ABSTRACT
Mine tailings invariably contain large quantities of fine sediment enriched in heavy metals. Unprotected from the elements by vegetation or artificial coverings such as concrete or geotex matting, these tailings deposits may be subject to wind, water erosion, or oxidation processes. The release of metals into the adjacent environment can occur over a range of timescales. In some cases, this may be a relatively slow process that is controlled by chemical dissolution, wind, or water erosion rates or it may be extremely rapid, such as that associated with the catastrophic collapse of tailings dams. In arid and semi-arid areas where vegetation growth is limited by climatic factors, wind erosion of tailings has contributed to human health problems due to the absorption of heavy metals associated with household dust.[1] However, water erosion and transport of tailings is usually the primary source of river and floodplain contaminated-sediment pollution. This can be particularly severe when tailings retention dams catastrophically fail, such as that which occurred in 1979 when a heavily contaminated uranium tailings dam collapsed on the Puerco River, New Mexico, resulting in serious contamination of the river with thorium-230.[2]
Acid mine drainage from both active mines and abandoned sulfide tailings dumps is also known to cause major environmental problems for river and floodplain water and sediment quality. Acid mine drainage results from the oxidization of sulfide minerals (such as pyrite), which when discharged release metal ions into the aqueous environment, e.g., lakes, rivers, and groundwater bodies. These metals are then transferred in solution through the system and may be precipitated with alluvial sediments where they can be stored for extended periods of time (101-104 yr) causing potential long-term environmental problems.[3]
THE TRANSPORT, STORAGE, AND AVAILABILITY OF METALS IN THE ALLUVIAL ENVIRONMENT
Contamination of river and floodplain environments is complex in time and space. Heavy metals are not readily dissipated in the natural environment and can have extremely long residence times in sediment, depending on their physical and chemical mobility within an
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affected sediment system. Heavy metals are transferred through a river system by four principal mechanisms: hydraulic sorting according to particle size and density; chemical dispersal-solution, adsorption, Fe and Mn complexes, and biological uptake; dilution with clean uncontaminated sediments; and loss and exchange with floodplain sediments.[3] These processes may occur in differing amounts depending on the prevailing physicochemical conditions and the hydroclimatic regime. For example, the dispersal of mining related metal contamination may, under some circumstances, be controlled by physical processes rather than chemical mobility. Metals such as cadmium, copper, lead, and zinc are often absorbed on grain surfaces or on oxides, hydroxides, and oxyhydroxides particularly in iron and manganese forms, which are then transported by physical processes. In particular, manganese oxides have been shown to be one of the most significant groups of substances that control heavy metal concentrations in alluvial sediment.[4] Where a catchment is rich in calcareous materials, the release of metals into the aqueous system may be buffered by an increase in sediment and water alkalinity. The chemical mobility of metals and their availability and dispersal in the environment may also be related to fluctuations in the water table and/or changes in pH. For example, a rising water table or decreasing pH may cause the dissolution of oxide substances resulting in the release of metals previously bound up. In contrast, under ambient conditions, oxide substances such as iron and manganese may act as long-term stores for heavy metals in alluvial sediment.[5]
One of the main issues surrounding systems affected by historical metal mining is the storage and dispersal of contaminants in floodplain and in-channel sediment sinks because they may pose an ongoing and a longterm risk to the environment.[6,7] Alluvial systems are notoriously complex in terms of their morphology and sediment dynamics and the spatial distribution and concentration of metals will often reflect the channel and floodplain depositional environments in which they are stored. The dispersal of metals within a river system may occur both laterally across the floodplain and longitudinally throughout the system. Generally, metal concentrations will diminish away from the channel towards the margins of a floodplain and also in a downstream direction. This general distance-decay pattern of metal concentrations away from the polluting source may vary according to channel and floodplain geomorphology. For example, sediment-metal concentrations may become elevated in slack water environments (e.g., paleochannels) on the floodplain. This is because metals in the fine sediment fractions have a greater surface area to volume ratio. When metals become adsorbed (fixed) onto particle surfaces, this produces a strong relationship between trace
metal concentration in the solid phase and decreasing particle size. This is not the case, however, in environments where the fine fraction may either be absent or have been transported downstream. Under these circumstances, a lag of larger, denser particles will give rise to the contaminant signal found in alluvium.[8]
Within channels, metal concentrations may be considerably lower in reaches that are characterized by high stream power values, due to the dominance of erosion processes. In contrast, elevated metal concentrations will tend to occur where alluvial deposition processes are dominant.
