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I think my soil/compost is contaminated!! What do I do?

Updated: Oct 18, 2021

There has been a lot of discussion and media attention about contaminated gardening products lately, but not a lot about what to do if you think you have the problem yourself. This blog post touches on some questions to ask and a bit about what to do if you suspect you have contaminated soil or organic matter/compost.

We realise that this blog post may not give you and your community definitive answers. We hope that you can use it as a guide to problem solving and for how to access reliable information.

Three members of our team worked together on this blog post to help provide information that crosses a range of issues and subject matter areas.

Sampling for contaminanted soil and water

Question 1: Do you know where the material has come from? The first question to be asked in assessing contamination is what are the sources of contamination in that environment. This means that knowing where your soil or compost has come from is an important step in narrowing down what the problem may be. For example, if your compost was produced from your own household waste:

  • have you put dead plants or leaves that could have been treated with herbicide in your compost bin?

If you have soil from agricultural fields:

  • Do you know whether herbicides or pesticides were actively used on the farm?

  • Does the soil come from a location near sheep dips or fuel tanks?

  • Does the soil come from a farm where chemicals such as arsenic or copper occur in naturally high, but toxic concentrations?

Some more details about this are in the first post in this series.

Question 2: Have you ruled out other problems?

In our second blog post in this series, we discuss common plant-soil maladies. Have a read of this post, and then make sure you have ruled out some common issues that may be limiting your plant growth or causing plant death and health issues.

Question 3: Do you have soil test results?

The supplier of the material in question should have confirmed the soil came from land with no sources of contamination or tested the soils themselves to meet local legislation (this is discussed in our first blog). Make sure you review a copy of this and that the soil tests meet your state or territory’s criteria – you might even know where the soil came from and have local knowledge about whether the soil’s source location had problems. If the supplier does not have this information or if the information supplied to you does not meet your state requirements, get in touch with your local regulator (e.g. EPA Victoria, NSW EPA, SA EPA etc).

Once you know where your material is from and you have ruled out other soil issues, it is time to consider soil contaminants.

Question 4: Time to talk to an expert and test your soil for contaminants

It’s important to state here that there are many things that need to be considered when sampling and testing soils for contaminants. These things make all the difference to whether any analysis of soils undertaken is meaningful or otherwise -- we have seen numerous examples of chemical analysis where the results were basically meaningless when some of these questions weren’t appropriately considered: How many samples do you need to collect? What temperature do they need to be stored at to ensure the contaminants in question are appropriately preserved? Do samples need to travel with a chemical of known concentration (called a trip spike) to make sure they don’t deteriorate in transport? From which depth and location should the samples be collected to be representative? And of the more than 40,000 chemicals and substances used in Australia, which are to be selected for analysis, at what concentrations should they be analysed ― and can you afford it (noting that analysing only one sample [which is not enough] for the standard suite of contaminants costs more than $200)?

Once the soil has been appropriately tested, and results are received, the interpretation of the results then needs some care. The presence of a chemical absolutely does not mean that this chemical is causing problems, even if the chemical is known to be toxic to plants. Boron, for example, is a plant nutrient. Plants suffer boron deficiency when concentrations in the soil are as low as 0.15 to 0.5 mg/kg. Boron is also toxic to plants, killing them at concentrations as low as 2.9 mg/kg. This example also shows that the absence of a chemical can inhibit plant growth just as chemical presence can, as well as that its presence doesn’t mean plants will die.

It is also important to note that the behaviour of organic contaminants (that is chemicals containing hydrogen and carbon) can be substantially different with only very small differences in chemistry and that means concentrations of chemicals and their impact must be carefully considered. Here’s an example of this that we noticed recently in the ACT’s guidelines for classifying soils and other materials for waste disposal:

The cut off for concentrations of 2,4,5 trichlorophenol in inert waste is 800 mg/kg.

The cut off for concentrations of 2,4,6 trichlorophenol in inert waste is 4 mg/kg.

2,4,5 trichlorophenol (Image:Wikimedia)
2,4,6 trichlorophenol (Image:Wikimedia)

These two chemicals are almost identical, except the chlorine in 2,4,6 trichlorophenol is evenly distributed (around a ring of 6 carbon atoms) and are not in 2,4,5 trichlorophenol. This truly small difference means that 200 times more 2,4,5 trichlorophenol is considered inert in soils sent to Canberra’s landfills than 2,4,6 trichlorophenol!

There are also a range of factors including the contaminant concentration, your soil type, climate etc that all factor into whether or not a chemical will be a problem for your soil or your home-grown food. These problems are well recognised by scientists; a chemical will generally (not always) have more potential to cause toxicity problems in sandy soils than in clay rich soils, for example, and this means one has to be able to tell the difference between a clayey sand and a sandy clay.

If all of this seems complicated and frustrating, you are not alone! It really is a complicated area of science that requires specific expertise. That’s why testing and analysis for soil contamination must be undertaken by an expert in a number of Australian jurisdictions.

If hiring a consultant is cost prohibitive for you, and you have ruled out all the other issues that may be affecting your plant growth, then we suggest contacting a local government agency or not-for-profits as a starting point. New South Wales Department of Primary Industries has a wealth of soils information, and many local Landcare groups have soil gurus who see and understand soil problems across whole districts. You can even band together as a community group to get soil testing done by a suitably qualified consultant. In the interim, the precautionary principle is a good one – stop using the material you are concerned about, and do not eat anything produced by it.

If you are ever worried or in doubt, always consult your local EPA or environmental regulator, or find a suitably qualified consultant to help you.

There are some reputable places for advice on what to do about contaminated soil in your backyard, including advice on arsenic in old gold mine areas. When using online sources, make sure they are from a reputable source such as a government department, a suitably qualified consultant, Landcare group or not-for-profit. Make sure that the author of the information has experience in understanding chemicals in the environment.

Question 5: What do I do with the contaminated material?

How contaminated material should be managed (as a waste or otherwise) is dependent on what contaminant is present, its concentration and its risk to the environment and human health. You will need to follow advice from a consultant or regulator about what to do with your material if contamination is present (see Question 4).

Removing contaminants from soil is complicated. There is no hard and fast method that can be prescribed for use in backyards.

We have seen some discussion of approaches people can use to get rid of contamination. These include solarisation, adding compost and organic matter, turning soil or compost, leaching with water, and treatment with slaters, fungi, crystals etc. There is a real risk that using these methods does not only not remove the contamination, but could increase the environmental and human health hazards. For example, you might spread the potentially impacted area of contamination further as you mix the soil; you could expose yourself to chemicals that are volatile; you could wash a highly toxic pesticide off your boots and into your septic tank, killing it in the process.

Remediation approaches need to be specific to the type of chemical problem or problems. Soils that are contaminated with petroleum are often treated by turning soils (landfarming), for example, but if these soils were contaminated prior to the year 1993, there is potential for lead to also occur with the petroleum chemicals. If this is the case, turning soils will not remediate the soil of all its hazards. Asbestos contaminated soils are often buried to reduce risks to human health; any asbestos contaminated soils that occur with petroleum cannot be buried, however, without creating a hazard from volatile petroleum chemicals. Like contamination assessment, remediation is a complex area of science requiring specific expertise. Solutions require careful consideration of the contamination problem in the context of the whole environment. If you want to undertake remediation of your soil or compost, please discuss this with an appropriately qualified consultant or government department.

If you are ever worried or in doubt, always consult your local EPA or environmental regulator, or find a suitably qualified consultant to help you.

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