CONSIDERATIONS OF OBSERVATIONAL SCALE WHEN EVALUATING THE EFFECT OF, AND REMEDIATION STRATEGIES FOR, A FLUVIAL TAILINGS DEPOSIT IN THE UPPER ARKANSAS RIVER BASIN, COLORADO, USA

The upper Arkansas River, Colorado, USA, has been the focus of a great deal of research on water-quality related impacts from past and current mining practices. We examined the water-quality impacts from a 0.1 km2 fluvially transported tailings deposit located along the flood plain of the upper Arkansas River, 13 km south of Leadville, Colorado. In our examination of the deposit, we used four different scales of observation. First, we collected surficial material from the upper 15 cm of the deposit and subjected it to batch water-leaching tests, which represent the short (minutes to hours) time scale of runoff from snow melt and rainfall. Second, we excavated an intact 8-inch-diameter (60 centimeters in length) core of the deposit, leached it under unsaturated conditions for 23 days, and collected the effluent at various times; this represents the impact of water infiltration through the deposit (days to months) and into the shallow ground-water system. Third, we examined the water quality of the shallow ground water beneath the 0.1 km2 deposit; and fourth, we monitored water quality along a 5-km reach of the adjacent Arkansas River. Results from both the batch leaching of surficial samples and the core leaching experiment indicate that there is a large solid reservoir of water-soluble material that yields elevated metal concentrations and high acidity, which could degrade water quality. The local ground water also exhibits lowered pH and elevated metal concentrations. In contrast, the river water-quality data indicate that there is little measurable effect from the fluvial tailings deposit. Since we did not evaluate storm events in our study, we do not know their short-term effects on river water quality. Our results illustrate the importance of observational scale in the interpretation of the impact of the fluvial tailings deposit on water quality. Attention to the importance of observational scale can lead to informed remediation decisions.