Risks associated with the transfer of toxic organo-metallic mercury from soils into the terrestrial feed chain

Although the transfer of organo-metallic mercury (OrgHg) in aquatic food webs has long been studied, it has only been recently recognized that there is also accumulation in terrestrial systems. There is still however little information about the exposure of grazing animals to OrgHg from soils and feed as well as on risks of exposure to animal and humans. In this study we collected 78 soil samples and 40 plant samples (Lolium perenne and Brassica juncea) from agricultural fields near a contaminated industrial area and evaluated the soil-to-plant transfer of Hg as well as subsequent trophic transfer. Inorganic Hg (IHg) concentrations ranged from 0.080 to 210mgkg(-1) d.w. in soils, from 0.010 to 84mgkg(-1) d.w. in roots and from 0.020 to 6.9mgkg(-1) d.w. in shoots. OrgHg concentrations in soils varied between 0.20 and 130μgkg(-1) d.w. representing on average 0.13% of the total Hg (THg). In root and shoot samples OrgHg comprised on average 0.58% (roots) and 0.66% (shoots) of THg. Average bioaccumulation factors (BAFs) for OrgHg in relation to soil concentrations were 3.3 (for roots) and 1.5 (for shoots). The daily intake (DI) of THg in 33 sampling sites exceeded the acceptable daily intake (ADI) of THg of both cows (ADI=1.4mgd(-1)) and sheep (ADI=0.28mgd(-1)), in view of food safety associated with THg in animal kidneys. Estimated DI of OrgHg for grazing animals were up to 220μgd(-1) (for cows) and up to 33μgd(-1) (for sheep). This study suggested that solely monitoring the levels of THg in soils and feed may not allow to adequately taking into account accumulation of OrgHg in feed crops and properly address risks associated with OrgHg exposure for animals and humans. Hence, the inclusion of limits for OrgHg in feed quality and food safety legislation is advised.