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TME levels and flows via the spreading of OWPs

The most abundant TMEs in the OWPs are zinc, copper and lead. Next are chromium, nickel, cadmium and mercury. MSW compost has the highest content as far as the majority of TMEs are concerned. BIO compost and manure display lower levels, with the exception of Zn in manure. The spreading of ROPs provides an input of TMEs representing between less than 5% and more than 15% of the stock of TMEs present in the soil.

TME levels in OWPs

The TME levels in the OWPs are shown in Figure 1. The maximum levels stated in standards on organic conditioners (manure, MSW, BIO) and sludge composts are also shown (standards NF U 44-051 and NF U 44-095).
The TME concentrations measured in the OWPs are, in ascending order:  Se, Hg < Cd < As <  Ni < Cr < Cu, Pb < Zn.
The average concentrations in OWPs vary as follows: less than 2 mg/kg DM for Se, Hg, Cd and As, from 15 to 33 mg/kg DM for Ni, from 42 to 108 mg/kg DM for Cr, from 63 to 180 mg/kg DM for Cu and Pb and from 269 to 417 mg/kg DM for Zn.
MSW compost has higher TME concentrations than other ROPs, except in the case of Zn, for which manure has the highest concentration, and As, for which GWS has higher concentrations. However, TME levels in the last MSW composts spread are lower than those in the first.
BIO compost and manure are the OWPs with the lowest levels for some TMEs, in the case of Cd, Cu and Zn for BIO and Cr, Hg and Ni for manure.
The average TME levels in the distributed products are within the maximum levels of the standards NF U 44-051 and NF U 44-095.



Figure 1. Average TME levels (mg/kg DM) in distributed OWPs from 1998 to 2007 (n=6, except for As and Se, measured since 2002: n = 4), comparison with the thresholds of the standards NF U 44-051 (organic conditioners) and NF U 44-095 (compost containing sludge). For Cd and Hg, the concentrations were multiplied by 10 in order to be visible on the graph.

TME flows via spreading

The TME flows via the spreading of ROPs are, in ascending order:  Se, Hg < Cd < As <  Ni < Cr < Cu, Pb < Zn.
The average flows via spreading of the ROPs vary as follows: less than 15 g/ha for Se and Hg, from 19 to 21 g/ha for Cd, from 24 to 242 g/ha for As, from 202 to 663 g/ha for Ni, from 564 to 1,332 g/ha for Cr, from 1,383 to 2,611 g/ha for Cu, from 1,047 to 2,233 g/ha for Pb and from 5,071 to 6,209 g/ha for Zn.
For the purposes of comparison at the maximum thresholds of the standards, these flows were spread out over 10 years (Figure 2). Although the average TME concentrations in the ROPs are within the thresholds of the standards, cumulative flows over 10 years for some products exceed the thresholds: for Cu (GWS, RMW and MAN), Ni (BIO), Pb (BIO, RMW and MAN) and Zn (GWS, BIO and MAN).
These excesses can be partially attributed to the doses of distributed ROPs, which are higher than those normally applied in farming.
Compared with TME stocks in the soil, these flows correspond on average to: less than 1% of the stock in the soil forCr and Ni around 2% for Cd, Pb and Hg and between 3% and 4% for Cu and Zn.
For the purposes of comparison, measurements of atmospheric effects in the farming area of Versailles were taken (Azimi et al, 2004): 39 g/ha/year for Cu, 15 g/ha for Ni, 22 g/ha/year for Pb, 165 g/ha/year for Zn and 0.47 g/ha/year for Cd. Spreadings therefore constitute a considerable source of TME in the soil compared with atmospheric effects.

Figure 2. Average TME flows (g/ha) during the period from 1998 to 2007, i.e. 10 years or 6 spreadings, comparison with the thresholds of the standards NF U 44-051 (organic conditioners) and NF U 44-095 (compost containing sludge). For Cd and Hg, the flows were multiplied by 10 in order to be visible on the graph.