Karen A. Hudson-Edwards

Mineralogy and geochemistry of alluvium contaminated by metal mining in the Rio Tinto area, southwest Spain

The Rio Tinto in SW Spain drains Cu and pyrite mines which have been in operation since at least the Bronze Age. Extensive metal mining, especially from 1873 to 1954, has resulted in contamination of the Rio Tinto alluvium with As, Cu, Pb, Ag and Zn. X-ray diffraction (XRD), wavelength-dispersive X-ray mapping, scanning electron microscope petrography and X-ray energy-dispersive (EDX) analysis has revealed that 4 major groups of contaminant metal and As-bearing minerals, including sulphides, Fe-As oxides, Fe oxides/hydroxides/oxyhydroxides, and Fe oxyhydroxysulphates, occur in the alluvium. Sulphide minerals, including pyrite, chalcopyrite, arsenopyrite and sphalerite, occur in alluvium near the mining areas. Iron hydroxides and oxyhydroxides such as goethite and possibly ferrihydrite occur in cements in both the mining areas and alluvium downstream, and carry minor amounts of As, Cu and Zn. Iron oxyhydroxysulphates, including jarosite, plumbojarosite and possibly schwertmannite, are the most common minerals in alluvium downstream of the mining areas, and are major hosts of Cu, Pb, Zn and of As, next to the Fe-As minerals. This work, and other field observations, suggest that (1) the extreme acidity and elevated metal concentrations of the river water will probably be maintained for some time due to oxidation of pyrite and other sulphides in the alluvium and mine-waste tips, and from formation of secondary oxide and oxyhydroxysulphates; (2) soluble Fe oxyhydroxysulphates such as copiapite, which form on the alluvium, are a temporary store of contaminant metals, but are dissolved during periods of high rainfall or flooding, releasing contaminants to the aqueous system; (3) relatively insoluble Fe oxyhydroxysulphates and hydroxides such as jarosite and goethite may be the major long-term store of alluvial contaminant metals; and (4) raising river pH will probably cause precipitation of Fe oxyhydroxides and oxides/hydroxides/oxyhydroxides and thus have a positive effect on water quality, but this action may destabilise some of these contaminant metal-bearing minerals, releasing metals back to the aqueous system.

The significance of pollution from historic metal mining in the Pennine orefields on river sediment contaminant fluxes to the North Sea

Abstract Historic lead and zinc mining in the Northern Pennines, Yorkshire Dales and Peak District significantly affects water and sediment quality in river basins in the LOIS study area. Sediment-associated contaminant metals are transported, often in pulses or slugs, as suspended load under flood conditions. Although downstream decreases in metal concentrations are observed in the river systems, a substantial proportion of the metals have been incorporated into alluvial deposits. These in turn are, and will continueu to act, as a major secondary source of metal contaminants over many hundreds of years. These processes need to be taken into consideration when modelling riverine metal fluxes to the North Sea, to avoid overestimation of metal contaminants from industrial and urban sources, and underestimation of contaminants from these mining-related sources.

Sources, distribution and storage of heavy metals in the Rio Pilcomayo, Bolivia

Hudson-Edwards, K. A., Macklin, M. G., Miller, J. R., Lechler, P. J. (2001). Sources, distribution and storage of heavy metals in the Rio Pilcomayo, Bolivia. Journal of Geochemical Exploration, 72(3), 229-250.

Fertilizing the Amazon and equatorial Atlantic with West African dust

Atmospheric mineral dust plays a vital role in Earths climate and biogeochemical cycles. The Bodele Depression in Chad has been identified as the single biggest source of atmospheric mineral dust on Earth. Dust eroded from the Bodele is blown across the Atlantic Ocean towards South America. The mineral dust contains micronutrients such as Fe and P that have the potential to act as a fertilizer, increasing primary productivity in the Amazon rain forest as well as the equatorial Atlantic Ocean, and thus leading to N2 fixation and CO2 drawdown. We present the results of chemical analysis of 28 dust samples collected from the source area, which indicate that up to 6.5 Tg of Fe and 0.12 Tg of P are exported from the Bodele Depression every year. This suggests that the Bodele may be a more significant micronutrient supplier than previously proposed.

How paleosols influence groundwater flow and arsenic pollution: A model from the Bengal Basin and its worldwide implication

In the Bengal Basin, the land surface exposed during the last lowstand of sea level around 20 ka, and now buried by Holocene sediment, is capped by an effectively impermeable clay paleosol that we term the Last Glacial Maximum paleosol (LGMP). The paleosol strongly affects groundwater flow and controls the location of arsenic pollution in the shallow aquifers of our study site in southern West Bengal and, by implication, in shallow aquifers across the Bengal Basin and As-polluted deltaic aquifers worldwide. The presence of the LGMP defines paleointerfluvial areas; it is absent from paleochannel areas. A paleosol model of pollution proposed here predicts that groundwater in paleochannels is polluted by arsenic, while that beneath paleointerfluvial areas is not: paleointerfluvial aquifers are unpolluted because they are protected by the LGMP from downward migration of arsenic and from downward migration of organic matter that drives As-pollution via reductive dissolution of As-bearing iron oxyhydroxides. Horizontal groundwater flow carries arsenic from paleochannels toward paleointerfluvial aquifers, in which sorption of arsenic minimizes the risk of pollution.

The impact of tailings dam spills and clean-up operations on sediment and water quality in river systems: the Rı́os Agrio–Guadiamar, Aznalcóllar, Spain

The Aznalcollar tailings dam at Boliden Apirsas Aznalcollar/Los Frailes Ag–Cu–Pb–Zn mine 45 km west of Seville, Spain, was breached on 25 April 1998, flooding approximately 4600 hectares of land along the Rios Agrio and Guadiamar with approximately 5.5 million m3 of acidic water and 1.3×106 m3 of heavy metal-bearing tailings. Most of the deposited tailings and approximately 4.7×106 m3 of contaminated soils were removed to the Aznalcollar open pit during clean-up work undertaken immediately after the spill until January 1999. Detailed geomorphological and geochemical surveys of the post-clean-up channel, floodplain and valley floor, and sediment and water sampling, were carried out in January and May 1999 at 6 reaches representative of the types of river channel and floodplain environments in the Rio Guadiamar catchment affected by the spill. The collected data show that the clean-up operations removed enough spill-deposited sediment to achieve pre-spill metal (Ag, As, Cd, Cu, Pb, Sb, Tl, Zn) concentrations in surface sediment. These concentrations, however, are still elevated above pre-mining concentrations, and emphasise that mining continues to contaminate the Agrio-Guadiamar river system. Dilution by relatively uncontaminated sediment appears to reduce metal concentrations downstream but increases in metal and As concentrations occur downstream, presumably as a result of factors such as sewage and agriculture. River water samples collected in May 1999 have significantly greater dissolved concentrations of metals and As than those from January 1999, probably due to greater sulphide oxidation from residual tailings with concomitant release of metals in the warmer early summer months. These concentrations are reduced downstream, probably by a combination of dilution and removal of metals by mineral precipitation. Single chemical extractions (de-ionised water, CaCl2 0.01 mol l−1, CH3COONH4 1 M, CH3COONa 1 M and ammonium oxalate 0.2 M) on alluvial samples from reaches 1 and 6, the tailings, pre-spill alluvium and marl have shown that the order of sediment-borne contaminant mobility is generally Zn>Cd>Cu>Pb>As. Pb and As are relatively immobile except possibly under reducing conditions. Much of the highly contaminated sediment remaining in the floodplain and channel still contains a large proportion of tailings-related sulphide minerals which are potentially reactive and may continue to release contaminants to the Agrio–Guadiamar river system. Our work emphasises the need for pre-mining geomorphological and geochemical data, and an assessment of potential contributions of contaminants to river systems from other, non-mining sources.

2000 years of sediment-borne heavy metal storage in the Yorkshire Ouse basin, NE England, UK

Floodplain overbank sediments are often used to evaluate the influence of environmental change on sediment and chemical fluxes within river basins. This paper presents the results of an investigation of heavy metal storage in seven floodplain reaches in the Yorkshire Ouse basin in north-east England. Floodplain heavy metal storage has been greatest since c. 1750, and many of the post-1750 sedimentary units exhibit heavy metal values that exceed recommended trigger and guideline values for contaminated land. Relatively high heavy metal storage occurs from ad 1250–1750 in most of the reaches. These patterns are related to 2000 years of Pb and Zn mining in the Yorkshire Dales and c. 250–300 years of industrial and urbanization activity around Leeds and Bradford, and increased delivery of fine-grained sediment during the last millennium, possibly owing to factors such as population growth and agricultural expansion during the Middle Ages, and climate change during the Little Ice Age. Copyright

Chemical remobilization of contaminant metals within floodplain sediments in an incising river system: implications for dating and chemostratigraphy

Metals such as Pb, Zn, Cd and Cu from historical mining activity have been used as stratigraphic markers for dating and provenancing vertically accreted, fine-grained floodplain overbank deposits. This study presents evidence for chemical remobilization of these metals within overbank sediments in the Tyne basin, UK. The evidence includes: breakdown of metal-bearing minerals (sulphides, carbonates, iron and manganese oxyhydroxides); shifts of chemical fractions within zones of relatively low pH towards more soluble and reactive phases; and accumulation of secondary iron and manganese oxyhydroxides at levels related to fluctuating water-table levels or to the breakdown of organic matter. All of this suggests that fine, centimetre-scale, chemostratigraphy using metal concentrations and ratios is unlikely to provide reliable data in river systems that have experienced, or are experiencing, major changes in water-table levels, or pedogenesis. Coarse tens of centimetre- to metre-scale, chemostratigraphy, when applied with caution, may still provide a means of delineating contaminated units.

Sources, mineralogy, chemistry and fate of heavy metal-bearing particles in mining-affected river systems

Abstract Heavy metal-bearing sediment particles enter river systems by discharge of mine or processing waste, tailings dam failures, remobilization of mining-contaminated alluvium and mine drainage. The mineralogy and geochemistry of these particles is dependent upon the original ore mineralogy,and on processes that have occurred in the source areas,during transport and deposition and during post-depositional early diagenesis. This paper reviews the research carried out to date on the sources, mineralogy,chemistry and fate of heavy metal-bearing particles in mining-affected river systems, and identifies six important avenues for further investigation.

Historic metal mining inputs to Tees river sediment

Historic metal mining has greatly influenced sediment delivery and metal contamination in the Tees River Basin. Investigations of metal-contaminated overbank river sediment show that sediment-borne metal concentrations decrease downstream of mining areas. Metalliferous mineralogy also changes: sulphides and carbonates are abundant in the upstream part of the basin, and both decline and disappear downstream where iron oxyhydroxides dominate. Mineral compositional and textural information can be used to trace sources of mining-related contaminated sediment. Sulphides, carbonates, and oxyhydroxides which pseudomorph ore deposit minerals, are interpreted to be derived from outcropping ore bodies and mine-waste tips. The relative abundance of these grains suggests that the ore bodies and mine-waste tips are still important sources of metal pollutants. This is corroborated by morphological mapping and coring of Tees floodplain sequences, which also suggests that metal-contaminated alluvium downstream has experienced limited re-working.