U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting Charleston South Carolina March 8-12,1999--Volume 1 of 3--Contamination From Hard-Rock Mining, Water-Resources Investigation Report 99-4018A

Considerations of Observational Scale when Evaluating the Effect of, and Remediation Strategies for, a Fluvial Tailings Deposit in the Upper Arkansas River Basin, Colorado

By Kathleen S. Smith, Katherine Walton-Day, and James F. Ranville

This report is available in pdf format : Smith.pdf 301KB

ABSTRACT

We examined the water-quality effects of a fluvial tailings deposit along the flood plain of the upper Arkansas River south of Leadville, Colorado. Fluvial tailings deposits are a possible diffuse source of acid and metal contamination to surface and ground water. We used four different scales of observation to evaluate the potential effect of fluvial tailings on water quality. First, we collected surficial material and subjected it to batch water-leaching tests. Second, we excavated an intact 8-inch-diameter (60 centimeters in length) core, leached it under unsaturated conditions for 23 days, and collected the effluent. Third, we examined the water quality of the shallow ground water beneath the fluvial tailings deposit; and fourth, we monitored water quality along a 5-kilometer reach of the adjacent Arkansas River. Our results illustrate the importance of observational scale in the interpretation of the effect of the fluvial tailings deposit on water quality. Leaching of surficial samples indicates that there is a large reservoir of readily water-soluble material yielding elevated metal concentrations and high acidity that could degrade water quality. However, the river-water-quality data indicate that there is no measurable effect from the fluvial tailings deposit. It is important to note that this data set does not include any stormwater sampling. Natural attenuation processes (including dilution) appear to contribute to our different findings at different observational scales. Attention to the importance of observational scale can lead to informed remediation decisions.