Kirsten Rügge

Fate of seven pesticides in an aerobic aquifer studied in column experiments.

The fate of selected pesticides (bentazone, isoproturon, DNOC, MCPP, dichlorprop and 2,4-D) and a metabolite (2,6-dichlorobenzamide (BAM)) was investigated under aerobic conditions in column experiments using aquifer material and low concentrations of pesticides (approximately 25 microg/l). A solute transport model accounting for kinetic sorption and degradation was used to estimate sorption and degradation parameters. Isoproturon and DNOC were significantly retarded by sorption, whereas the retardation of the phenoxy acids (MCPP, 2,4-D and dichlorprop), BAM and bentazone was very low. After lag periods of 16-33 days for the phenoxy acids and 80 days for DNOC, these pesticides were degraded quickly with 0.-order rate constants of 1.3-2.6 microg/l/day. None of the most probable degradation products were detected.

Characterization of an old municipal landfill (Grindsted, Denmark) as a groundwater pollution source: landfill hydrology and leachate migration

The migration of leachate from an old municipal landfill (Grindsted, Denmark) was investigated by intensive mapping of groundwater potentials and groundwater quality at the downstream borders of the landfill and beneath the landfill. A groundwater mound controlling the migration of the leachate into the aquifer was observed beneath the landfill, especially after the wet season The lateral spreading in the leachate plume was significantly increased due to the water table mound present and the seasonal variations of the mound. Also, a significant vertical transport of leachate in the aquifer was observed below the landfill. Detailed informa tion about the spatial and seasonal changes in groundwater flow directions and location of intensive leaching is needed in order to design a cost-effective delineation of the plume in the downstream aquifer and to establish appropriate remedia tion.

Fate of herbicides in a shallow aerobic aquifer: A continuous field injection experiment (Vejen, Denmark)

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 m 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 l m 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 m long monitoring network. However, the mass of the phenoxy acids passing the 10–25 m 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.

An anaerobic field injection experiment in a landfill leachate plume, Grindsted, Denmark: 1. Experimental setup, tracer movement, and fate of aromatic and chlorinated compounds

A continuous, natural-gradient field injection experiment, involving 18 xenobiotic compounds and bromide as tracers, was performed in the anaerobic region of a leachate plume downgradient from the Grindsted Landfill, Denmark. The injection lasted for 195 days, and within this period a continuous cloud was established. Over a period of 924 days the cloud movement was monitored in approximately 70 discrete sampling points in the central part of the cloud, and the spatial distribution was described by seven cloud snapshots involving 400–700 sampling points. The bromide cloud movement closely followed the varying flow direction predicted by the water table measurements. Moment analysis showed decreasing tracer flow velocities and reduced capture of bromide mass with time, which may be explained by varying flow conditions (direction, hydraulic gradient) and the heterogeneous geological conditions in the sandy aquifer. Naphthalene, having the highest log Kow value, was the most retarded compound, with a retardation of less than 10%. Therefore sorption was not considered to be a significant attenuation process for any of the compounds studied. Transformation under iron-reducing conditions was observed for toluene, o-xylene, TeCM, 1,1,1-TCA, PCE, and TCE, while transformation of benzene and napthalene was not detected in the aquifer within the time frame of this study. First-order transformation rates were in the range of 0.028–0.039 d−1 and 0.0014–0.0028 d−1 for the aromatic compounds toluene and o-xylene, respectively. The rates for the chlorinated aliphatic compounds, tetrachloromethane, 1,1,1- trichloroethane, tetrachloroethylene, and trichloroethylene, were >0.7 d−1, 0.0044–0.0054 d−1, 0.0012–0.0038 d−1, and 0.0003–0.001 d−1, respectively. Long lag periods and slow transformation rates were observed for some of the compounds, suggesting that lack of transformation reported in the literature may be attributable to short experimental periods in those studies.

Fate of MCPP and atrazine in an anaerobic landfill leachate plume (Grindsted, Denmark)

The fate of the two pesticides MCPP (mecoprop) and atrazine was investigated in an anaerobic aquifer downgradient from the Grindsted Landfill, Denmark, by means of a field injection experiment, in situ microcosms and laboratory batch experiments. The redox conditions in the experiments were dominated by iron reduction, but also methanogenesis and sulfate reduction occurred closest to the landfill. No degradation nor sorption was observed for the two pesticides in any of the biologically active experiments. In microbially inhibited laboratory experiments, supplied with sodium azide as a sterilizing agent, a chemical degradation of atrazine occurred.

Degradation of the (R)- and (S)-enantiomers of the herbicides MCPP and dichlorprop in a continuous field-injection experiment.

An aerobic field-injection experiment was performed to study the degradation and migration of different herbicides at trace levels in an aerobic aquifer at Vejen, Denmark. Mecoprop (MCPP) and dichlorprop monitored in a dense network of multilevel samplers were both degraded within a distance of 1 m after a period of 120 days. The study showed that no preferential degradation of the (R)- and (S)-enantiomers of MCPP and of dichlorprop took place as the enantiomeric forms of the phenoxy acids were degraded simultaneously in the aquifer.

An anaerobic field injection experiment in a landfill leachate plume, Grindsted, Denmark: 2. Deduction of anaerobic (methanogenic, sulfate‐, and Fe (III)‐reducing) redox conditions

Redox conditions may be environmental factors which affect the fate of the xenobiotic organic compounds. Therefore the redox conditions were characterized in an anaerobic, leachate-contaminated aquifer 15–60 m downgradient from the Grindsted Landfill, Denmark, where an field injection experiment was carried out. Furthermore, the stability of the redox conditions spatially and over time were investigated, and different approaches to deduce the redox conditions were evaluated. The redox conditions were evaluated in a set of 20 sediment and groundwater samples taken from locations adjacent to the sediment samples. Samples were investigated with respect to groundwater chemistry, including hydrogen and volatile fatty acids (VFAs) and sediment geochemistry, and bioassays were performed. The groundwater chemistry, including redox sensitive species for a large number of samples, varied over time during the experimental period of 924 days owing to variations in the leachate from the landfill. However, no indication of change in the redox environment resulting from the field injection experiment or natural variation was observed in the individual sampling points. The methane, Fe(II), hydrogen, and VFA groundwater chemistry parameters strongly indicated a Fe(III)-reducing environment. This was further supported by the bioassays, although methane production and sulfate-reduction were also observed in a few samples close to the landfill. On the basis of the calculated carbon conversion, Fe(III) was the dominant electron acceptor in the region of the aquifer, which was investigated. Because of the complexity of a landfill leachate plume, several redox processes may occur simultaneously, and an array of methods must be applied for redox characterization in such multicomponent systems.

Attenuation of Organic Pollutants in Redox of Landfill Leachate Plumes

Organic pollutants are considered to to constitute the major risk to the groundwater quality of old landfills. Studies at two Danish landfills indicate that natural attenuation is substantial, limiting the distribution of the organic pollutants to a few hundred meters distance from the landfills. Detailed mapping of the redox zones downgradient from the landfills indicates that the major attenuation takes place in the anaerobic parts of the plumes, in particular in the iron reducing zone. Dilution and sorption cannot account for the observed fate of dissolved carbon and specific organic compounds, indicating that degradation is the major attenuation process. Microbial bioassays and sediment analysis support the observations.