How safe is your vegetable patch?

As part of an ongoing effort to manage the level of my family’s exposure to contaminants in the food chain, my wife and I began an backyard vegetable patch in 2015.

ourgarden
A section of our garden in 2015. All organic. All delicious.

The basic parameters were to grow those vegetables and herbs we use the most and to minimize or eliminate the level of contaminants entering our garden and eventually ourselves. The outcome, hopefully, were better quality seasonal vegetables grown with no herbicides or chemicals.

Like most people that start this type of activity, our aspiration was an organic farmlet in the middle of suburbia but is this actually possible?

I live in the Western suburbs of the second largest city in Australia, Melbourne; essentially smack bang in the middle of the industrial belt. Historically there was ammunitions factory to the north, chemical factories to the south and a foundry to the east. The people that live in my suburb primarily work in light-industrial or heavy-industrial factories situated between 2 and 5 kilometres of their homes.

The natural environment is all but gone. The extensive temperate grasslands replaced by farms then factories, or developed into extensive housing estates. The original vegetation, Plains Grassland, is now endangered of becoming extinct with less than 1% remaining. For the most part the soil is relatively intact, shallow basalt soils over a vast volcanic plain. The hydrology however has been severely disrupted. As urbanisation has occurred the water has been redirected to the few creeks and rivers in the area. Consequently these waterways have changed dramatically due to the change in the frequency, quantity and quality of water entering the systems. In the urban areas the soil systems have also changed with little or no water entering the soil directly. Traditional sinks for regional underground aquifers rarely get surface flow, so natural seepages have all but dried up.

So this sets the scene — a veggie patch in a shallow clay soil, not ideal for growing vegetables, in an area where there might have been or still could be significant contaminants in or entering the system.

So what do I mean when I talk about contaminants? For me a contaminant is anything in the soil that you would not expect to find there that could effect the growth or quality of your vegetables or animals, and/or if consumed, could potentially be harmful to your health. These contaminants may be present in the soil before you considered establishing a vegetable patch, entered the soil as a consequence of something you did and/or could still be entering the soil (e.g. in rainfall, via the tap water you use to water your garden or via things to add to the soil to help your plants grow).

So what about my place? My garden is on natural soil — no external fill. There is no evidence of any factories occurring on site or within 500 metres of my house (I checked historical aerial photos of the area). I have not and I am unaware of anyone historically storing chemicals on the property. So, so far so good.

The house however is relatively old. Established in the 1950s it would have been painted with Lead paint. So potentially, there could be Lead in the soil under or immediately around the house. Several years ago, the house was renovated and debris was stored for short periods on areas now being used to grow vegetables so potentially some Lead may have entered the soil at this time. The fences, although now replaced, were made with treated-pine that contained Copper, Chromium and Arsenic. So these heavy metals could also potentially be in the soil, especially around the fenceline or areas where the demolished fence was stockpiled before going to the tip.

So there you have it, that’s all the potential contaminants that might be in my soil that I can think of at present; but how do I establish if they are in my soil? Heavy metals are toxic and can cause all sorts of health problems, so if there are any of these chemicals present, I want to know what they are and what to do about it!

Well luckily, the Department of Environmental Sciences at Macquarie University has established a free service called VegeSafe that tests your soil for heavy metals commonly found in the urban environment. The program is run by Professor Mark P. Taylor and the principle staff/students giving their time and expertise for free are Mr. Steve George, Mr Marek Rouillon, Mr. Paul Harvey and Ms. Louise Kristensen. The group in my mind should be commended for providing a valuable community service.

Participation in the VegeSafe program has only one condition. The results of your assessment, after being de-identified and location generalised to within 25 metres, can be used by the University for ongoing research. Not an unreasonable request for a research institution; at least in my mind.

So how does it work? For exact details and procedures I suggest you visit the VegeSafe page. In a nutshell you collect 5 soil samples and post them to the group with details of your property and the signed consent form. Then you wait. For me, it took 8 days from sending them the soil samples to getting an email with my results. The email contained a table with the level of heavy metals for each of the samples provided in mg/kg. A separate image was provided listing the upper thresholds published in a variety of documents and links back to the VegeSafe page that helps you understand the results and provides some suggestions if you happen to have high levels of any particular contaminant. To be honest, I found it hard to easily visualise the outcome of the tests so consolidated the data and thresholds into a composite table (Table 1) and cross-referenced to those of thresholds that were still current and updated those that were recently updated . This table is presented below — click on the thumbnail to enlarge.

VegeSafe_Results
Table 1. The levels of heavy metals in my vegetable garden

The upshot is that all chemicals were below the thresholds listed in the national legislation (CoA 2013) for soil in the Residential A category. This category includes accessible garden soil, childcare centres, preschools and primary schools and would be what my house would fall under. The Residential B category included areas where soil is not readily accessible and Residential C parks and other types of public open space.

The problem is understanding what this actually means. Well, what I found interesting is that the two main organic standards in Australia (AO 2013, NASAA 2016) will only certify you if the levels of these metals in the soil are lower than the specified thresholds in the standards. OK. I buy certified organic when I can so this is a useful yard stick. Looking at Table 1 you can see that the soil in  my garden is lower than all the thresholds except for Zinc. For some reason I have a slightly elevated level of Zinc near the back fence. Being significantly lower than the rest of the thresholds listed I am not particularly worried.

The only other point worth raising is that the Arsenic and Chromium VI levels in my soil are higher than the Canadian and the USA thresholds (Table 1). These countries and European countries usually have lower thresholds for most environmental contaminants both in soil and in food because they have higher levels of exposure than people living in Australia. So again, I am not particularly concerned.

The take away message I got from this exercise is that don’t assume you don’t have contaminants in your vegetable patch. These toxins are ubiquitous and while it is preferable to never have them in your diet, this seems impossible as these toxins are ubiquitous and reducing your overall exposure is probably the best interim strategy. My results show that even in urban environments patches of soil can be found that could be certified organic if all other criteria are met.

References

OEHHA. 2010. Soil- and Soil-Gas Screening Numbers (California Human Health Screening Levels or CHHSLs). California: Office of Environmental Health Hazard Assessment. http://oehha.ca.gov/chhsltable.
NPCA. 2009. Soil Contamination in Day-Care Centers and Playgrounds. Norweigian Pollution Control Authority. http://www.miljodirektoratet.no/old/klif/publikasjoner/2550/ta2550.pdf.
NASAA. 2016. National Association for Sustainable Agriculture Australia Organic and Biodynamic Standard. Stirling South Australia: National Association for Sustainable Agriculture Australia Limited. http://www.nasaa.com.au/data/pdfs/NASAA%20Organic%20Standard%20Feb%202016.pdf.
EPA. 2016. Regional Screening Levels. Washington DC,  The United States of America: Environmental Protection Agency. http://www.epa.gov/region9/superfund/prg/.
CoA. 2013. “Schedule B1: Guideline on Investigation Levels For Soil and Groundwater.” In National Environment Protection (Assessment of Site Contamination) Measure 1999 (as Amended). Canberra: Commonwealth of Australia. http://www.comlaw.gov.au/Details/F2013C00288/Html/Volume_2.
CCME. 2013. “Soil Quality Guidelines for the Protection of Environmental and Human Health.” In Canadian Environmental Quality Guidelines. Canadian Council of Ministers of the Environment. http://st-ts.ccme.ca/.
AO. 2013. Australian Certified Organic Standard. Nundah Queensland Australia: Australian Organic Ltd. http://austorganic.com/wp-content/uploads/2013/11/ACOS-2013-final.pdf.

Leave a Reply

Your email address will not be published. Required fields are marked *

*

This site uses Akismet to reduce spam. Learn how your comment data is processed.