An Empirical Technique for Predicting the Chemistry of Water Seeping from Mine-Rock Piles

Abstract

Mine-rock piles are complex hydrogeologic systems. As a result, the current knowledge of their physical and chemical hydrogeology is too limited to permit accurate predictions of water chemistry through time based on detailed simulations of their internal processes. However, a simplistic empirical model based on general knowledge and available data can be used to obtain rough estimates of seepage chemistry through time. This empirical model is based on five factors: (1) the production rates of metals, nonmetals, acidity, and alkalinity under acid and pHneutral conditions, (2) the volume rate of flow through the rock pile based on infiltration of precipitation, (3) the elapsed time between infiltration events, (4) the residence time of the water within the rock pile, and (5) the percentage of mine rock in the pile flushed by the flowing water. The last factor is most difficult to define at many minesites, but can be assumed due to its apparently frequent narrow range of 5 to 20%. A hypothetical example illustrates how to use the model and highlights other complications like secondary-mineral precipitation that may also have to be considered. A field example based on data from an actual minesite demonstrates the accuracy of the model as compared to measured concentrations. Again, this model ignores many complexities of mine-rock piles and is thus only useful for rough estimates of future chemistry.

Mine Wastes

“…A simple mathematical model for predicting the chemistry of water seeping from waste rock piles has been presented by Morin and Hutt (1994). All presently available mathematical and computational models have limitations and rely on good field and laboratory data obtained from solid mine wastes and mine waters.…”

“…In Morin and Hutt's (1994) example, a hypothetical waste rock pile is 600 m long, 300 m wide, and 20 m high, and contains 6.5 Mt of rock. The long-term production rate of zinc has been obtained from kinetic test data at 5 mg kg -1 per week (factor 1).…”

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

“…Initially, this may be surprising in view of the previous temporal relationships; however, there is more than the one variable of flow that determines chemistry. Other variables are discussed by Morin and Hutt (1994) and Morin et al (1994b).…”

“…Mine-rock dumps are recognized as physically and geochemically complex systems (e.g., Morin et al 1991, Morin andHutt 1994). In a general sense, precipitation and/or snowmelt initiates infiltration into a dump, the water is then focused into preferential channels as it collects metals and nonmetals from rock-particle surfaces, and the 'Paper presented at the International Land Reclamation and Mine Drainage Conference and the Third International Conference on the Abatement of Acidic Drainage, Pittsburgh, PA, USA, April [24][25][26][27][28][29]1994.…”

Non-invasive flow path characterization in a mining-impacted wetland

“…Heterogeneity and preferential flow path development in AMD settings has been shown to decrease the e ciency of contaminant attenuation (Malmström et al, 2008), likely because preferential flow paths reduce the residence time of solutes in the subsurface and contact with attenuating agents (Brusseau, 1994). Deposition of mining waste piles typically results in graded bedding, through which most discharge is concentrated into a small of the total rock volume (Morin & Hutt, 1994;Smith, 1995). Unfortunately, the subsurface is rarely mapped to a su cient extent to identify and characterize flow paths, especially at historical mine sites, where e↵orts generally contend with a lack of site data and highly disturbed aquifer material (e.g., Nordstrom, 2011b;Oram et al, 2010).…”