Use of Natural and Applied Tracers to Guide Targeted Remediation Efforts in an Acid Mine Drainage System, Colorado Rockies, USA

Rory Cowie,Mike Wireman,Mark Williams,Robert Runkel

doi:10.3390/w6040745

Abstract

Stream water quality in areas of the western United States continues to be degraded by acid mine drainage (AMD), a legacy of hard-rock mining. The Rico-Argentine Mine in southwestern Colorado consists of complex multiple-level mine workings connected to a drainage tunnel discharging AMD to passive treatment ponds that discharge to the Dolores River. The mine workings are excavated into the hillslope on either side of a tributary stream with workings passing directly under the stream channel. There is a need to define hydrologic connections between surface water, groundwater, and mine workings to understand the source of both water and contaminants in the drainage tunnel discharge. Source identification will allow targeted remediation strategies to be developed. To identify hydrologic connections we employed a combination of natural and applied tracers including isotopes, ionic tracers, and fluorescent dyes. Stable water isotopes (δ18O/δD) show a well-mixed hydrological system, while tritium levels in mine waters indicate a fast flow-through system with mean residence times of years not decades or longer. Addition of multiple independent tracers indicated that water is traveling through mine workings with minimal obstructions. The results from a simultaneous salt and dye tracer application demonstrated that both tracer types can be successfully used in acidic mine water conditions.

Highlights

Acidic, metal-rich drainage from abandoned hard-rock mines can produce both acute and chronic environmental problems [1]
The two locations of measured discharge alone were inconclusive on determining if Silver Creek was only gaining or only losing water as it moved past the mine complex, and suggest that both inflows and outflows may be occurring within the study reach
The slug addition techniques produced varying results ranging from 7% to 29% loss across the study reach during the sampling event with loss rates being smaller after precipitation events

Summary

Introduction

Metal-rich drainage from abandoned hard-rock mines can produce both acute and chronic environmental problems [1]. An end of the pipe (e.g., at the mine discharge point) treatment strategy has been employed to handle AMD prior to mixing with local surface waters. This strategy is very expensive and treatment must occur in perpetuity, which does not represent a permanent solution to the problem. Remediation efforts may be most successful when the source of water producing the AMD can be targeted and separated, isolated, or removed from the area of a mine most prone to AMD production

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Conclusion