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Nedbrydning og sorption af dichlobenil og BAM-litteraturopsamling samt laboratorieforsøg

Summary and conclusions

The present report concludes the subproject of The Danish Environmental Protection Agency project "Pesticides and Waterworks" and contains the results of an investigation of sorption and degradation of the herbicide dichlobenil (2,6-dichlorobenzonitrile) and its degradation product BAM (2,6-dichlorobenzamide). BAM is found in 24% of the water supply wells in Denmark, and therefore BAM has caused a large problem for waterworks. Data such as sorption coefficients and degradations rates are therefore important for the prediction of the environmental impact of BAM.

First, the existing knowledge of the compounds was reviewed. Information of the herbicide chlorthiamid (2,6-dichlorothiobenzamide), another parent compound of BAM, was included. The review shows that the sorption of chlorthiamid is limited, whereas the degradation potential is high. In Denmark, chlorthiamid has not been used for more than 20 years, and therefore detection of the compound in soil and groundwater is unlikely. As a consequence, the compound was not been included in the experimental work. The review shows a lack of data on the sorption of dichlobenil at concentration levels relevant in aquifers. Furthermore, it is unclear how dichlobenil is degraded, how fast the degradation proceeds, and whether or not dichlobenil accumulates in the soil zone. With regards to BAM, data from the literature is generally limited.

In order to enable reasonably accurate predictions of the long-term effects of BAM contamination, corresponding sorption- and degradation experiments with dichlobenil and BAM have been carried out. The main purposes of these experiments are the determination of distribution coefficients and half-lives at relevant concentration levels and to investigate the interactions between sorption and degradation as well as the influence of sediment characteristics and water chemistry.

Sediment and groundwater were collected from six sites in Denmark. These sites were chosen in order to represent a broad range of typical Danish sediments. Two sites represent glacial sand deposits (Staurbyskov and Eskærhøj), three sites represent till deposits (Strøby Egede, Avedøre, and Kirke Syv), and one site represents a chalk aquifer covered by till (Hvidovre). The sediments were characterised with regards to surface area, total organic carbon, and grain size distribution. Groundwater for the experiments was characterised with regards to content of cations and anions, dissolved oxygen, pH, conductivity, and dissolved organic carbon. The sorption experiments were carried out as batch experiments, and the sorption was measured by scintillation counting on ¹⁴C-labelled dichlobenil and BAM. Degradation experiments were batch incubations, where the degradation over time was measured by scintillation counting on ¹⁴C-labelled dichlobenil and BAM, and for incubations with dichlobenil also by measuring BAM with a quantitative enzyme-linked immunoassay done by GEUS.

The sorption experiments with dichlobenil and BAM show that both dichlobenil and BAM sorb to sediments from the unsaturated and saturated zone. The sorption of dichlobenil and BAM in sandy sediments depends primarily on the organic matter content. For dichlobenil the following correlation between the linear distribution coefficient (K_d) and the total organic carbon content (TOC) is obtained: Kd,Dic (L/kg) = 9.0 · TOC (wt. %), R2 = 0.93, whereas the similar correlation for BAM is: Kd,BAM (l/kg) = 0.33 · TOC (wt. %), R2 = 0.92. For dichlobenil, the measured K_d-values in sandy aquifers correspond to retardation factors between 2.2 and 8.6 and for BAM the measured K_d-values correspond to retardation factors between 1.2 and 1.6. In clays, a higher sorption of both dichlobenil and BAM was found, corresponding to retardation factors between 22 and 984 for dichlobenil and between 1.5 and 8.3 for BAM. The sorption in this sediment was much higher than expected from the correlation between K_d and TOC for sandy sediments, and therefore the sorption in anaerobic clay may primarily be controlled by the clay mineralogy or alternatively by a different composition of the organic matter.

The dichlobenil degradation experiments show that the degradation of dichlobenil to BAM primarily will take place in the first few meters below surface. However, in the top soil (0-0.25 mbs) the BAM formation is slow, in particular in the sediment from Staurbyskov where only 5% of the dichlobenil was found as BAM after 436 days. The reason for this can be, that the degradation is inhibited by the large sorption of dichlobenil in this depth interval. In the sediments from 0.25-0.75 mbs the formation of BAM is fast with up to 88% of the dichlobenil degraded to BAM within 436 days. In sediments from below 4 mbs the degradation of dichlobenil is limited (<2%) and in most saturated sediments the dichlobenil degradation is insignificant. In general therefore the degradation rate decreases rapidly with depth. The dichlobenil degradation is well described by both 0.- and 1.-order rate law equations, but in general the 1.-order equation yields a slightly better correlation. In the control incubations, the dichlobenil degradation is insignificant. This strongly indicates a microbially mediated hydrolysis. The lack of a lag-phase indicates a co-metabolic reaction.

For incubations with BAM the ¹⁴C-koncentration was measured over time. Because the side chain is ¹⁴C-labelled, loss of ¹⁴C activity from the water phase can be caused by sorption or mineralisation of the side chain resulting in ¹⁴CO₂ formation. To be able to distinguish between the effect of sorption and the effect of degradation, the theoretical concentrations in the water phase at equilibrium, are calculated from the K_d-values determined in the sorption experiments. In most experiments a good agreement between calculated and observed concentrations was obtained, with the exception of controls where the autoclaving affected the sorption capacity. Therefore, in most cases a significant degradation of BAM cannot be detected. However, there is a small but significant degradation of BAM in the upper 0.75 m of the sediments from Staurbyskov and Eskærhøj and in the sediment 4.96-6.18 mbs from Staurbyskov. The degradation of BAM is probably microbial catalysed, because no degradation was observed in the control incubations. The degradation of BAM in these sediments has as a rough estimate been calculated as the relative difference between the expected concentration in the water phase after sorption equilibrium and the measured ¹⁴C-concentration after 436 days. The estimated DT₅₀-values is in the range 3-16 years, with the lowest DT₅₀-value in the top soil (0-0.25 mbs), after which the DT₅₀-value increases with depth.