DERIVATION AND FIELD TESTING OF AIR-MILK AND FEED-MILK TRANSFER-FACTORS FOR PCBS

Detailed field experimental data on the air to herbage transfer of PCB s was combined with data on feed to milk transfers from a detailed fee ding trial with lactating cows to derive congener-specific air to milk and feed to milk transfer factors (TFA:M and TFF:M) The variability a nd uncertainties in these factors are discussed largely with reference to UK field conditions. TFA:M values were 2.4, 54, and 650 m(3) of ai r g(-1) of milk fat for congeners 18, 74, and 170, respectively. The u sefulness of the transfer factors as predictive tools was tested on (i ) data from two milk and feed surveys (in late spring 1996 and winter 1997) of farms in Northwest England; (ii) data from a long-term monito ring study conducted throughout the 1996 growing season; and (iii) dat a from the literature. TFA:M and TFF:M gave excellent predictions of t he milk PCB concentrations for all tested data sets, with milk concent rations of the persistent congeners predicted to within a factor of si milar to 2-3 at the local level and to well within an order of magnitu de at the regional level. The main requirements of using TFA:M are tha t (i) pasture is the dominant feed; (ii) winter-fed silage is grown lo cally, and (iii) there is no local intermittent source. Survey results showed that levels of persistent PCB congeners in silage are directly correlated with milk output fluxes. Bioconcentration factors (BCFs) a nd carry-over rates (CORs) calculated for both study approaches were v ery similar to those found in ''uncontaminated'' feeding studies. Alth ough CORs are theoretically preferable to BCFs the variability found f or each showed that there is likely to be little practical advantage i n collecting the extra data required for the calculation of CORs.