5. Potential Environmental Impacts of Tyre Leachate
The literature reviewed on the environmental impact of tyre leachate is summarised in Sections 5.1 and 5.2. It should be noted, however, the review was not an in-depth analysis of the literature.
5.1 Recent Laboratory Research
Recent laboratory research on tyre leachate reviewed in this study is briefly summarised in Table 5.1.
Table 5.1: Summary of reviewed laboratory research on tyre leachate
View summary of reviewed laboratory research on tyre leachate (large table)
In the laboratory tyre leachate is often generated by inundating a tyre sample (plug, shred, chip, or crumb) with water. There are limitations when comparing the results of the reviewed laboratory studies as different makes of tyres were used and different compounds were measured; however the following general observations can be made about the tyre leachate generated in the laboratory:
- it may be toxic to some fish species (eg rainbow trout but not minnow), bacteria, invertebrates and green algae
- levels of aluminium and manganese are likely to be elevated, especially where steel is exposed
- levels of mercury and lead may be elevated; however most studies reported negligible levels
- levels of zinc and organic compounds are likely to be dependent on individual circumstances as a wide range of levels have been reported in the studies reviewed
- levels of other substances are likely to below United States Drinking Water Standards
- levels of leachate compounds (metals and organic compounds) are likely to increase with time of inundation, increase proportionally with amount of tyre and decrease proportionally with size of tyre exposed to inundation.
5.2 Recent Field Trials
Recent field trials on tyre leachate reviewed in this study are briefly summarised in Table 5.2.
Table 5.2: Summary of reviewed field trials on tyre leachate
View summary of reviewed field trials on tyre leachate (large table)
In field trials tyre leachate is generated by water percolating through the tyre sample (whole tyre, shred or chip). There are limitations when comparing the results of the reviewed field trials as different makes of tyres were used and different compounds were measured; however the following general observations can be made about the nature of tyre leachate generated in the field:
- levels of manganese and iron are likely to be elevated in groundwater, especially when steel is exposed
- levels of aluminium, zinc and organic compounds may be elevated in groundwater; however the majority of studies reported negligible levels
- level of cadmium and lead may be elevated in soil; however no studies reviewed reported elevated levels in groundwater
- levels of other substances measured are likely to be below United States Drinking Water Standards
- level of leachate compounds in groundwater are likely to decrease down gradient of the tyre site.
5.3 General Observations
The potential environmental impacts of tyre leachate are contamination of soil, surface water and groundwater on the site and surrounding area. Based on the reviewed literature and previous MWH experience in site contamination, factors that may affect the rate of leaching and/or the concentration of tyre leachate compounds in soil, surface water and groundwater include:
- tyre size: leaching from whole tyres is likely to be slower than leaching from tyre chips or shreds - this is because of the differences in the surface area to volume ratio
- amount of exposed steel: if steel is exposed, say in tyre chips, there is likely to be faster leaching of manganese and iron than from whole tyres where the steel is not exposed
- chemical environment: leaching of metals is likely to be more rapid under acidic conditions while leaching of organic compounds is likely to be more rapid under basic conditions
- permeability of soil: leaching is likely to be faster when soils are permeable
- distance to groundwater table: the greater the vertical distance to the groundwater table, the less likely the contamination of groundwater
- distance from tyre storage site: the further the downstream distance from the tyre storage site, the lower the contaminant concentration in the soil and groundwater
- contact time with water: the longer the tyres are in contact with water, the greater the risk of groundwater contamination
- vertical water flow through soil: the greater the water flow through the soil (eg, from rainfall), the greater the dilution of contaminants
- horizontal groundwater flow: the greater the groundwater flow, the greater the spread of the contaminant plume
- leached compounds at site: levels of manganese and iron are likely to be elevated in groundwater when steel is exposed. Levels of aluminium, zinc and organic compounds may be elevated in groundwater. Levels of zinc, cadmium and lead may be elevated in soil.
