Sources and fates of heavy metals in a mining-impacted stream: temporal variability and the role of iron oxides

Abstract

Heavy metal contamination of surface waters at mining sites often involves complex interactions of multiple sources and varying biogeochemical conditions. We compared surface and subsurface metal loading from mine waste pile runoff and mine drainage discharge and characterized the influence of iron oxides on metal fate along a 0.9-km stretch of Tar Creek (Oklahoma, USA), which drains an abandoned Zn/Pb mining area. The importance of each source varied by metal; mine waste pile runoff contributed 70% of Cd, while mine drainage contributed 90% of Pb, and both sources contributed similarly to Zn loading. Subsurface inputs accounted for 40% of flow and 40-70% of metal loading along this stretch. Streambed iron oxide aggregate material contained highly elevated Zn (up to 27,000 μg g(-1)), Pb (up to 550 μg g(-1)) and Cd (up to 200 μg g(-1)) and was characterized as a heterogeneous mixture of iron oxides, fine-grain mine waste, and organic material. Sequential extractions confirmed preferential sequestration of Pb by iron oxides, as well as substantial concentrations of Zn and Cd in iron oxide fractions, with additional accumulation of Zn, Pb, and Cd during downstream transport. Comparisons with historical data show that while metal concentrations in mine drainage have decreased by more than an order of magnitude in recent decades, the chemical composition of mine waste pile runoff has remained relatively constant, indicating less attenuation and increased relative importance of pile runoff. These results highlight the importance of monitoring temporal changes at contaminated sites associated with evolving speciation and simultaneously addressing surface and subsurface contamination from both mine waste piles and mine drainage.

Keywords: Heavy metals; Iron oxides; Metal loading; Mine drainage; Mine waste piles; Sequential extractions.

Figure 1

Map of sampling locations. Shaded areas show outline of mine waste piles, based on an aerial photograph.

Figure 2

Chemical constituents in Tar Creek water samples. (a)-(d) Total acid soluble and dissolved metal concentrations and total acid soluble metal fluxes; (e) pH and acid neutralizing capacity (ANC); and (f) dissolved SO₄²⁻ and Cl⁻ concentrations. Locations of surface inputs of mine waste pile runoff (R), mine drainage (M), and Lytle Creek (LC) are indicated with arrows showing their junctions with Tar Creek. The shaded area shows Stretch #2, visibly impacted by iron oxides. Note differences in scales of y-axes.

Figure 3

Contributions to water flow and metal fluxes originating from upstream Tar Creek (as measured at location A), surface and subsurface inputs along Stretches #1 and #2. Horizontal bars separate Stretches #1 and #2. Iron flux decreased slightly (0.18 kg day⁻¹) along Stretch #1; for all other stretches, increases in flux and flow rate were observed. Note differences in scales of y-axes.

Figure 4

Backscatter field-emission scanning electron microscope image and x-ray maps of iron oxide aggregate material filtered from Tar Creek. A detrital quartz core serves as a host for iron oxides precipitated on its surface. X-ray maps note two geochemically distinct iron oxide phases. Note 6-micron scale bar in BSE image. Regions (a) and (b) are described in the text.

Figure 5

Sequential extraction results for (a) Fe, (b) Mn, (c) Pb, (d), Zn, and (e) Cd at locations E and H, shown as a percentage of total extracted. Data represent the average of three replicate extractions from each location. Total extracted concentrations are presented under the columns.

Figure 6

Dissolved metals and other water quality parameters in (a) mine drainage discharge and (b) mine waste pile runoff from the 1980s and 2000s (current study). Comparisons between 1980s and 2000s data were evaluated using the Mann-Whitney U test. Significance level: * (p<0.05), ** (p<0.01), *** (p<0.001), n.s. (not significant). SpC = specific conductance, ANC = acid neutralizing capacity. In the absence of ANC data, alkalinity data were used when available. Statistical comparisons were not conducted for Fe, Pb, and Ni in MWP runoff due to data limitations described in text.