Fe(III) oxide is an important heavy-metal sink, and bacteria are responsible for much of the Fe(III) reduced in nonsulfidogenic aquatic environments, yet factors governing the bacterial reduction of heavy metal-contaminated iron oxide are largely unknown. In this study with a stabilized bacterial consortium enriched from metal-contaminated sediments, we demonstrate that Cu toxicity impedes anaerobic carbon oxidation and bacterial reduction of hydrous ferric oxide (HFO). In the enrichment culture, a Clostridium sp. fermented lactate to propionate and acetate and Fe(III) reducers coupled acetate oxidation to HFO reduction. Increasing the amount of Cu in the culture medium significantly extended the time before Fe(III) reduction occurred and decreased the reduction rate, but did not affect the extent of HFO reduction. The Clostridium had a higher Cu-complexation capacity than the Fe(III) reducer Shewanella alga. Iron reduction was inhibited until almost all of the dissolved Cu was removed from solution and occurred two to seven times faster if the sediment enrichment culture was fed lactate instead of acetate. Our findings suggest that fermentative bacteria play a role in ameliorating heavy metal toxicity to iron-reducing bacteria. Fermenters may therefore enhance metal release in sediments by facilitating the bacterial reductive dissolution of heavy metal-contaminated HFO.

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For most chemicals, evaluation of ecological risk typically does not address inhalation because ingestion dominates exposure. However, burrowing ecological receptors have an increased exposure potential from inhalation at sites contaminated with volatile chemicals in the subsurface. Evaluation of ecological risk from contaminants like volatile organic chemicals (VOCs) is constrained by a lack of relevant ecological screening levels (ESLs). To address this need, inhalation ESLs were developed for 16 VOCs: Acetone, benzene, carbon tetrachloride, chloroform, chloromethane, dichlorodifluoromethane, 1,1‐dichloroethane, 1,2‐dichloroethane, 1,1‐dichloroethene, methylene chloride, tetrachloroethene, toluene, 1,1,1‐trichloroethane, trichloroethene, trichloro‐fluoromethane, and total xylene. These ESLs are based on Botta's pocket gopher (Thomomys bottae) as a representative fossorial receptor. The ESLs are presented with an emphasis on the process for developing inhalation toxicity reference values to illustrate the selection of suitable toxicity data and effect levels from the literature. The resulting ESLs provide a quantitative method for evaluating ecological risk of VOCs through comparison to relevant exposure data such as direct burrow‐air measurements. The toxicity reference value development and ESL calculation processes and assumptions detailed here are provided as bases from which risk assessors can use or refine to suit site‐specific needs with respect to toxicity and exposure inputs.

The weathering of coal combustion products (CCPs) in a lotic environment was assessed following the Tennessee Valley Authority (Kingston, TN) fly ash release of 2008 into surrounding rivers. Sampled materials included stockpiled ash and sediment collected from 180 to 880 days following the release. Total recoverable concentrations of heavy metals and metalloids in sediment were measured, and percent ash was estimated visually or quantified by particle counts. Arsenic and selenium in sediment were positively correlated with percent ash. For samples collected 180 days after the release, total concentrations of trace elements downstream of the release were greater than reference levels but less than concentrations measured in stockpiled ash. Total concentrations of trace elements remained elevated in ash-laden sediment after almost 2.5 years. A sequential extraction procedure (SEP) was used to speciate selected fractions of arsenic, copper, lead, nickel, and selenium in decreasing order of bioavailability. Concentrations of trace elements in sequentially extracted fractions were one to two orders of magnitude lower than total recoverable trace elements. The bulk of sequentially extractable trace elements was associated with iron-manganese oxides, the least bioavailable fraction of those measured. By 780 days, trace element concentrations in the SEP fractions approached reference concentrations in the more bioavailable water soluble, ion exchangeable, and carbonate-bound fractions. For each trace element, the percentage composition of the bioavailable fractions relative to the total concentration was calculated. These SEP indices were summed and shown to significantly decrease over time. These results document the natural attenuation of leachable trace elements in CCPs in river sediment as a result of the loss of bioavailable trace elements over time.

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