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Science for Watershed Decisions on Abandoned Mine Lands: Review of Preliminary Results, Denver, Colorado, February 4-5, 1998

Determination of Pre-Mining Background Using Sediment Cores from Old Terraces in the Upper Animas River Watershed, Colorado

By S.E. Church,¹ D.L. Fey,² and E.M. Brouwers³

Determination of the pre-mining background concentrations of metals in mining districts, particularly those that were heavily mined near the turn of the century, has proven to be challenging. The approach used in the upper Animas River watershed is to search for pre-mining surfaces using established geomorphological techniques, to core the sediment preserved in these settings, to examine the sediment in these cores for metal content and for signs of pre-mining aquatic life, and to compare metal concentrations in the pre-mining sediment to those in bed sediment from active stream channels today. In this report, we discuss the results from five sites within the upper Animas River watershed where pre-mining metal concentrations have been determined: (1) from the upper Animas River near Eureka and from a terrace above Brendel Gulch about one-half mile below the Cunningham Gulch confluence, (2) from pre-mining sediment exposed in the stream bank on Cement Creek halfway between the confluences of Porcupine and Ohio Gulches, (3) from Mineral Creek about one-quarter mile south of Chattanooga, (4) from Elk Park about 5 miles south of the confluence of the Animas River with Mineral Creek, and (5) from the Animas River about one-half mile south of the confluence with Cascade Creek.

Chemical data from the total digestions of the samples show that metals concentrations are systematically enriched in bed sediment today relative to the pre-mining bed sediment collected from old terraces. The data indicate that: (1) the enrichment factor for arsenic in bed sediment today is generally 1.5 to 2, with an enrichment factor of about 6 at Chattanooga; (2) copper enrichment is generally about 6, but only about 2.5 in sediment from Cement Creek; and (3) lead and zinc enrichment factors are highly variable, ranging between 2.3 and 60. Silver, cadmium, and molybdenum also show systematic enrichments in bed sediment today relative to pre-mining sediment at various sites within the basin; however, the data for the pre-mining sediment are censored, so enrichment factors cannot be calculated for each stream reach. Historic mining has significantly changed the metal loading in bed sediment of the upper Animas River watershed above Silverton.

Terrace-sediment samples were examined for microscopic traces of terrestrial and aquatic biota, particularly for any microscopic traces of vertebrate life in the upper Animas River watershed. The terrace sediment ranges from silt to fine pebbles, with most sediment being sand-sized particles. Environments where finer-grained sediment occur tend to be better suited for preservation of microscopic biotic remains. Localities reflecting lower energy levels, such as overbank deposits, pond sediment, and abandoned meander channels have been identified in the watershed for additional examination. To date, biotic remains are dominated by terrestrial plant debris, including stems, roots, and few seeds. Much more rarely, terrace sediment includes terrestrial insect parts, as represented by the heavily chitinized and readily preservable body parts. A few isolated fish scales were recovered, which indicate the presence of aquatic life in the watershed prior to mining.

¹U.S. Geological Survey, MS 973, P.O. Box 25046, Denver Federal Center, Denver, CO 80225 (schurch@usgs.gov)

²U.S. Geological Survey, MS 973, P.O. Box 25046, Denver Federal Center, Denver, CO 80225 (dfey@usgs.gov)

³U.S. Geological Survey, MS 911, P.O. Box 25046, Denver Federal Center, Denver, CO 80225 (brouwers@usgs.gov)

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