Effects of subsurface heterogeneity on natural bioremediation at a gasoline spill site.

A test cell of 3-m by 6-m located at the mid-point of a gasoline spill site was selected to test the hypothesis that the rate of hydrocarbon biodegradation is influenced by the spatial distribution of the electron acceptors, aqueous geochemistry, and microbial population. Multilevel samplers (MLSs) were installed at four corners of the test cell for groundwater sampling. Sampling ports were placed at 0.3-m intervals from 1.5 to 4.8 m below land surface (bls). A 0.91-m by 12.7-cm sediment core (from 3.3 to 4.2 m bls) in the center of the MLSs was collected. The core was cut into 7 sections, and each was used for sediment extractions, microbial enumeration, grain size distribution, and microcosm studies. Groundwater analytical results indicate that iron reduction was the dominant biodegradation process within this test cell. Iron-reducing process caused the preferential removal of certain compounds. Microbial enumeration results show that the distribution of microbial population varied with depth and sediment materials. Lower microbial population was observed in those sections with higher portion of clayey materials. The less permeable materials would limit the bacterial transport, decrease the bioavailability of Fe(III) to iron-reducing bacteria, and thus cause the low biodegradation activity. Results suggest that using blended sediments for biodegradation rate measurements may provide misleading results.

Mineralization of 1,4-dioxane in the presence of a structural analog.

A mixed culture with the ability to aerobically biodegrade 1,4-dioxane in the presence of tetrahydrofuran (THF) was enriched from a 1,4-dioxane contaminated aquifer. This consortium contained 3-4 morphologically different types of colonies and was grown in mineral salts media. Biodegradation of 1,4-dioxane began when THF concentrations in batch experiments became relatively low. No biodegradation of 1,4-dioxane was observed in the absence of THF and the measured cell yield was similar during degradation of 1,4-dioxane with THF or with THF alone. However, when the consortium was grown in the presence of 14C-1,4-dioxane plus THF, 2.1% of the radiolabeled 1,4-dioxane was present in the particulate fraction. The majority of the 14C (78.1%) was recovered as 14CO2, while 5.8% remained in the liquid fraction. This activity is interesting since the non-growth substrate is mineralized, yet only minimally assimilated into biomass. Using THF as the growth substrate, the consortium also degraded 1,3-dioxane, methyl t-butyl ether, ethyl t-butyl ether and t-amyl methyl ether.

Encoding and retrieval effects of dual sensory-semantic cues.

This experiment was designed to explore the relative effects of dual and embedded sensory-semantic retrieval cues (e.g.,GLAMOUR GIRL). Cues and targets shared both first-letter and associative features, only first-letter features, or only associative features. The cues were presented during the single test trial or they were presented during the single study trial and again at test. Finally, subjects in the dual-cue condition were or were not told about the addition of the first-letter contingency. The results indicated that dual were superior to single cues with each feature contributing independently and additively to recall. This dual facilitation was obtained when the cues were shown only at test or during both study and test and regardless of whether or not the subjects were explicitly aware of the additional sensory contingency. These findings were expected and explained by the assumptions of the coding-access-recognition model.

Sensory features and the reduction of imaginal interference.

Interference generated among the sensory features of PA stimulus words is not eliminated by processing at the imaginal level. This experiment was planned to test the reverse possibility. Will interference generated by similar imaginal referents be eliminated by sensory overlap between the stimuli and their responses? Thus, stimulus words did or did not evoke similar images, pairs did or did not share initial letters, and lists were acquired either under an instructional set t o form interacting images or to repeat the items aloud. The results indicated that the imagery strategy was superior to the repetition strategy in all conditions, and that stimulus interference was statistically eliminated when pairs shared the same initial letters. The findings were interpreted as consistent with a levels of processing conceptualization.