The biogeochemical processes controlling the reductive transformation of contaminants in an anaerobic aquifer were inferred from the relative reactivity patterns of redoxsensitive probe compounds. The fate of five nitroaromatic compounds (NACs) was monitored under different redox conditions in a landfill leachate plume of a sandy aquifer. Results of field experiments (continuous injection and in situ microcosms) were compared to the findings of laboratory batch and column experiments (using aquifer matrix and model systems for sulfate-and iron-reducing conditions). NACs were transformed within 2-70 days in the leachate plume as well as in microbially active and in microbially deactivated experiments. Generally, aromatic amines were the predominant reduction products, and these compounds were stable within the time frame and under the conditions of our experiments. Despite the presence of various potential reductants (e.g., H 2 S/HS -, Fe(II) aq , reduced organic matter, microorganisms), the patterns of relative reactivity of the probe compounds indicated that ferrous iron associated with iron(III) (hydr)oxide surfaces was the dominant reductant throughout the anaerobic region of the plume. Our results suggest that Fe(II) associated with ferric iron minerals is a highly reactive reductant in anaerobic aquifers, which may also determine the fate of other classes of reducible contaminants such as halogenated solvents, azo compounds, sulfoxides, chromate, or arsenate.

Abstract. A continuous, natural gradient, field injection experiment, involving six herbicides and a tracer, was performed in a shallow aerobic aquifer near Vejen, Denmark. Bentazone, (_+)-2-(4-chloro-2-methylphenoxy) propanoic acid (MCPP), dichlorprop, isoproturon, and the dichlobenil metabolite 2,6-dichlor-benzamide (BAM) were injected along with 2-methyl-4,6-dinitrophenol (not discussed in this paper) and the tracer bromide. The injection lasted for 216 days and created a continuous plume in the aquifer. The plume was monitored in three dimensions in 96 multilevel samplers of 6-9 points each for 230 days, with selected individual points for a longer period. The bromide plume followed a complex path through the monitoring network downgradient of the injection wells. The plume movement was controlled by spatially varied hydraulic conductivities of the sand deposit and influenced by asynchronous seasonal variation in groundwater potentials. An average flow velocity of 0.5 m/d was observed, as depicted by bromide. Bentazone, BAM, MCPP, and dichlorprop retardation was negligible, and only slight retardation of isoproturon was observed in the continuous injection experiment and a preceding pulse experiment. No degradation of bentazone was observed in the aerobic aquifer during the monitoring period. BAM and isoproturon were not degraded within 5 rn downgradient of the injection. The two phenoxy acids MCPP and dichlorprop were both degraded in the aerobic aquifer. Near the source a lag phase was observed followed by fast degradation of the phenoxy acids, indicating growth kinetics. The phenoxy acids were completely degraded within 1 rn downgradient of the injection wells, resulting in the plumes being divided into small plumes at the injection wells and pulses farther downgradient. During the lag phase, phenoxy acids had spread beyond the 25 rn long monitoring network. However, the mass of the phenoxy acids passing the 10-25 rn fences never matched the corresponding bentazone or bromide masses, and the pulse was observed to shrink in size. This indicates that this pulse of phenoxy acids was being partially degraded at a low rate as it traveled through the aquifer.

The potential for aerobic biotransformation of the herbicides MCPP [(+/ ‐)‐2‐(4‐chloro‐2‐methylphenoxy)propionic acid] and Atrazine [2‐chloro‐4‐ethylamino‐6‐isopropylamino‐s‐triazine] under natural aquifer conditions was investigated. A natural gradient experiment was carried out in an aerobic ground‐water zone at the Canadian Forces Base Borden, Borden, Ontario, Canada. Over 90 days, a continuous plume was created by weekly injections of the herbicides (average concentrations of 400 jug/1 each), together with a conservative tracer, chloride. The fate of the herbicides was monitored at multilevel piezometers located downgradient from the injection well. In parallel, experiments with in situ testers (Gillham et al., 1990) were set up in the aquifer and microcosms were set up in the laboratory. For MCPP, the natural gradient experiment showed an adaptation period of 42‐56 days followed by transformation from 400 to 30 μg/1. As predicted, MCPP was not retarded compared to the conservative tracer. No transformation was observed in the in situ testers over 47 days. In the laboratory experiments no transformation was observed over 74 days. Atrazine was recalcitrant within the experimental period of 96 days in the natural gradient experiment. A retardation factor R = 1.2, within the predicted range, was found for Atrazine in the Borden aquifer. No transformation was observed either in the in situ testers or in the laboratory experiments after a period of 96 and 74 days, respectively. These results suggest that both MCPP and Atrazine are mobile in sand aquifers. Atrazine appears persistent, at least for 96 days, while MCPP undergoes considerable transformation.

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