Composition and arsenic-attenuating capacity of biogenic iron (hydr)oxide flocs at the Lava Cap Mine Superfund Site, Nevada County, CA.
Abstract
The Lava Cap Mine Site (LCMS) is on the National Priority List due to the elevated human health risk presented by the catastrophic release of several thousand cubic meters of arsenic (As) enriched tailings (average: 500 ppm As ) from the site. These tailings were released into a creek and lake (former tailings retention pond) in a low-density residential area where ground water is the primary source of drinking water. Although oxidation of iron (Fe) sulfides (pyrite and arsenopyrite) from tailings are the main sources of As and Fe, buffering by carbonate minerals prevents formation of acidic waters. Macroscopic accumulations of fluffy Fe (hydr)oxide are observed suspended in the water column or at the sediment-water interface in creeks, ponds, and seeps of the LCMS. Microscopic analysis indicates that the Fe (hydr)oxide is predominantly associated with the sheaths of bacteria identified as members of the genus Leptothrix, which are known to enzymatically oxidize Fe and manganese (Mn) under oligotrophic, near-neutral, sub oxic conditions. Both Fe- encrusted Leptothrix sheaths (which are largely devoid of cells) and free aggolmerations of Fe hydr(oxide) support morphologically distinct Eubacteria whose identity is currently under investigation. Dried biogenic Fe (hydr)oxide averages 4.4 % organic carbon, 20.2 % Fe, and 0.91% As (9100 ppm), making it attractive as a potential natural biosorbent for As and Fe. Water flow rate is a very important control on the amount of As retained in biogenic Fe (hydr)oxide flocs, based on monitoring of a natural passive bioreactor system. In addition, a pond with nearly stagnant water accumulated approximately one order of magnitude more As (dried) than a seep site with faster-running water, even though there was only a 5-fold difference in their median filtered (0.45 micron) arsenic concentrations. Most Probable Number estimates and analysis of PCR amplicons of Eubacterial DNA indicate that populations of Fe-, As-, and sulfate- reducing microorganisms are present within the Leptothrix-supported Fe (hydr)oxide flocs, but their abundances are low under the oxic conditions prevailing in surface creeks. These microbial populations could potentially re-mobilize As if prevailing conditions changed from sub- to anoxic, and if more organic carbon entered the system.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.B11B0351F
- Keywords:
-
- 0418 Bioremediation;
- 0448 Geomicrobiology;
- 0461 Metals;
- 0463 Microbe/mineral interactions;
- 0465 Microbiology: ecology;
- physiology and genomics (4840)