THE USE OF MULTI-SCALE GEOCHEMICAL TECHNIQUES IN THE ASSESSMENT OF LEGACY MINE LAND REMEDIATION

Thousands of legacy mine land (LML) sites negatively affect water quality and ecosystems in the western US. Remediation is costly and effectiveness can vary depending on individual site characteristics. Our goal is to develop screening methods that identify watersheds moderately impacted by LML sites with metal concentrations slightly above aquatic life standards where limited, targeted remediation could greatly improve water quality and ecosystem health. A first-pass GIS approach focused on central Colorado, integrating existing regional datasets containing water chemistry, known mine features (USGS USMIN database), and satellite-based hydrothermal alteration (from multispectral data) to identify LML sites for further investigation. Five sites were chosen for preliminary investigation in 2019. Of those, North Quartz Creek in Gunnison County was chosen for more focused study to test a set of field methods to identify contaminant levels and discern natural from mine-related sources.

North Quartz Creek hosts the draining Bon Ton Mine, where mining occurred in the late 1800s (primary commodities Cu, Pb, Mo, Au, and Ag). The mine and dump piles are located on the north side (left bank) of the creek, but a large outcrop of altered rock with some small mine prospects is located on the south side (right bank), upstream from the mine. We collected stream water, groundwater, and soil samples and performed an instream synoptic tracer study. Stream and groundwater data show that metals, primarily Cu and Zn, are sourced from natural weathering of the altered zone and the Bon Ton mine, with a greater influence on stream load from the natural weathering zone. In addition to elevated Cu and Zn, soil data also shows elevated Pb, Mo, and Ag in fine-grained, organic-rich, saturated soils near the stream downslope from the naturally weathered altered zone. Molybdenum values are highest in fine-grained sediments along the adit discharge path and are also slightly elevated in some left bank soils downslope from the mine dump. Although each LML-impacted watershed will have unique characteristics that will require detailed sampling to characterize metal contamination, screening techniques using existing data and targeted follow-up sampling can be an effective strategy to identify remediation candidate sites.