Environ. Sci. Technol. 1996, 30, 72-80
Processes of Formation and Distribution of Pb-, Zn-, Cd-, and Cu-Bearing Minerals in the Tyne Basin, Northeast England: Implications for Metal-Contaminated River Systems K A R E N A . H U D S O N - E D W A R D S , * ,† MARK G. MACKLIN,‡ CHARLES D. CURTIS,† AND DAVID J. VAUGHAN† Department of Earth Sciences, University of Manchester, Manchester M13 9PL, U.K., and School of Geography, University of Leeds, Leeds LS2 9JT, U.K.
Historic mining has produced widespread Pb, Zn, Cd, and Cu contamination in the fluvial deposits of the Tyne River Basin, northeast England. Detailed mineralogical analysis of contemporary overbank river sediment, mining-age alluvium, and mine-waste tips and of suspended solids in river waters has defined a general weathering reaction paragenesis of Pb-, Zn-, Cd-, and Cu-bearing minerals: sulfides f carbonate, silicate, phosphate, and sulfate weathering products f iron and manganese oxyhydroxides. Textural and chemical evidence suggests that the sulfides alter to carbonates in high pH/pCO2, limestone-dominated source terrains. These minerals and other contaminant metal-rich minerals such as silicates and manganese oxyhydroxides decline and disappear downstream in lower pH shale/sandstone-dominated environments. The concomitant decrease in total Pb, Zn, Cd, and Cu sediment contents in the Tyne and possibly other metal contaminated rivers may be related to these essentially chemical weathering and dispersion processes. These are augmented by physical, hydrodynamic processes that to a large extent effect dilution by premining Quaternary sediment and by uncontaminated sediment from tributaries.
Introduction Integration of textural and chemical mineralogy with geochemistry is fundamentally important for understanding the speciation and migration of contaminant (e.g., Pb, Zn, Cd, and Cu) metals in the environment. In river systems, * To whom correspondence should be addressed; fax: +44 0161 275 3947; e-mail address:
[email protected]. † University of Manchester. ‡ University of Leeds.
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sediment contaminant metal concentrations tend to decrease downstream from pollution sources (1-7). These patterns have been attributed to both hydrodynamic processes (dilution of contaminated by uncontaminated sediments, abrasion, hydraulic sorting, resuspension of contaminated sediment) (2, 8-11) and chemical processes (12). The role of the latter, in terms of integrated mineralogy and geochemistry, has been investigated to only a limited extent. Studies have focused largely on geochemistry, mineralogy being inferred by using chemical extraction procedures (e.g., refs 6, 13, and 14). Knowledge of the solid phases present and their textural characteristics constrains sources, sinks, and pathways of contaminant metals in fluvial systems. To this end, the aims of this study have been to (1) document the mineralogical forms of Pb, Zn, Cd, and Cu in mine-waste tips, contemporary overbank river sediment, mining-age alluvium, and suspended solids and precipitates from river waters of the Tyne Basin; (2) infer the formation processes of these minerals; and (3) evaluate the distribution of these minerals along the Tyne River and its tributaries and the implications of these findings for other metal-contaminated river systems. In this paper, ‘oxyhydroxide’ refers to oxides and hydroxides, and ‘contaminant metals’ refer to Pb, Zn, Cd, and Cu.
Study Area The Tyne River Basin (catchment area 2927 km2) drains the most productive area of the Northern Pennine lead-zincfluorite-baryte orefield in northeast England (15) (Figure 1). Mineralization is largely restricted to the major headwater tributaries of the South Tyne, but significant orebodies are also found in the lower reaches of the South Tyne between Haltwhistle and Hexham. These areas are underlain mainly by Carboniferous limestones, with the remainder of the basin underlain by sandstones, shales, and coal measures (Figure 1). Mineralization occurs as veins and replacement bodies (16, 17). Galena [PbS], sphalerite [ZnS], and, locally, chalcopyrite [CuFeS2] are the major sulfide ore minerals (16, 18-22) occurring with pyrite [FeS2] and nickeliferous pyrite, marcasite [FeS2], and pyrrhotite [FeS]. Gangue minerals include fluorite [CaF2], baryte [BaSO4], quartz/chalcedony [SiO2], ankerite [Ca(Mg,Fe,Mn)(CO3)2], siderite [FeCO3], calcite [CaCO3], and witherite [BaCO3] (18). Several secondary minerals formed by oxidation of the primary ore and non-ore minerals have been documented by Dunham (18). The oxidation of galena to cerussite [PbCO3] (23), anglesite [PbSO4] (20), and lesser pyromorphite [Pb5(PO4)3Cl]; sphalerite to smithsonite [ZnCO3] and hydrozincite [ZnCO3‚3Zn(OH)2]; chalcopyrite to malachite [Cu2CO3(OH)2] and azurite [Cu3(CO3)2(OH)2]; and the iron sulfides to ankerite, siderite, and goethite [FeO·OH] is widespread. Locally, cerussite, smithsonite, and particularly the oxidized iron products were found in sufficient concentrations to make them exploitable (16, 18). From the Roman occupation to the present day, 1.6 × 106 t of Pb concentrates containing 60-75% Pb, 2.9 × 105 t of Zn concentrates containing 56-60% Zn, and 1.6 × 103 t of Cu concentrates were produced from this area (18). Most of this production was from 1815 to 1920. Crude and often inefficient seventeenth to nineteenth century methods
0013-936X/96/0930-0072$12.00/0
1995 American Chemical Society
FIGURE 1. Drainage network and mining areas of the Tyne Basin, northeast England, showing location of contemporary overbank river sediment, mining-age alluvium, mine-waste tip, and water samples.
of ore extraction, sorting, and dressing resulted in a large input of metal-bearing and other mineral material into the Tyne Basin (7, 24). Attempts to control this pollution have been made through legislation (25) and through rehabilitating old mine workings and spoil heaps (7). Water quality data (26) show low ‘dissolved’ metal contents that do not exceed EC or WHO guidelines (7) except in streams immediately downstream of old mine workings (27-29). A basinwide survey of Pb, Zn, Cd, and Cu concentrations in contemporary overbank river sediment in the Tyne catchment was carried out by Newcastle University (30) from 1983 to 1986 to build on earlier and more localized work on sediment-associated metal in the Derwent Reservoir (31) and the River Team (32). This work and subsequent studies (6, 33) have revealed that sedimentborne metal contamination extends down the course of the entire river system (some 80 km) decreasing in intensity downstream. It was demonstrated that at present the principal source of contaminant metal in contemporary overbank river sediment is metal-contaminated alluvium, deposited in the late nineteenth and the first half of the twentieth century, during the peak of mining activity (29, 34-36). Macklin (7) estimated that more than 18 km2 of the alluvial valley of the Tyne River and its tributaries have soil Pb, Zn, and Cd concentrations above those considered acceptable by the U.K. Inter-Departmental Committee on the Redevelopment of Contaminated Land (37, 38). Preliminary chemical extraction investigations (6) on Pb-, Zn-, Cd-, and Cu-bearing contemporary overbank river sediment in the River Tyne suggests that these metals are associated with an iron/manganese oxyhydroxide phase, but little work has been done on the solid phases present.
Methods Four mine-waste tip samples, 13 contemporary overbank river sediment samples deposited during a flood on May 14, 1993, and four samples of mining-age alluvium were collected from the Rivers South Tyne and Tyne and their major tributaries (Figure 1) upstream from Newcastle upon Tyne. The samples were air-dried at 20 °C, sieved to
