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

Abstract

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.

Stimulation of aerobic degradation of bentazone, mecoprop and dichlorprop by oxygen addition to aquifer sediment

“…No dichlorprop degradation was observed under aerobic conditions with any of the groundwater samples from the monitoring or drinking water abstraction wells ( Figure 2C). This persistence of dichlorprop is in accordance with the findings of Pedersen (2000) at an initial concentration of 50 µg L -1 under aerobic conditions, but it contrasts with the complete degradation of dichlorprop in a laboratory column experiment (Tuxen et al 2000) and a continuous field injection experiment (Broholm et al 2001) in an aerobic aquifer.…”

“…Bentazone mineralisation was lower than 1% in all the groundwater samples from Well I and Well II, but this was higher than in the autoclaved control incubations (0.25%), thus indicating microbial mineralisation. Even though bentazone is usually considered to be recalcitrant in aquifers (Broholm et al, 2001), it was significantly removed at low concentrations in laboratory experiments with groundwater only. Thus, groundwater provides degradation potential.…”

“…For instance, bentazone is biodegradable in topsoils under aerobic conditions (Huber and Otto, 1994;Leistra et al, 2001;Li et al, 2008;Rodriguez-Cruz et al, 2008;Larsbo et al, 2009), but it appears to be recalcitrant in aquifers under aerobic conditions (Tuxen et al, 2000;Broholm et al, 2001). In contrast, the degradation of mecoprop and dichlorprop has been observed both in topsoil (Smith, 1989) and in aquifers under aerobic conditions (see the reviews of Buss et al, 2006;Reitzel et al, 2004;Janniche et al, 2010).…”

In Situ Exposure to Low Herbicide Concentrations Affects Microbial Population Composition and Catabolic Gene Frequency in an Aerobic Shallow Aquifer

“…In a natural gradient field injection experiment, a part of the Vejen aquifer (Denmark) was continuously exposed to herbicide concentrations of Ͻ40 g l Ϫ1 (8) (equivalent to Ͻ7 g kg Ϫ1 of sediment). After an initial lag phase of 80 to 100 days, fast degradation of mecoprop and dichlorprop was observed and herbicide concentrations decreased to below detection levels within the first meter from the injection wells (8). This indicated that the microbial community acclimated to the contamination.…”

Removal kinetics of organic compounds and sum parameters under field conditions for managed aquifer recharge

“…Within the NASRI project 7 retardation factors were determined during column experiments (Jekel, 2006, Licht et al, 2005. Referenced values for comparable conditions (sandy aquifer, low sedimentary organic carbon) are used for further 6 parameters (Braids, 2001, Broholm et al, 2001, Buss et al, 2006, Grischek et al, 1997, Heberer and Jekel, 2006, Jørgensen et al 1998, Mersmann et al, 2003, Scheytt et al, 2006, Stuyfzand et al, 2007, Tuxen et al, 2000. Eight retardation values are applied based on similarity considerations regarding the substances' structural characteristics (Table 2).…”