risks to aquatic biota might be associated with exposure to the streambed sediment of Miller Creek. These risks are basin-wide and appear to be influenced more by the local geology than a specific source.

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