Determination of pre-mining geochemical conditions and paleoecology in the Animas River Watershed, Colorado

Determination of the pre-mining geochemical baseline in bed sediments and the
paleoecology in a watershed impacted by historical mining activity is of utmost
importance in establishing watershed restoration goals. We have approached this
problem in the Animas River watershed using geomorphologic mapping methods to
identify old pre-mining sediments. A systematic evaluation of possible sites resulted in
collection of a large number of samples of pre-mining sediments, overbank sediments,
and fluvial tailings deposits from more than 50 sites throughout the watershed. Chemical
analysis of individual stratigraphic layers has resulted in a chemical stratigraphy that can
be tied to the historical record through geochronological and dendochronological studies
at these sites.
Preliminary analysis of geochemical data from more than 500 samples from this
study, when coupled with both the historical and geochronological record, clearly show
that there has been a major impact by historical mining activities on the geochemical
record preserved in these fluvial bed sediments. Historical mining activity has resulted in
a substantial increase in metals in the very fine sand to clay sized component of the bed
sediment of the upper Animas River, and Cement and Mineral Creeks. Enrichment
factors for metals in modern bed sediments, relative to the pre-mining sediments, range
from a factor of 2 to 6 for arsenic, 4 to more than 10 for cadmium, 2 to more than 10 for
lead, 2 to 5 for silver, and 2 to more than 15 for zinc. However, the pre-mining bed
sediment geochemical baseline is high relative to crustal abundance levels of many orerelated
metals and the watershed would readily be identified as a highly mineralized area
suitable for mineral exploration if it had not been disturbed by historical mining activity.
We infer from these data that the water chemistry in the streams was less acidic prior to
historical mining activity in the watershed.
Paleoentologic evidence does not indicate a healthy aquatic habitat in any of the
stream reaches investigated above the confluence of the Animas River with Mineral
Creek (fig. 1) prior to the impact of historical mining activity. The absence of
paleoentologic remains is interpreted to reflect the poor preservation regime of the bed
sediment materials sampled. The fluvial sediments sampled in this study represent higher
energy environments than are conducive to the preservation of most aquatic organisms
including fish remains. We interpret the sedimentological data to indicate that there has been substantial loss of riparian habitat in the upper Animas River above Howardsville as
a result of historical mining activity.