MEND Report 2.36.2b
March 1999

EXECUTIVE SUMMARY

This report documents the results of the second part of a research project jointly funded by Noranda Inc. and the Mine Environment Neutral Drainage program. The first part is covered in a separate report entitled Hydrogeochemistry of Oxidised Waste Rock from Stratmat Site, N.B., published concurrently with this report (MEND 2.36.2a). The overall objective of the project was to understand the geochemical and hydrological interactions between partially oxidised waste rock and water, and to improve our capabilities and techniques in the prediction of acidic drainage from waste rock piles.

The main objective of this part of the research was to understand the hydrology and solute transport within waste rock piles during infiltration and drainage events in response to precipitation. Large column tests were conducted to achieve this objective. Three columns measuring 1.2 m in diameter and 2 m in height and containing up to 3.7 t of partially oxidised Stratmat waste rock were subjected to ten rain simulations. The bottom area of each column was divided into drainage partitions. During and after each rain simulation, the volume and the chemistry of the drainage in each partition was monitored independently over time.

Geochemically, the experimental results suggest that the concentrations of Ca, Pb, and Al in the drainage are solubility-controlled by gypsum, anglesite, and jurbanite, respectively. In contrast the concentrations of Zn, Fe, and SO₄^2- in the drainage are not subject to solubility controls. A dilution hypothesis is proposed to explain the concentration variations of Zn and SO₄^2-. The hypothesis states that the variations of these concentrations and the pH are a result of successive dilutions and/or intermixing of various-stage dilutions of the original pore water, subject to the regulation by redox reactions and mineral precipitation. All soluble zinc seems to originate from the pore water and not from dissolution of secondary minerals. The zinc loading in the drainage is a function of the mass transfer that occurred during dilution and mixing processes.

Hydrologically, the experiments have demonstrated that channelling is a ubiquitous phenomenon in the waste rock studied. Large channels representing < 5% of the total drainage area conduct 20-30% of the total drainage flow. Intermediate-size channels accounting for ~20% of the drainage area carry ~ 40% of the total flow. About 50% of the drainage area has background or matrix flows that carry ~30-40% of the total flow. Finally, ~30% of the column base area does not intercept any flow. Channelling is more pronounced in earlier stages of drainage events and tends to attenuate as the draining process continues. Channel stability is influenced by variables related to the rock bed properties and simulated rain characteristics.

On solute transport, the Zn mass balance shows an efficiency of Zn removal from the pore water that is comparable to the efficiency of a well-mixed system. Whereas the mechanism giving rise to this observation is unclear, it is unlikely that the transport of solutes takes place by pore water displacement. There is no simple relation between solute concentrations and drainage flow rates. A conceptual dendritic-reticulate channelling model is proposed on the basis of the experimental observations.

Statistical analysis suggests that the flow density and the zinc loading can be appropriately described by the lognormal distribution, whereas the Zn concentrations are distributed normally.

The main challenges in flow and solute transport modelling are channelling and interactions between flows and geochemical processes. In this study, porous media flow rock is differentiated from channelling flow rock based on hydraulic properties. Furthermore, five basic component structures that make up a waste rock pile are identified. Each structure has distinct characteristics and should be modelled with different approaches. Factors influencing water flows and flow effects contributing to flow heterogeneity are discussed. A mathematical representation of channelling phenomena is developed, which can be coupled with statistical relationships between solute concentrations and flow rates to model solute transport in waste rock piles.

The kinematic wave model, recommended by an earlier MEND study, was applied to the experimental data. The model did not appear to adequately predict the channelling flow characteristics of the waste rock. It over-predicted the extent of larger channel flows at the price of smaller channel flows. The lack of applicability probably stems from the fact that the kinematic wave model precludes merges and splits of flows within waste rock. The model may be more appropriate to coarser waste rocks.

Further fundamental research and case studies are needed to advance our understanding of flow and solute transport in waste rock piles to a point where concentrations and loadings in the drainage can be reliably modelled.

Last Modified: 2003-11-26		Important Notices

Français | Contact Us | Help | Search | Canada Site
Home | What's New | CANMET-MMSL | MMS Site | NRCan Site