Constructed wetlands have the potential to trap and remove metals in mine wastewater. To determine the effectiveness of constructed wetlands for treating selected heavy metals in neutral mine effluent typical of lead mines, eight laboratory-scale constructed wetlands were set up to treat a synthetic, slightly alkaline, mine water containing 34.2 mg/L sulfate (SO 4 2Ϫ ), 50 g/L lead (Pb), and 300 g/L zinc (Zn). After 45 days, one of the wetlands was switched to treat a synthetic smelter effluent with a much greater load of SO 4 2Ϫ , sodium (Na ϩ ), and Pb.Temperature, hydraulic loading, and substrate composition typically did not affect treatment efficiency. The pH of the effluent was reduced from 8.0 to 8.5 to near neutral. The average removal in the eight wetlands was 90% for Pb and 72% for Zn. In wetlands operating on synthetic mine water, SO 4 2Ϫ was completely removed, likely by conversion to sulfide by sulfate-reducing bacteria. In the wetland operating on synthetic smelter effluent, only approximately 25% of 6 g/L influent sulfate was removed, and a breakthrough period of 4 days for Na ϩ was observed. Whole effluent toxicity assays on undiluted wetland effluent from wetlands treating mine and smelter water had 100% survival of fathead minnows and Daphnia magnia. Survival of Ceriodaphnia dubia was zero in undiluted effluent, but 75 to 100% survival was observed when the effluent was diluted to one-half strength. Water Environ. Res., 73, 37 (2001).

Abstract--Fe 57 Mfssbauer spectroscopy has been used to determine the nature of iron-containing minerals in Lower Greensand samples from an experimental lysimeter at Uffington, Oxfordshire, both before and after a three-year irrigation with a synthetic heavy metal leachate. Analysis of the spectra measured at 300 ~ 77 ~ and 4.2~ and at 4.2~ in an applied field of 45 kOe shows that iron is present in the uncontaminated sandstone in fine-grained goethite (a-FeOOH) and glauconite. In the irrigated samples iron is precipitated as a fine-grained ferric hydroxide gel having values for the hyperfine field at 4.2~ of 435 and 470 kOe. The stability of the gel over the three-year period of irrigation may be explained by surface energy considerations.

Summary In Britain most groundwater resources are held in bedrock aquifers and the unsaturated zone forms an important buffer between landfill disposal sites and the water table. The processes which control the transfer of (1) persistent organic compounds, (2) inorganic anions and (3) heavy metals, in the unsaturated zone have been evaluated using experimental lysimeters constructed in the Lower Greensand at a site near Uffington, Oxfordshire. Complementary monolith and column experiments were conducted. Biodegradation is the primary mechanism for the attenuation of the inorganic anions and the organic species, but the migration of heavy metals is limited by interactions with the sediment. Sequential, selective chemical extraction indicates that the stability of the geochemical phases retaining heavy metals controls their rate of movement. Although the processes which regulate the partition of the heavy metals are complex, uptake by sesquioxides and clay minerals are particularly important.

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Unsaturated zone attenuation of contaminants common to landfill leachate, in particular heavy metals and carboxylic acids, has been studied in an in situ lysimeter constructed in calcareous clayey-sandstone (Lower Greensand), for a period of nearly 6 years. Considerable attenuation of heavy metals was found to be attributable to a combination of processes including adsorption and precipitation. Leaching with a blank carboxylic acid solution (volatile fatty acids containing no heavy metals) resulted in limited remobilization and the order of mobility of the metals (Ni > Cd > Zn > Cu > Cr > Pb) was found to be unchanged, with only nickel showing extensive remobilization. High heavy metal concentrations at the surface of the lysimeter were found to cause inhibition of carboxylic acid degradation. However at depths in the lysimeter, below the limit of heavy metal penetration, anaerobic decomposition rates of approximately 100 mg 1 -1 day -1 for acetate, propionate and butyrate were obtained by extrapolation to input concentrations of 5000 mg 1 -1 . The degradation of the carboxylic acids was found to be dependent upon the establishment of an adapted population of micro-organisms and a single pulse of carboxylic acids was released before leachate-degrading micro-organisms could colonize the sediment.