Retention and compartmentalization of lead and cadmium in wetland microcosms

Abstract

This report describes results of a 14 month study of the fate of trace-metal contaminants in wetland microcosms. Duplicated wetland microcosms contained either cattail, Typha domingensis or duckweed, Lemma minor, as the dominant macrophyte. Each wetland microcosm received 14.6 cm/day of dilute landfill leachate amended with lead (measured average: 396 μg/l) and cadmium (measured average: 105 μg/l). At intervals of approximately 3 months, sediments and macrophytes were sampled and analyzed for heavy metal content. Differences between measured trace-metal concentrations in inflow and outflow samples indicated that approximately half of the added lead and cadmium were retained in wetland microcosms. Of the fraction removed from the water, less than half could be accounted for in calculated sediment and plant tissue concentrations, based on analysis. The apparent discrepancy in mass balance calculations probably arises from failure to accurately measure bulk density of sediment detritus floc, which contained a substantial amount of the trace metals. Measurement of sediment acid volatile sulfides (AVS), and comparisons of these concentrations to those of the added lead and cadmium, showed that nearly all of these trace metals were present in the sediments as sulfides, limiting bioavailability and toxicity of these elements. Results of analyses and a lack of overt biological effects suggest that in wetlands containing organic sediments, sediment chemistry dominates cycling of these trace metals. Plant accumulation is of secondary importance. Comparison of heavy-metal uptake by cattails and duckweed shows that duckweed, on a whole plant basis, accumulates lead and cadmium more effectively than does cattail. If plants were to be used in a biomass-harvesting process for metal bioremediation, duckweed appears to be a more suitable candidate than cattail because of high productivity and ease of harvesting, coupled with a significant bioaccumulation of lead and cadmium. Potential metal removal rates by duckweed are 3–8 mg Pb/m 2 day and 2–4 mg Cd/m 2 day.