Margarete Kalin and William N. Wheeler
This is an advance summary of a forthcoming article in the Oxford Research Encyclopedia of Environmental Science. Please check back later for the full article.
The first treatise on mining and extractive metallurgy, published in 1556, mentioned the side effects of mining, namely dead fish and poisoned water. These same side effects are still with us today, even though our knowledge of extractive techniques and chemical processes has grown tremendously. The dead fish and poisoned water, we now know, are caused by oxidative weathering of minerals, resulting in acidic and metal-laden water. The weathering is exacerbated by microbes that break chemical bonds in pyrite to derive their energy.
To compound the problem, our insatiable appetite for metals and energy, combined with our development of industrial tools, has allowed us to dig mines vastly larger than those envisioned in 1556. This exponentially increases the weathering area available in waste rock and finely ground rock (tailings). Through infiltration of atmospheric precipitation, severely polluted seepages emerge from these mining wastes to surface and ground water.
Since metals are essential products needed in society, cost-effective remediation measures need to be developed. New sustainable approaches to mining need to be established. Currently, engineered covers and dams contain and reduce the infiltration of atmospheric precipitation, slowing the weathering process. However, weathering will continue for millennia. With this much time, covers will break down and dams will leak. Currently accepted practice is to integrate basic neutralizing agents (lime) to wastes or seeps in perpetuity. These and other stop-gap measures do not show any resemblance to sustainable mine development and reclamation.
What is needed is a paradigm shift in thinking about mine waste management. Waste rock and tailings need to be thought of as primitive ecosystems, characterized by harsh physical and chemical conditions. These harsh environments are similar to those encountered in the vicinity of hot springs characterized by highly acidic, or alkaline and saline conditions. These ecosystems are populated by thermophilic, acidophilic, and halophilic microbes (as a group called extremophiles), all of which can modify their surroundings. If managed properly, based on ecological principles, mines and these ecosystems will provide the resources of the future.
Ecological engineering utilizes ecological, geo-microbiological, and physical processes to change the conditions within the wastes to favor microbial remediation. To counter oxidative conditions, reductive environments and their microbes are supported with the ecological measures introduced. Reducing conditions can be generated in sediments and on the water-rock or water-sediment interphases through microbial growth. Gradually, contaminated acidic or alkaline water is cleansed by indigenous biota. These organisms sequester metal ions on or inside their cells and neutralize aquatic waste streams. Eventually, biomass (and metals) are relegated to the sediment, where they are bio-mineralized - forming new biogenic ore bodies. Re-oxidation of bio-mineralized metals is prevented by the introduction of underwater and emerging vegetation, which reduce mixing and consume oxygen above and at the sediment-water interphase. Natural cycles of oxidation and reduction have been operating on the planet for millennia, producing biogenic ore bodies, and are ecologically sound, sustainable approaches.
Gerrit de Rooij
Henry Darcy was an engineer who built the drinking water supply system of the French city of Dijon in the mid-19th century. In doing so, he developed an interest in the flow of water through sands, and, together with Charles Ritter, he experimented (in a hospital, for unclear reasons) with water flow in a vertical cylinder filled with different sands to determine the laws of flow of water through sand. The results were published in an appendix to Darcy’s report on his work on Dijon’s water supply. Darcy and Ritter installed mercury manometers at the bottom and near the top of the cylinder, and they observed that the water flux density through the sand was proportional to the difference between the mercury levels. After mercury levels are converted to equivalent water levels and recast in differential form, this relationship is known as Darcy’s Law, and until this day it is the cornerstone of the theory of water flow in porous media. The development of groundwater hydrology and soil water hydrology that originated with Darcy’s Law is tracked through seminal contributions over the past 160 years.
Darcy’s Law was quickly adopted for calculating groundwater flow, which blossomed after the introduction of a few very useful simplifying assumptions that permitted a host of analytical solutions to groundwater problems, including flows toward pumped drinking water wells and toward drain tubes. Computers have made possible ever more advanced numerical solutions based on Darcy’s Law, which have allowed tailor-made computations for specific areas. In soil hydrology, Darcy’s Law itself required modification to facilitate its application for different soil water contents. The understanding of the relationship between the potential energy of soil water and the soil water content emerged early in the 20th century. The mathematical formalization of the consequences for the flow rate and storage change of soil water was established in the 1930s, but only after the 1970s did computers become powerful enough to tackle unsaturated flows head-on. In combination with crop growth models, this allowed Darcy-based models to aid in the setup of irrigation practices and to optimize drainage designs. In the past decades, spatial variation of the hydraulic properties of aquifers and soils has been shown to affect the transfer of solutes from soils to groundwater and from groundwater to surface water. More recently, regional and continental-scale hydrology have been required to quantify the role of the terrestrial hydrological cycle in relation to climate change. Both developments may pose new areas of application, or show the limits of applicability, of a law derived from a few experiments on a cylinder filled with sand in the 1850s.