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

Abstract

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.