Superfund Research Program
Transport of Trace Metals in a Polluted Aquifer
Project Leader: Martha H. Conklin
Grant Number: P42ES004940
Funding Period: 1995 - 2000
- Project Summary
For the 1999-2000 period, researchers continued our efforts to elucidate the processes that control the movement of metals in the Pinal Creek Basin near Globe, AZ. The investigators have determined the reactions that control the release of Mn(II) to the aqueous phase from alluvial sediments in contact with the mining-waste plume, and have identified a reduced-manganese containing intermediate phase similar in structure to the mineral jacobsite (MnFe2O4). To do this, a flow-through reaction cell was developed to allow in situ, real-time spectroscopic data to be collected regarding complex geochemical reactions. Project investigators have also identified and quantified the major controls on the microbial oxidation of Mn(II) within the stream sediments (pH, Mn(II) concentration and dissolved oxygen), and they have shown that concentrations of other trace metals present in the system (Ni, Zn and Co) are greatly affected by the precipitating Mn oxides. Laboratory results for Mn(II) oxidation agree with field-scale data and residence time and pH in the sediments have been identified as the overall control for Mn(II) removal in Pinal Creek. In addition, aquatic vascular plants, algae, and moss have been shown to be effective scavengers of Mn and other metals (Zn, Ni, Co, Cu, and Fe). Bioaccumulation of the trace metals in these species are directly correlated to the Mn concentration. Field studies have shown that stream morphology is affected by the precipitation of Mn oxides that cement the stream sediments, as well as the growth of aquatic plants that widen the active channel and decrease the average flow velocity. This process is interrupted by flood events that scour the cemented sediments as well as the vegetation approximately once every three years, renewing the cycle. Field work has also defined diel cycles involved in the uptake of Mn, Zn, and Ni and indicates that lateral flows into the banks of the creek have a diel variation that must be considered in future model development and data analysis. Synthesis of laboratory and field work and modeling during this year has generated a more complete description of fundamental and field-scale processes controlling the movement of metal contaminants through a polluted aquifer.