Predicting Future Water-Quality Impacts from Mining: A 52-Year-Old Field Analog for Humidity Cell Testing, Copperwood Deposit, Michigan

Modern mine development requires consideration of environmental management, including long-term, post-closure conditions. A pile of ore from test mining of the Copperwood deposit in 1957 subsequently weathered under ambient local conditions for 52 years prior to this study. This study used the ore pile as a long-term field analog to evaluate conceptual models for scaling of bench-scale humidity cell tests commonly used to assess the potential for mine wastes to generate acidic and/or metalliferous drainage waters. The Copperwood ore is simple, with only one sulfide mineral (chalcocite) hosted by gray to black shale. As part of the new Copperwood development work, a drainage system was established beneath the ore rock pile to collect water that had infiltrated through and weathered the ore material. The composition of the drainage waters today, after approximately 52 years of weathering, can be used to test conceptual predictive models of water-rock interaction. Using inferred “steady-state” rates of release from a 40-week humidity cell test and a simple conceptual model of weathering, the predicted release of elements from the ore rocks was greater than the actual composition of present-day drainage waters. This discrepancy is interpreted to indicate that the steady-state release rates inferred by the humidity cell test are not actually steady state but are the initial stages of a long-term progressive decline. As a result, use of the humidity cell test without calibration to field conditions would significantly overestimate the potential for mine rocks to affect water quality. While progressive decline of release rates has been documented in some multiple-year laboratory testing and heap and dump leaching, it is not yet possible to estimate the ranges in rates of decline as a function of waste mineralogy, fragment size, and other factors, because sufficient long-term studies such as the one presented here are lacking. This case study shows how a variety of methods can be applied to help properly calibrate humidity cell testing to field conditions, so that mining projects can reliably predict the chemistry of waters draining mined products decades into the future.