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TOC levels in soils

The most abundant OTCs in soils are PAHs; the concentration of the sum of 16 congeners varies from 182 to 1,045 µg/kg DM. PCBs content is lower, at around a few µg/kg DM. Other OTCs were only measured in 2006 and 2007, at around 100 µg/kg DM in 2006 and at a trace level in 2007. No significant effect from the treatments is observed. Neither is any accumulation observed for PAHs and PCBs, which have been monitored since the start of the field experiment.

OTC concentrations in soils in 2006

OTC concentrations in the soils in 2006 are shown in Figure 1. Despite the significant flows of certain compounds, particularly phthalates and LAS, there is no significant difference in concentrations in the soils between the treatments after four spreadings. The cause of this may be the degradation of these compounds in the soils. This is because the most abundant compounds in the composts are those with the shortest half-lives.


Figure 1. TOC concentrations in µg/kg DM in the soils in 2006, after four spreadings (n=4, 4 plots per treatment

PAHs are the most abundant OTCs in the soil for all treatments. The concentration of the sum of the 16 compounds varies from 182 to 1,045 µg/kg DM. LAS and DEHP are frequently detected, but at concentrations of below 250 µg/kg DM. DEPs and DIBPs, two other phthalates, are detected at less than 17 µg/kg DM. The sum of the seven PCBs is lower still, at between 3 and 7 µg/kg DM. Lastly, the other phthalates (DMP, DEP, DINP, DIDP) and nonylphenol are not detected.
In 2007, other soil analyses show a similar level of PAHs and PCBs, also phthalates and LAS are not detected. These OTC concentrations are consistent with data in the literature.

PAHs and PCBs profiles

4- to 6-ring PAHs are the most abundant in the soil. Fluoranthene, benzo(b)fluoranthene and benzo(ghi)perylene are the most abundant. Each represents 12% to 15% of the total of the 16 PAHs.
PCB 138 is the most abundant of the 7 PCBs measured. It represents 38% of the PCB total.

Change in PAHs and PCBs concentrations

PAHs and PCBs have been regularly measured since the start of the field experiment. This makes it possible to monitor their change in the soil and verify that there is no accumulation as spreadings were carried out.
Figure 2 shows the PAHs concentrations from 1998 to 2006. )


Figure 2. Change in the concentration of the sum of PAHs in the soil (µg/kg DM) from 1998 to 2006 (n=4, 4 plots per treatment).

There are variations in PAHs concentrations in two-year intervals, although there is no relation with the treatments. At the start of the test there was a variation in plots, with higher concentrations in block 3, which persisted over time.
For PCBs, no significant difference was observed between the treatments. The temporal effects are more significant. A significant decline is observed between 1998 and 2006. The concentrations of the sum of the seven PCBs falls from 30 to 5 µg/kg DM during this period. Two hypotheses can be proposed to explain this phenomenon: an analytical artefact or an actual dissipation of PCBs in the soil by volatilisation. Further analyses should enable one of these hypotheses to be validated.