ABNORMALLY high levels of the toxic metal cadmium accumulate in the
kidneys and livers of sheep grazing on pasture fertilised for years with sewage
sludge, according to disturbing new research.
If the practice of spreading sewage sludge on pasture intensifies, as is
likely in Europe after 1998 when dumping at sea is banned, people who eat lambs鈥�
livers or kidneys regularly might be at risk of chronic cadmium poisoning.
Preliminary findings will be presented next week in Scarborough at the annual
meeting of the British Society of Animal Science.
Mike Wilkinson of the consultancy Chalcombe Agricultural Resources in
Lincoln, working with Julian Hill of Writtle College in Chelmsford, Essex,
measured levels of heavy metals accumulated by sheep when they grazed on one
field that was treated with sewage sludge and another that was untreated. After
150 days, average levels of cadmium in sheep livers were 1.24 milligrams per
kilogram of dried tissue for the animals that had grazed on heavily treated
pasture, eight times higher than the level found in sheep grazing on untreated
land. In sheep kidneys, the levels in animals that had grazed on the treated
land averaged 2.57 milligrams per kilogram of dried tissue, six times as high as
in the sheep that had grazed on clean pasture.
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Toxicologists do not know for certain whether these levels of cadmium would
be dangerous if regularly consumed by people. The WHO has set a recommended safe
daily intake for individuals of 70 micrograms of cadmium. John Groten of the TNO
Toxicology and Nutrition Institute in Zeist, the Netherlands, calculates that
people鈥檚 cadmium intake could breach this limit if they frequently consumed a
dish made from the contaminated organs. 鈥淚f you were a regular consumer of these
dishes鈥攎ore than once a week鈥攖hen there might be a problem,鈥� he
says.
But an official at the Ministry of Agriculture, Fisheries and Food, which
commissioned Wilkinson鈥檚 research, disputes Groten鈥檚 calculations. He estimates
that people would consume less than the WHO limit 鈥渆ven if they ate the meat
with the observed levels for the rest of their lives鈥�.
The sludge left behind after sewage treatment is sometimes rich in cadmium
because sewage contains residues of the metal from industrial processes such as
electroplating.
At very high doses, the effects of cadmium poisoning are debilitating. The
world鈥檚 worst incident took place in Japan four decades ago when thousands of
people in the Fuchu region fell ill with 鈥渋tai itai鈥�, which translates
as 鈥渋t hurts, it hurts鈥�. Victims had consumed as much as 2 milligrams of cadmium
per day over long periods in rice irrigated with water contaminated by effluent
from mines. They suffered painful bone and kidney disorders.
No one eating liver from sheep grazing on fields treated with sludge would
receive anything approaching the doses suffered by the itai itai
sufferers. And Wilkinson stresses that the treated field he studied represents a
鈥渨orst case scenario鈥� because it had received sludge regularly between 1981 and
1995. The average concentration of cadmium to a depth of 25 centimetres below
the surface was 2.72 milligrams per kilogram of dried soil, close to the legal
limit of 3 milligrams. Nearer the surface, in soil likely to be eaten with grass
by sheep, the concentration was 4.97 milligrams per kilogram of dried soil.
Wilkinson鈥檚 findings pose a problem for Britain鈥檚 water companies, which face
a ban on dumping sewage at sea from 1998. Water companies would like to hand the
sludge they now dump at sea over to farmers to fertilise their pasture because
this is cheaper than incinerating the sludge or burying it in landfill sites. At
present, a third of the 30 million tonnes of sewage sludge produced each year in
Britain goes into the sea, a third to agriculture and a third to incineration or
landfill.
Given his findings, Wilkinson says that sludge should not be dumped on
pasture that has already been used for many years for the same purpose. 鈥淭he
water companies know where the most contaminated fields are,鈥� he says.
Norman Lowe, chair of the sludge group at the Water Services Association and
chief environmental scientist at Welsh Water, says that the companies will
investigate Wilkinson鈥檚 findings. 鈥淢y immediate response is that these levels of
cadmium in the soil are exceptionally high, and the likelihood that there are
lots of fields like this is low,鈥� he says.