Millions of tires are discarded each year, and the scrap piles of rubber this creates pose a serious fire hazard. Rubber tires also leach hazardous materials into the environment as they decompose. Recycling rubber materials for use as mulch has been one solution that many view as environmentally friendly, but the effects of using rubber as mulch material are similar to spreading the problem around your garden.
Decomposition and Leachates
While rubber mulch is often touted as being a permanent mulch material, rubber is like any other organic material in that it does break down when subjected to sunlight, bacteria and fungi. As it decomposes, the chemicals in the rubber leach into the soil and nearby water sources. Many of these chemicals, as well as heavy metals, are dangerous to plants, soil and aquatic systems.
Ornamentals and vegetables mulched with rubber can accumulate high levels of metals used in rubber tires, such as zinc. These metals can build up in plant roots, leaves, or fruit, depending on the plant, and this can lead to plant death. Removing the mulch only solves part of the problem because although the source of the problem is gone, the metals that have leached into the soil are still being taken up by plant roots.
Rubber mulch used near water sources, such as lakes or backyard ponds, can leach materials that may kill aquatic life such as algae, zooplankton, snails and fish. The toxic nature of rubber mulch is due to its mineral content. The tires used to make rubber mulch contain aluminum, copper, cadmium, iron, chromium, magnesium, sulfur, selenium, manganese, molybdenum, and zinc. In addition to these leachates, tires may have been exposed to other contaminants during their useful lifetime, such as heavy metals like lead, and these contaminants may also leach into groundwater supplies.
Plants mulched with rubber may be exposed to high levels of zinc, which can lead to plant death. As rubber decomposes, its zinc content is released into the soil. Acidic soils and aquatic environments are particularly sensitive to zinc toxicity, because heavy metals and other positively charged elements are more available to plants in these soils.
Rubber mulch not only contains mineral and organic building blocks, it may also contain plasticizers and accelerators used during tire manufacturing that are harmful to the environment. Some of these break down quickly, but others persist in the environment. For example, a common rubber leachate is 2-mercaptobenzothiazole, which is an accelerator. This material is highly persistent in the environment and toxic to aquatic organisms. Rubber softeners such as polyaromatic hydrocarbons are also toxic to aquatic life and are released continually as the rubber decomposes.
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