Potential sources of contamination: cultivation and growing

The first installment of this series gave an overview of why testing cannabis and hemp for heavy metal contaminants is so important and how the pharmaceutical industry can play a critical role in preparing the cannabis industry for federal oversight.

Parts two and three will focus on how growers, cultivator, processers, and manufacturers need to be actively investigating all the potential sources of elemental contamination before they can even hope to minimize them – a process the pharmaceutical industry took over 20 years to fully understand.

The series has been summarized from two chapters in Robert Thomas’ upcoming book, Measuring Heavy Metal Contaminants in Cannabis and Hemp: A Practical Guide, which will be published by CRC Press this September. The book, including its table of contents, is now available for pre-ordering from the publisher

Environmental pathways

Cannabis and hemp are known to be hyper-accumulators of contaminants in the soil, which is why they’ve been used to clean up toxic waste sites where other kinds of remediation attempts have failed. In the aftermath of the Chernobyl nuclear melt down in the Ukraine in 1986, industrial hemp was planted to clean up the radioactive isotopes that had leaked into the soil and ground waters1. Of course, Chernobyl is an extreme example of heavy metal and radionuclide contamination. But as a result of several anthropogenic industrial activities over the past few decades, including mining, smelting, use of fertilizers and pesticides, and waste treatment, heavy metal pollution has become one of the most serious environmental problems today.

Considerations about where to grow cannabis and hemp is therefore going to be critically important because it could have serious implications on the level of heavy metals that are absorbed by the plant and its resulting safety for human consumption of cannabinoids. Historically, apart from the Western and Southern regions of the US, much of the cannabis in the US has been grown indoors in greenhouses under controlled growing environments, so the absorption of heavy metals into the plant has been kept in check reasonably well.

But in 2020, it’s legal to grow hemp for CBD production anywhere in the US. As a result, it will become more challenging to keep the levels low, because most of the hemp plants will be grown outdoors on farms where the soil might be an additional source of contamination. Part two of this series of articles will look at potential sources of elemental contaminants in cannabis and hemp, from a growing and cultivation perspective, while part three will focus on the extraction, manufacturing and packaging processes.

Phytoremediation properties of cannabis and hemp

Plant-based phytoremediation is emerging as a cost-effective technology to concentrate and remove elements, compounds and pollutants from the environment. And within this field, the use of cannabis and hemp plants to concentrate metals from the soil into the harvestable parts of roots and above-ground shoots (phytoextraction) has great potential as a viable alternative to traditional contaminated land remediation methods. However, the natural inclination of these plants to absorb heavy metals from the soil could potentially limits its commercial use for the production of medicinal cannabinoid-based compounds. A number of studies provide convincing evidence that cannabis is an active accumulator of heavy metals such as lead, cadmium, arsenic, mercury, magnesium, copper, chromium, nickel, manganese, and cobalt.

Other pathways for contamination

An added complication is that cannabis and hemp plants not only absorb heavy metals from the soil and growing medium, but also from contaminants found in fertilizers, nutrients, pesticides, and growth enhancers, as well as from other environmental pathways. Additionally, the process of cutting, grinding and preparing the cannabis/hemp flowers for extraction can often pick up elemental contaminants from the manufacturing equipment.

It should also be emphasized that the extraction/concentration process has the potential to extract/concentrate varying amounts of heavy metals, depending on the solvents and the super/sub critical extraction temperatures/pressures used. It’s therefore critically important that an optimized extraction/concentration process is carried out in order to minimize the carry-over of heavy metals, even though it might not maximize cannabinoid yield. It’s also worth pointing out that the equipment used to package and deliver these cannabinoid products to consumers, such as inhalers, vaporizers, transdermal patches, bottles and containers can mean the user is exposed to additional sources of elemental contaminants, apart from what’s in the cannabinoid compound itself.

Main factors for metal uptake from the growing medium

The health and growth of all plants rely on the absorption of essential nutrients and minerals being available in the dissolved, ionic form in the soil. To maintain enough water to survive and thrive, the plant’s primary means of facilitating the movement of water is through transpiration, which is a highly efficient means of drawing a concentrat­ed solution of minerals and nutrients out of the soil. Transpi­ration works by the evaporative loss of water from the shoots, which is controlled by the opening and closing of specialized pores (known as stoma­ta) embedded in the surface of the leaves that initiates gas exchange. When the stomata are open, the pressure potential of the plant becomes very negative, creat­ing a vacuum effect that draws water and nutrients into the plant, moving it from the roots to the shoots. Unfortunatelythis is also the mechanism that the plant draws in heavy metal contaminants in addition to the nutrients.

The rhizosphere is the region of soil in the vicinity of plant roots in which the chemistry and microbiology is influenced by their growth, respiration, and nutrient exchange. Unfortunately, under certain conditions the plant’s root system will preferentially absorb heavy metals over other minerals, which cannot be explained exclusively by passive ion uptake. The hyper-accumulating properties of cannabis and hemp aren’t fully understood, but are partially dependent upon other factors, including soil pH, availability of other metal/mineral ions in solution, the N/P/K (nitrogen/phosphorus/potassium) ratio and the ability of the plant’s natural metalloproteins, which act as chelating compounds to bind with the heavy metals to reduce the rate of absorption and overcome their toxic effects.

It should also be noted that the plant’s natural polyamine compounds will strengthen the defense response of plants and enhance their defense against diverse environmental stressors including toxicity, and oxidative stress. Bode wrote an excellent article on the underlying mechanisms of heavy metal uptake by cannabis plants, which should be essential reading for the cannabis grower.

Based on evidence in the public domain, there are in the order of 15-20 heavy metals found in polluted ecosystems that could be potential sources of contaminants, including Lead, Arsenic, Mercury, Cadmium, Nickel, Vanadium, Cobalt, Copper, Selenium, Boron, Thallium, Barium, Antimony, Silver, Gold, Zinc, Tin, Manganese, Molybdenum, Tungsten, Iron, and Uranium. Many of these elements exist as different species (eg. metalloids), based on their oxidation state, organic/inorganic/ionic form, or more recently as engineered nanoparticles that could find their way into wastewater streams. Their levels of toxicity would need to be investigated further, but there is a compelling case to be made that the many of these elements, metalloids, and speciated forms could be the future basis of a federally-regulated panel of metal-based contaminants in cannabis and hemp.

Outdoor growing sources

Let’s take a closer look at some of the ‘real-world’ sources of elemental contaminants that are supported by evidence in the public domain. It’s only when cannabis and hemp cultivators and growers have a good understanding of this problem, can they hope to reduce or even eliminate them. This isn’t an exhaustive list, but it represents a good starting point to begin investigating these issues.

  • When gown outdoors, if possible, the soil should be characterized to make sure that the elemental contaminants are at acceptable levels. Explore the use of natural chelating agent such as humic acid or synthetic ones like biochar to bind with the harmful metallic contaminants to minimize their uptake by the plant’s root system.
  • In areas where gold and silver mines are found, there is the potential for high mercury levels, as mercury amalgamation is a well-accepted extraction method. These mines are particularly prevalent in the growing states of California, Oregon and Washington, which have some of the highest density of outdoor cannabis farms in the US.
  • All metal smelting plants will experience heavy metal contamination in surrounding areas. Lead and copper ores in particular contain high levels of arsenic.
  • Environmental Protection Agency (EPA) superfund sites, especially those involved in the manufacture of weapons, could have high levels of heavy metal/radioisotope contaminants in the soil.
  • Fly ash waste from coal-fired power plants is extremely high in heavy metals from the coal combustion process. Surrounding areas where the fly ash has been dumped will likely be contaminated.
  • Decades of using leaded-gasoline has contaminated much of the soil close to and around major highways and roads.
  • Many industries are known to emit elemental mercury into the atmosphere, including coal fired power plants, metal refineries/smelters, petrochemical plants, and cement works. It’s well documented by the Clean Air Act that approximately 100 tons of mercury are emitted by US industries annually, much of it being converted into the highly toxic methyl mercury.
  • Wood preservation chemicals contain high levels of copper, arsenic, and chromium. Areas around these plants are likely contaminated.
  • Low-grade fertilizers/nutrients made from phosphate rocks contain significant amounts of elemental impurities.
  • Nickel has been promoted as a cannabis flower/bud-enhancer and silicon as a way of increasing shoot size, which means that higher levels of nickel and silicon will invariably end up in the cannabis product. These elements typically aren’t required by the vast majority of states, so they would escape the scrutiny of most state regulators.
  • Some inorganic pesticides contain arsenic, copper, lead, and mercury.
  • Although there are no elemental species of arsenic and mercury defined in the current state-based limits for heavy metals, the pharmaceutical industry has shown that inorganic and organic forms of these elements should be monitored if the maximum limits for the total amount are exceeded. In addition, depending on where the cannabis/hemp plants are grown, will also dictate whether other speciated forms should be monitored. Examples of this might include the highly toxic hexavalent chromium (CrVI) compared to the relatively innocuous trivalent species (CrIII).
  • At some point in the future, nanoparticle characterization in the soil and uptake by the cannabis plant may be needed particularly when there are regulated methods for environmental and food-based assays.
Indoor growing sources


Although an indoor or greenhouse growing environment is far more controlled than cultivating plants outdoors, there are stillmany opportunities for picking up elemental contaminants. Nutrients, fertilizers and potable water are three of the potential sources of metal contamination. As a result, indoors plants cultivated in a soil-based growing medium or hydroponically-grown, are highly dependent on the nutrients, minerals and water used. For that reason high quality fertilizers and a source of clean water are essential for healthy plants. Here are three possible areas of concern when growing plants indoors.


  • Last year the EPA estimated that 30 million people in the US live in areas where drinking water violated safety standards. The EPA defines a list of primary and secondary elemental maximum contaminants levels (MCLs) in drinking water and overseas all local water authorities/municipalities in the US to ensure compliance. It’s important to know these levels in your region to make sure they are well below the limits for metal impurities (they will be posted online by your local water authority). Also keep in mind that these are levels for samples taken at the water treatment plant and not the levels measured at your home or growing site. EPA mandates that a municipality only has to measure water quality at its customers sites every two years and has to take remedial action only if 10 percent of those sampled are above the MCL. Look what happened with lead-contaminated drinking water in Flint, Michigan, because the source was changed from lake water (Lake Huron) to the local Flint River, without understanding the chemistry and its corrosion properties. As a result, the water dissolved the inside of the old lead pipes and ended up contaminated the drinking water supply.
  • Decades of using lead, cadmium, and arsenic based pigments in residential and commercial paint has meant that many of the older homes and buildings still contain these types of paints, which have often been painted over, but still produce dust/particles that can potentially be problematic. If in doubt, scrape some paint off and get it tested.
  • Some plasterboard used in the construction industry is made from gypsum-based flue gas desulfurization (FGD) waste products mixed with a silicate material known as clinker. FGD is produced by scrubbing particulate emissions from coal-fired power plants, which are notoriously high in heavy metals.
Final thoughts

The pharmaceutical industry has shown that the only way to reduce elemental impurities is to fully understand all the potential sources of contamination. It took over 20 years to investigate every facet of drug production, including analyzing the raw materials, and active pharmaceutical ingredients, together with the entire manufacturing and packaging process. The US cannabis industry is currently nowhere near this level of knowledge. This is even more disturbing because our insatiable demand for cannabis products is being fulfilled by other parts of the world. This was exemplified in a couple of recent newspaper editorials. One of them highlighted the gross soil contamination produced my metal refineries in many parts of China. That in itself isn’t surprising, but the other editorial talked about Yunnan Province in southern China, which is now producing CBD-products for the US market. So the US cannabis and hemp industry must begin to find ways to reduce levels of heavy metal contaminants, otherwise consumers will turn away. If cannabinoid products on the market are considered unsafe to use, it might be too late by the time the FDA comes knocking at the door.

Copyright © 2020 From Measuring Heavy Metals Contaminants in Cannabis and Hemp: A Practical Guide by Robert Thomas. Reproduced by permission of Taylor and Francis Group, LLC, a division of Informa plc. FDA comes knocking at the door.