Tuesday, 5 May 2009

Removing lead from contaminated soil.

Lead mining existed as long ago as 6000BC. It became commonly used in jewellery as it is more malleable than many metals. Toxicity problems associated with lead became apparent as early as 200BC when colic and gout was blamed on leaden drinking vessels and white lead used in make-up. Most health problems though, came from lead dust caused by mining. The miners’ symptoms of chronic lead poisoning include neurological problems, such as temporary reduced cognitive abilities, gastrointestinal problems, hair loss, insomnia, reproductive difficulties and in extreme cases seizures. Although the Romans used lead pipes for plumbing ( the word plumbing comes from the Roman word for lead: plumbus) this was not a major cause of lead poisoning as the hard water in Rome coated the inside of the pipes with calcium.

Lead poisoning in the last century or more was largely caused by industrial processes and airborne lead particles from car exhaust fumes. Lead accumulation in soil is partly due to the airborne particles and partly due to lead in paint flakes being deposited over time. The control of the use of lead in paint and the removal of lead additives in petrol by the western world reduced soil contamination although some countries continued to use leaded petrol for many years.

Soil contamination is still a major cause for concern. Lead remains in the soil for hundreds if not thousands of years. Children are most affected by contaminated soil as they are most likely to be in contact with it during play and will even ingest it. As with affected adults, intelligence is impaired and this may become permanent if the contamination is high enough or prolonged. There are concerns that the lead from the soil will migrate in to food crops but studies of lead contaminated soil have shown that the fruit of crops contains far less lead than the foliage, although root crops may be contaminated by the presence of soil dust on the surface. Leafy plants such as cabbage and lettuce are of concern. Lead levels remain high in the soil and lead is not taken up by the plant if the the ph level is high and there is organic vegetable matter in the soil.

This may seem unimportant to the homeowner who would think: “why plant crops in contaminated soil anyway?” but the very soil in your garden may be contaminated if there is an older building or shed nearby that will have been painted with lead paint at some stage in its history.

“How will I know if there is lead in my soil?” There are many lead testing kits available on the internet. If the soil lead level is too high there are a few ways in which you can reduce the likelihood of plant contamination. Firstly you can add lime to keep the ph levels above 6.5. This reduces the amount of lead taken up by the plants. Add organic matter such as leaf compost, manure and peat substitutes as organic materials combine with lead to make it less available to the plant. If contamination is high it may be necessary to physically remove the top layer of soil and replace it with new uncontaminated soil. Finally, grow food crops away from roads and buildings where lead paint may have been used. In addition to the above suggestions for reducing the uptake of lead, don’t forget to thoroughly wash the fruit and vegetables as the soil and dust on the surface of the fruit or vegetable may be contaminated.

There is another method for lead removal: bioremediation. There are plants that are very successful at removing cadmium, copper and zinc but lead removal has proved to be more challenging. It has been discovered that certain grasses have proved to be promising and the grass that has the highest uptake of lead is Vetiveria zizanioides. The grasses, which have little decorative value, are planted in the polluted area and then harvested and removed. The plain appearance of this unassuming plant hides a multitude of talents and in addition to its lead removing properties, vetiver oil is antiseptic and can be used for cosmetic and medicinal purposes. The roots can be used for basketry and screen making.

Below: faux lead planter

We have discussed removing lead from soil. We have been accidentally adding lead to our gardens over many years by pollution and with the use of lead paint and indeed have actually added lead on purpose in the form of lead planters. Decorative lead planters were associated with grand country houses and period properties. Little did those proponents of container gardening realise that they were actually poisoning the very earth they cherished. Lead planters are still available today and apart from the toxicity problem they are prohibitively expensive. There is a lead free alternative, however. Lead style planters made from faux lead (actually made from GRP or glass fibre) in the style of the classic containers can be purchased from leading landscape contractors. The classical style and beauty is retained without the inherant drawbacks. They can also be used as water tanks or rain butts.


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