Three-dimensional reactive transport simulations were undertaken to study the sorption and degradation dynamics of three herbicides in a shallow aerobic aquifer with spatially variable pH during a 216 days injection experiment. Sorption of two phenoxy acids [(+/-)-2-(4-chloro-2-methylphenoxy) propanoic acid] (MCPP) and [(+/-)-2-(2,4-dichlorophenoxy)propanoic acid] (dichlorprop) was found to be negligible. Degradation of the phenoxy acids was rapid after an initial lag phase. Degradation of the phenoxy acids could only be reproduced satisfactorily by growth-linked microbial degradation. The model fit to the field data was slightly improved if degradation was assumed to be influenced by the local pH that was observed to increase with depth ( approximately 4.5--5.7). In the observed pH-range the nitroaromatic herbicide [2-Methyl-4,6-dinitrophenol] (DNOC) was partly dissociated (pK(a)=4.31) and present in both the neutral and ionized form. The model simulations demonstrated that most of the observed spatial variation in sorption of DNOC could be explained by assuming that only the neutral form of DNOC was subject to sorption. A varying flow field was observed during the injection experiment and the model simulations documented that this most likely resulted in different migration paths for DNOC and the non-sorbing solutes. The model simulations indicated that degradation of DNOC was an important process. The degradation rate of DNOC remained constant over time and was simulated adequately by first-order kinetics. Again, the model fit to field observation was slightly improved if local pH was assumed to influence the degradation rate. Only the maximum utilization rate was estimated from the field data, while the remaining degradation parameters where successfully transferred from the laboratory study.
[Show abstract][Hide abstract] ABSTRACT: A reactive transport model is developed to evaluate the potential of contam-ination of drinking water wells by surface water pollution. The model is validated using data of a tracer experiment. The fate of MCPP, glyphosate and its degradation product AMPA is investigated. Global sensitivity analysis using the Morris method is used to identify model dominant parameters. Results show that the existence of a clay aquitard, pollutant properties and the well depth are the crucial factors when evaluating the risk of drinking water well contamination from surface water.
[Show abstract][Hide abstract] ABSTRACT: Data obtained from a field study of an aquifer contaminated by landfill leachate and related laboratory experiments suggest that natural attenuation of phenoxy acid herbicides such as mecoprop (MCPP) occurs in the transition zone between the anaerobic plume core and the overlying aerobic water body. The location of this transition zone is assumed to be primarily controlled by vertical transverse dispersion processes occurring downstream of the pollution source. A reactive transport modeling study was carried out to evaluate this conceptual model. The transport was simulated for a two-dimensional vertical cross section to quantify the combined physical, geochemical, and microbial processes that affect the fate of the phenoxy acid herbicides. The simulations, showing removal of phenoxy acids, an increase of phenoxy acid degraders in the fringe zone, and a dependency of the results on vertical transverse dispersivity, are compatible with the hypothesis of fringe-controlled aerobic biodegradation of the phenoxy acids.
[Show abstract][Hide abstract] ABSTRACT: Laboratory batch and column experiments were performed to better understand the sorption and transport behaviour of commercial-grade methamidophos (Tamaron SL 600) in clay loam (CL) and sandy loam (SL) soils. The batch sorption experiments show that the soil texture and methamidophos concentration play a major role in the sorption and migration behaviour of methamidophos. At low surface coverage (q 0.6 mg g, the SL soil exhibits a much higher sorption affinity for methamidophos than the CL soil. The equilibrium isotherms for the sorption of methamidophos onto the SL and CL soils were non-linear, and were best described by the Freundlich equation. The results of column experiments indicate that the recovery of methamidophos during desorption was incomplete due to either partially irreversible sorption to high-energy surface sites or strongly rate-limited desorption. Methamidophos was more readily leached out from the SL soil column as consistent with the batch isotherm data.
International Journal of Environmental Analytical Chemistry 12/2006; 86(15):1127-1134. DOI:10.1080/03067310600922261 · 1.30 Impact Factor
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