The Loam Ranger – Soil remediation
Our reader Lynn Collins sent us this query:
I found the cadmium item useful as, following analysis of our garden soil with your company, we have a bit of the stuff polluting what was our veggie patch, along with other nasties.
- The soil we inherited is polluted with benzopyrene, lead, PAHs and zinc (and, sadly, but to a lesser degree, manganese, copper and arsenic) and we’d like to assist Mother Nature [to] repair this soil – any suggestions for neutralizing agents; stimuli for organic agents going about their regenerative/curative work; plants that might absorb. I had heard that water hyacinth and bracken may be particularly useful cleansing plants: an old wives’ tale?
- Are home-kits available for ongoing testing of areas of the garden for poisons? I noticed a basic lead-sensing device in the paint department of my hardware store the other day.
- Are fruit trees – say, orange, banana, blueberry, plum – likely to uptake lead and other poisons in a season’s crop sufficient to harm (very young) humans?
Avoiding the problem
The first question you need to answer is whether it is even necessary to remediate the soil. If you would like to reuse your former vege patch to grow veges again (because it has a good aspect, for example), you can use the raised-bed design favoured by Peter Cundall, formerly of Gardening Australia, by organisations that cater to the disabled and elderly (e.g. Sustainable Canberra), and by the late Esther Deans in her “no-dig” gardening design.
If you don’t intend to use the garden to produce food, the question then becomes whether anyone is likely to ingest any soil. If you have a crawling baby who likes to eat dirt (a behaviour called pica), obviously this is a concern, and it will be several years before any child playing in the garden stops getting caked with dirt (which then goes into the mouth with food). A simple solution to this is to turf the area or otherwise plant it with groundcovers that will prevent access to the soil.
If you really are keen to remediate the soil, the key factor is bioavailability: how available are the contaminants to plants (and thus humans)? Many heavy metals can be rendered unavailable to plants by adjusting the soil pH to above 5.5 (usually with lime). Plants then can’t take them up (or take up less).
Some plant species exclude heavy metals at their roots. Other species take up metals and then lock them up in their cells. Fruiting and seed structures such as tomatoes, grains and citrus fruits are largely protected in one of these ways from metal accumulation.
In contrast, foliage crops, such as silver beet and spinach, are known accumulators. Those that take up a lot of contaminants, and are sometimes used in soil remediation, are known as hyperaccumulators. Wikipedia gives a comprehensive list of hyperaccumulator species and the contaminants (mostly metals) they take up. It can take several years of raising crops and removing them before the contaminant level is reduced to one suitable again for food production.
Alternatively, soil washing has been developed as a way to remove the heavy metals from soil in a water-based solvent that contains iron chloride. This is time-consuming, labour-intensive and costly and is applicable only to small, usually urban sites. So it might be worthwhile investigating this. SESL can suggest possible firms.
Water hyacinth is used in some countries to take up contaminants from water, but here in Australia it is illegal to grow it. Bracken is not mentioned as an accumulator; one study (Water, Air, and Soil Pollution 2004 157: 257–267) found it unsuitable.
For any contaminant to pose a threat, four things have to be true:
- The contaminant has to be present in a sufficient quantity to be of concern.
- The contaminant has to be available (bioavailability).
- There needs to be a receptor (either a plant or an animal or a human) present.
- There needs to a mode of transfer (i.e. some way of getting the contaminant from where it is now into the receptor).
If any one of these four things is missing, then effectively all you have is a potential rather than an actual toxicity issue. The overall question is complex. Plant tissue testing can help untangle the factors.
Copper and zinc: Elevated Cu and Zn are commonly found in urban and industrial landscapes as a result of their common use in alloys, galvanising and a myriad of other things. Neither is toxic to humans except at very large acute doses but, because they have a role in plant biochemistry, they can become toxic to plants if they become available in excessive amounts. There are many ways to lock them up in soil, including adding lime, organic matter and phosphate fertiliser, all of which produce insoluble complexes. Zn toxicity in plants is the most common metal toxicity we see. Even then, it is uncommon.
Manganese is commonly elevated in natural soils of warm temperate regions, especially Australia, where high Mn is common and normal. Mn toxicity is common in susceptible plants (e.g. beans and peas) where Mn is elevated and the soil is acidic, waterlogged or both. Liming to raise the soil pH to 5.5 and improving drainage is an effective cure. Mn is not toxic to humans by any conceivable exposure route.
Lead is not particularly toxic to plants, and most plants are very good at excluding it. Fruiting structures (e.g. tomatoes and grain) are strongly protected and are usually safe to eat from plants grown in contaminated soil. On the other hand, it would be unwise to grow and eat silver beet and spinach grown in heavily contaminated soils, but keep in mind that for Pb poisoning to occur in humans, a chronic (continued frequent) exposure is needed, which usually involves direct ingestion of contaminated soil (e.g. by babies and young children). Eating the odd spinach leaf is extremely unlikely to cause Pb poisoning, particularly as humans excrete lead quickly (the half life is about 2 weeks).
Cadmium poisoning in plants is very uncommon, but Cd can accumulate in bone, and hence chronic exposure is of concern. Interestingly, human disease from Cd is extremely uncommon, mainly because living organisms are protected from Cd uptake by Zn. In natural systems there is sufficient Zn to effectively protect living things from Cd absorption. Therefore, if your soil is also contaminated with Zn you probably don’t need to worry about Cd unless the ratio of Zn to Cd is less than about 60:1. Nevertheless, authorities are very cautious with regard to Cd contamination of soils, and there are strict controls on Cd levels in fertiliser, for example. As a precaution, don’t eat root vegetables grown in soils with more than about 10 mg/kg of Cd, and don’t eat leaf vegetables from soil with more than about 20 mg/kg.
Arsenic again is not particularly toxic to plants, and in soils is generally not readily available to humans. (Nevertheless, arsenic poisoning from drinking water is a grave concern in Bangladesh, where degrading organic matter from ponds makes it soluble in deep groundwater.) Up to 100 mg/kg is permissible in domestic gardens where exposure to soil is minimal. A rate of 5–20 mg/kg in soil is fairly normal.
Polycyclic aromatic hydrocarbons (PAHs), including benzo[a]pyrene, are the products of incomplete combustion and are released from burning coal and wood. Large quantities are released naturally in bushfires. The most common human exposure route is smoking. Plants do not take them up, so this exposure route is of no concern. The general guideline for domestic soils is 20 mg/kg.
- Fruit growing (such as orange, banana, blueberry, plum) is of little or no concern.
- Removing metals in hyperaccumulators is slow and usually impractical.
- Make sure the soil is not acidic (by adding lime) and has adequate organic matter content (by adding manure, for example) to reduce availability.
- The test kit for lead paint is designed to pick up a Pb content of 5% to 20%, and would probably not be sensitive enough to pick a few hundred mg/kg (0.01%) in soils.
If you are still worried, grow silver beet and have the leaves analysed for metals.
Wikipedia: List of hyperaccumulators.
SESL: The Loam Ranger – Heavy metal content of fertilisers.