High-Frequency Geochemical Monitoring of Toe Seepage from Mine-Rock Dumps, BHP Minerals' Island Copper Mine, British Columbia

Abstract

Recent studies of mine-rock dumps have shown that water can pass through a dump in hours to days. As this water initially flows downward and is focused into preferential channels within the dump, it collects dissolved metals from the rock. The water then either flows as surface seepage from the toe or enters the groundwater system beneath the dump. In the case of toe seepage, the water is often collected in ditches and diverted out of the area. Because of the variability in flow and chemistry expected in toe seepage, questions can arise as to the appropriate monitoring frequency.This paper addresses monitoring frequency based on actual data from the Island Copper Mine in British Columbia. Data on flow and chemistry were repeatedly collected as frequently as every 15 minutes and every four hours, respectively, for six months in 1991-92 and again in 1992-93. The chemical parameters collected at 10 monitoring stations included pH, conductivity, alkalinity, acidity, sulfate, copper, zinc, cadmium, calcium, magnesium, and aluminum.The combined database for this ongoing study currently contains tens of thousands of values and continues to grow. This paper presents some of the findings, correlations, and relationships noted to date. For example, distributions of the chemical data in 1991-92 and 1992-93 typically resemble lognormal or normal statistical distributions. The statistical analyses show that the results of hypothetical hourly sampling, for example, can be estimated from the actual results of weekly sampling. Based on this, a parallel is drawn with hydrology in which there is an annual peak chemical concentration of a particular duration. Also, relationships of chemical concentrations to both flow and time show the gradual depletion of reaction products that accumulated during summer months, as well as the later flushing of short-term accumulations that accrue between storm events. However, correlations of chemical parameters to flow alone are poor, reflecting the significant influence of other factors on chemistry. These factors are discussed further in other papers at this conference.

Modeling Multiphase Reactive Transport in a Waste Rock Pile with Convective Oxygen Supply

“…Studies of waste rock piles are much more limited compared with mine tailing sites. Initial studies mostly focused on the interpretation of temperature and gas concentration profiles (e.g., Harries and Ritchie, 1985; Lefebvre et al, 1993) and an evaluation of the quality of drainage water leaving the base of waste rock piles (Morin et al, 1994). However, the outflow at the base of a pile integrates drainage from the pile and gives only limited information about the spatial and temporal distribution of geochemical processes.…”

Internal Hydrogeologic Monitoring of an Acidic Waste Rock Dump at Westmin Resources' Myra Falls Operations, British Columbia

“…Despite its relative unimportance on an annual basis, most infiltration through the dump occurs during the wet season from October to March and periodic storm events carry acid-generation products downward through the dump to the water table, affecting the chemistry of the Inner Drain water (Northwest Geochem 1990, Northwest Geochem 1992. Concepts behind such flushing of waste-rock dumps are discussed in Morin and Hutt (1994b) and Morin et al (1994).…”