5. Implications for risk assessment
Results from this environmental study indicate that there is uptake
of IHg from soils by roots of L. perenne and subsequent translocation of
IHg to above ground plant parts, although plant roots can to a certain
extent retain IHg and reduce its translocation. No accumulation of IHg
in shoots or roots in relation to soil IHg concentrations was observed
(BAFs b 1). On average, 99% of THg in shoots is in the form of IHg.
Thereby, and since significant relationships between BAFs for IHg in
edible plant parts and the corresponding concentration of IHg
in soils were established from experimental data, it is possible to
derive site-specific thresholds for IHg in soils from available quality
criteria for feed crops (THg), as described by Rodrigues et al.
(2012b). However, since IHg and OrgHg have different uptake and
accumulation mechanisms in L. perenne (as also previously reported
for other plants by Zhang et al. (2010b)) and since OrgHg is known
to be much more toxic than IHg, the establishment of soil thresholds
for IHg (or THg) may not fully address the actual health risks for
animals (and subsequently for humans).
This study indicated that despite the relatively low percentage of
OrgHg in plants, this pool must not be overlooked since it represents a
relatively high concentration in edible plant parts (up to 13 μg kg−1
d.w.) and includes the most toxic and mobile forms of Hg, primarily
MeHg (Fernández-Martínez et al., 2005). Traditionally, feed control
quality only included measurement of THg and disregarded organometallic