Water & Soil Remediation
“Bacteria are the most nutritionally diverse of all organisms, meaning they can eat almost anything.” – The Greater Victoria Composting Education Center.
Versions of this sentence pop up on a number of different composting sites, but unlike some claims that circulate the web, this one is true. Some micro-organisms, we have discovered, can digest oil and can help clean up oil spills on water or on land.
Modern industrial processes have left wide-spread and dangerous pollution in soil and water across the globe. Efforts to decontaminate or remediate these sites are both costly and time-consuming. Compost has turned out to play a key role in a number of quicker and cheaper alternatives. Frequently it is the microbes in compost that do the dirty work.
Bioconcentration, Bioaccumulation, and Biomagnification
The most troublesome pollutants are heavy metals and persistent organic pollutants (POPs). One reason they’re so toxic is that they bioconcentrate, bioaccumulate, or biomignify in the body.
It’s widely known that mercury is dangerous largely because it accumulates in the body. We can’t get it out of our systems. As a result, long-term, low-level exposure can be as dangerous (though in different ways) as a single, high-level exposure. The same is true for a variety of other substances.
All of these terms apply to toxins that most animals cannot efficiently eliminate from their bodies. The difference is the route through which the organism is exposed to the toxin.
Bioconcentration occurs after exposure by respiration only, whether through gills for a trout or lungs for a rabbit. A chemical bioconcentrates if levels in an organism rise consistently due to breathing contaminated air or water.
Bioaccumulation refers to exposure by any means, including diet, but it still refers to levels in a particular organism — a single fish or rabbit.
Biomagnification involves the passing on of chemicals from prey to predator, or across trophic levels. Bobcats and coyotes, both carnivores, are at the same trophic level: both eat rabbits, (when they can) which are herbivores, a different trophic level.
(See David Alexander’s Encyclopedia of Environmental Science, 1999.)
As a result of bioconcentration alone, toxic levels in a mussel or fish can be thousands of times higher than the levels in the surrounding water. When another fish — or a bear — eats that fish — it absorbs all of that accumulated toxin. And the larger animals are likely to eat more than one fish. Rarely does someone eat a single oyster. And while one good-sized trout may be enough for today, an otter or bear will want more tomorrow. As a result, levels in higher carnivores and omnivores (humans, for instance) can again be thousands of times higher than those in the surrounding environment.
A number of heavy metals, as well as a whole raft of complex organic compounds such as DDT, dioxins and persistent organic pollutants (POPs) such as PAHs and PCBs, all fall into this class of chemicals. Removing them from our soils and waters is a high priority. Compost can help do this.
Lifting Heavy Metals
The idea that compost can help remove or break down pollutants from contaminated soil and water may sound like wishful thinking. But the use of compost to decontaminate soils is a well-established field in which scientists have been working for decades. Phytoremediation actually has several branches. (All are summarized on pages vii and viii of the EPA’s 1999 Phytoremediation Resource Guide – PDF format.)
Phytoextraction, for instance, involves a process in which plants take up toxins from polluted soil or water and accumulate them in their foliage. Plants that can absorb over a hundred times as much of a particular chemical as can run-of-the-mill plants qualify as superaccumulaters. Once the toxins have been accumulated in the plant stems or foliage, they can be harvested and removed for further processing. This is generally much less expensive that removing and treating soil, though it only works for some chemicals.
The most unlikely plants turn out to tolerate and to lift toxins out of the soil. The lowly alpine pennycress (Thlaspi caerulescens), whose habitat includes several European countries and a wide swath of western North America, is one of these. It can absorb high levels of cadmium and zinc without suffering from phytotoxicity (damage through toxins).
When compost is applied to contaminated soil where alpine pennycress is grown, phytotoxicity falls even more. Precisely why this happens isn’t yet completely understood. But scientists suspect that what’s at work is the variety of ways that compost boosts plant health — by introducing and supporting beneficial micro-organisms, by binding nutrients in the soil and by making them more available to plants — rather through any one ingredient or attribute.
Whatever the mechanism, the effect has been documented in a number of studies. One 1997 study used alpine pennycress to remove zinc and cadmium from a heavily contaminated Pennsylvania site. It reported in suitably restrained tones that “compost treatment reduced metal phytotoxicity.” (The study abstract can be found on p.53 of the EPA Phytoremediation Resource Guide – PDF format.)
While compost helps hyper-accumulators absorb toxins it can, conversely, bind others in soil, keeping them out of plants and helping to protect crops (and people) from contamination. Arsenic is a key example.
The dangers of what is known as pressure-treated wood are well known. Arsenic can contaminate soil near wood treated with chromated-copper-arsenate as a preservative and can be taken up by vegetables such as carrots and lettuce. In two recent studies by scientists at the University of Florida, one in 2003, the other in 2004, phosphate added to soil increased plant uptake of arsenic while compost reduced it. In the second study, arsenic uptake increased by as much as 79-86% compared to uptake in uncomposted soils.
Both of those examples involve the application of compost to soil. But some contaminants can actually be broken down by the composting process itself. To the uninitiated (i.e., most of us), this sounds even more unlikely. Who would guess that composting could directly transform 2,4,6-trinitrotoluene, which most of us know as the explosive TNT, into less toxic minerals? Yet this is so well established that scientists are testing not whether it is possible but which composting method does the job best. A study published in 2004 found that compost started with a long anaerobic period degraded TNT more completely than did compost that was constantly aerated. (See “Bioremediation of 2,4,6-trinitrotoluene-contaminated soils by two different aerated compost systems.”)
Persistent Organic Pollutants
Up and down twenty miles or so of the upper Hudson River, a heavily wooded stretch of seemingly pristine beauty, signs warn fishermen not to eat their catch. They are restricted to catch-and-release not for the fish’s sakes but for their own. For almost 200 miles up from where the Hudson reaches the Atlantic, the river, its bed, and all aquatic life in it are heavily contaminated with PCBs (polychlorinated biphenyls) that were dumped in the river over a quarter of a century ago.
Over several decades, General Electric dumped an estimated 1.3 million pounds of PCBs into the Hudson from its manufacturing plants at Hudson Falls and Fort Edward. Dumping ceased in 1977 and 197 miles of the river were declared a Superfund site in 1983. Ensuing legal battles dragged on so long that General Electric will only begin dredging out the contaminated soil in the spring of 2009.
PCBs, like dioxins, DDT and others, are persistent organic pollutants or POPs. POPs rate as some of the world’s most toxic and troublesome pollutants, for reasons explained in part by their name. They persist in the environment for a very long time, resisting most natural degradation processes. Yet vermicompost, which has a very high level of microbial activity, effectively remediates POP-contaminated soils, reducing them to their constituent and less dangerous pieces and parts.
The health problems caused by POPs are many and serious. In “A Global Issue, a Global Response,” the EPA reports that “In people, reproductive, developmental, behavioral, neurologic, endocrine, and immunologic adverse health effects have been linked to POPs.” The language is cautious but the list long. The World Bank puts it more poetically, if more bluntly: “They [POPs] are highly toxic, causing an array of adverse effects, notably death, disease, and birth defects among humans and animals.” This source then goes on to list some of those “adverse effects: “cancer, allergies and hypersensitivity, damage to the central and peripheral nervous systems, reproductive disorders, and disruption of the immune system.”
“Persistent” means that these chemicals are extraordinarily stable. It also means that they tend to accumulate in both animals and humans because most organisms can neither eliminate or break them down. In other words, they usually bioconcentration, bioaccumulation and biomagnification. As a result, in fatty tissue, POP “concentrations can become magnified by up to 70,000 times the background levels.” (See “What are POPs?” The World Bank.)
Two of the chemical groups classed as POPs — PAHs (polycyclic aromatic hydrocarbons) and PCBs (polychlorinated biphenyls) — are objects of particular concern.
PAHs, naturally present in most fats and oils, are produced by a wide array of chemical processes. They’re also produced whenever an organic substance — wood, coal, tar, tobacco, gasoline, kerosene, cotton, wool, paper — burns incompletely. They’re in the charred material that builds up on the inside of the grill and the charred material on the steak that comes off the grill. They’re probably what’s behind the commonly repeated truth that char-broiled meats are carcinogenic. They are also found in all fossil fuels as well as in some edible fats and oils. Many PAHs cause cancer or birth defects.
PCBs had wide industrial use in the decades after WWII, especially in the electrical industry, despite having been recognized as extremely toxic as early as 1937. Manufactured in the U.S. solely by Monsanto, they entered the environment through manufacturing pollution and routine if irresponsible disposal as well as through numerous spills and accidents. Though they were banned in the US in the 1970s and are increasingly restricted around the world, they’re still found in rivers, lakes and soils where they leaked or were dumped.
A spate of studies in the past decade or so testifies to the extraordinary ability of composts, worms and their castings to help detoxify even these most resistant and dangerous chemical compounds. Some studies have tested vermicomposting itself, which consists essentially of adding worms and a food source (usually bio-sludge) to contaminated sites or contaminated soils. The soils are then tested for contaminant levels, as are the worms themselves. (A 2008 overview of such studies can be found here.)
Other studies have added compost or vermicompost to treatment processes, still others have used either bacteria or various mycelia (molds and mushrooms) isolated from composts. Again, results are usually positive. In these cases, the micro-organisms that flourish in composts eat the toxins, producing simpler, less toxic compounds.
Why does compost detoxify pollutants?
The last few words in the title of one scientific paper help to explain why compost and vermicompost are so effective. The paper explores how “a non-acclimated, complex source of microorganisms” was used to remediate the soil at a seriously contaminated site.
The point about composts and vermicomposts is that they support such a rich, varied and robust range and quantity of microbes, fungi and other organisms. Instead of trying to refine and purify a particular strain of microbe to tackle a particular pollutant, scientists in some areas are relying more and more on the sheer diversity of microbes in compost to do the work. Put this non-specific mix of microbes in contact with contaminated soil or water and the microbe capable of digesting the contamination will proliferate, digesting the pollutant.
This approach makes especially good sense when we consider that very few sites are in fact polluted with only one thing, and that the chemical stew in one place is unlikely to be reproduced anywhere else. In other words, tailoring a treatment to a site would be expensive and time-consuming. There are places for such tailoring, but in many cases it’s quicker, cheaper and more efficient to just use compost.