Beneficial Reuses of Waste Tires for Pollution Reduction

中国环境学会  2011年 06月21日

  Roy R. GU (顾若川)
  College of Environmental Sciences and Engineering, South China University of Technology, Guangzhou, China;
  Department of Civil, Construction and Environmental Engineering, Iowa State University, Ames, IA 50010, USA
  Abstract: Waste tires are a high-profile waste material.  The growing interest in utilizing waste materials in engineering applications has opened the possibility of constructing hydraulic structures with scrap tires.  Applications of scrap tires in hydraulic engineering benefits the environment by reducing the waste, yet one may ask whether scrap tires leach compounds that may adversely affect the environment.  This paper focuses on state-of-the-knowledge about the reuses of scrap tires in hydraulic engineering projects and discusses existing and new utilization methods for potential source reduction, which may be effective in pollution reduction.  The problems associated with scrap tires are identified, the physical and chemical properties of tire material are summarized.  Existing scrap tire applications in hydraulic engineering are reviewed, and new applications are developed and analyzed.  Technical feasibilities are investigated, and environmental impact is assessed.  
  Keywords:  environmental impact; leachates; scrap tires; toxicity; waste recycling; water quality

  Scrap tires represent one of several special wastes that are difficult for municipalities to handle.  Over 242 millions scrap tires are generated each year in the United States.  In addition, between 2 and 3 billion waste tires have accumulated in stockpiles or uncontrolled tire dumps across the country.  Millions more are scattered in ravines, deserts, woods, and empty lots.  Stockpiles of scrap tire are located in many communities, resulting in public health, environmental, and aesthetic problems [1, 2].   The continuing accumulation of waste tires has led to several concerns of varying severity [1]: breeding sites for rodents and mosquitoes, fire hazards, taking up landfill space and disposal problems, environmental impact, and conservation of natural resources.  Because rubber tires do not easily decompose, economically feasible and environmentally sound alternatives for scrap tire disposal must be found.  Recycling, reuse, and recovery of scrap tires are among existing source reduction measures.  Current recycling/reuse alternatives include: (a) reuses of whole tire in civil engineering; (b) applications of split or punched tire in mats, belts, dock bumpers, washers, insulators, etc.; (c) shredded tire applications in lightweight road construction material, gravel substitutes, and sludge composting; and (d) ground rubber products.  Whole tire recycling does not require extensive processing.  All of the recycling alternatives listed above are being used to varying degrees [1].
  However, the total usage of discarded tires for recycling into new products and reuses in engineering projects have not reduced the amount of tires in landfills and illegal dumps.  It is estimated that less than 7% of scrap tires generated annually were beneficially used in various engineering projects, including whole tire applications (0.1%) and processed tire products (6.5%).  The whole tire applications in civil engineering include bank and shore protection structures, artificial reefs, septic system drain fields, breakwaters, dock bumpers, playground equipment, and highway crash barriers [1, 2].  Scrap tire applications in civil engineering do not hold the potential to completely solve the scrap tire problem, but they do have the potential to consume more than they consume now (0.1%).  Increasing available recycling and reuse measures to their full potential and exploring new reuse alternatives will significantly reduce the amount of scrap tires.  A need still exists for the development of more practical uses for scrap tires.  As a part of the possible solution, scrap tire applications in hydraulic engineering can help in easing the waste tire problem.  In addition to existing applications, scrap tires may be utilized in various hydraulic engineering practices involving surface water and groundwater, such as erosion control, soil conservation, drainage, habitat enhancement, and rain trap. 
  In the past, there were some barriers to increased scrap tire utilization.  Economic barriers are the high costs or limited revenues associated with various waste tire utilization methods, which make the uses unprofitable.   A number of constraints on utilization are non-economic barriers, including technical concerns such as lack of technology information or concerns regarding the quality of products or processes.   The barriers also include aesthetic reason, health, safety, and environmental issues [1].   The barriers need to be overcome in order to have more widespread uses of scrap tires.  To do so, technical, economic, and environmental questions need to be answered.  There is also a need to perform research on new methods of recycling and reusing tires. 
  Hydraulic projects, including bank/shore protection, harbor protection, water drainage, rain trapment, and habitat enhancement, may have great potentials of consuming a considerable portion of scrap tires generated in may areas of the country if economic and technical feasibilities and minimum adverse environmental impact can be shown.  Before uses of scrap tires in hydraulic engineering projects, it is important to consider any possible environmental implications.  Such implications include potential surface and ground water contamination and associated risks.  In this paper, issues to be addressed on scrap tire reuses in hydraulic engineering include description of scrap tire characteristics, evaluation of existing scrap tire applications, development of new methods of scrap tire reuses, technical analysis for engineering design, economic feasibility with respect to cost and benefit, and assessment of environmental impact of hydraulic structures made of scrap tires.
  2     Characteristics of Scrap Tires and Beneficial Reuse Methods

  Laboratory measurements of passenger car and truck tires were conducted in previous investigations by Gu et al. [3] and Kjartanson et al. [4].  The widths of truck tire tread studied range from 18 to 24 cm, while passenger car tires have tread widths in the range of 16-20 cm.  The outside diameters of truck tires investigated range from 99 to 107 cm and passenger car tires measured from 59 to 65 cm.  Scrap tires have retaining strength and flexibility—characteristics that make them potentially attractive as a basic construction material.  They are relatively small and easy to handle, and their geometric configuration lends itself to a “building block” approach in constructing larger modules.
  Scrap tires have desirable strength characteristics for hydraulic structures, but are not individually massive in size.  Tires also have the characteristics of being elastic; they are capable of deforming up to 30% without permanent change in shape [5, 6].   Due to the relative inertness of tires, internal structural breakdown is slow.  Therefore, they are capable of maintaining strength and flexibility characteristics over long periods of time, even under unfavorable conditions.  The capacity of the tire to deform under large forces gives it potentially greater dissipative capabilities than more rigid materials.  For instance, in bank/shore protection by tire structures, erosion can be substantially reduced if the majority of the attacking wave energy can be dissipated by breakwaters before reaching the shoreline. 
  The very characteristics that make them desirable as tires, long life and durability make disposal almost impossible.  The fact that tires are thermal-set polymers means that they can not be melted and separated into their chemical components.  Tires are also virtually immune to biological degradation.   Some ingredients used in the manufacture of tires are toxic to aquatic organisms [7, 8, 9].  There may be a potential for toxic materials to leach from tires to water.  Some potentially toxic constituents released into the aquatic environment may be from weathering of tires.
  The basic ingredients of a tire include fabric, rubber, reinforcing chemicals (carbon black, silica, resins), anti-degradants (antioxidants/ozonants paraffin waxes), adhesion promoters, curatives, and processing aids.  Typical materials composition of a tire are synthetic rubber, natural rubber, sulfur and sulfur compounds, silica, phenolic resin, oil (aromatic, naphthenic, paraffinic), fabric (polyester, nylon, etc.), petroleum waxes, pigments (zinc oxide, titanium dioxide, etc.), carbon black, fatty acids, inert materials, and steel wire.  Listed in Table 1 are the major classes of materials used to manufacture tires by the percentage of the total weight of the finished tire that each material class represents.   Scrap tire ash analysis results show a list of compounds:  aluminum, arsenic, cadmium, carbon, chromium, copper, iron, lead, magnesium, manganese, magesium dioxide, nickel, potassium, silicon, sodium, sulfur, tin, and zinc. 
  Table 1 Typical Composition of Passenger Tire by Weight



Natural rubber

14 %

Synthetic rubber


Carbon black




Fabric, fillers, accelerators, antiozonants, etc.


Average weight

New 8-11 kg,  Scrap 6-9 kg

    Tires contain various types of additives in addition to varying proportions of natural and synthetic rubber.  These additives include organic polymers, such as ozone scavengers (paraphenyldiamines), oil-based plasticizers, paints, and pigments (e.g. zinc oxide, titanium oxide).  Metals, such as copper and zinc, are usually present in trace amounts in many steel-belted tires.  Some types of addictives are used in rubber curing during tire manufacture.  For example, benzothiazoles result from the degradation of substances used as antioxidants and from curing compounds used in rubber manufacturing [10].   Spies et al [10] also identified benzothiazoles in estuarine sediments and attributed their presence to the weathering of tires in the watershed.  Chemical tracers present in tire leachates may be useful in tracking the fate of toxic chemicals from tires and of urban runoff.
  The problem of waste tires can be turned into an opportunity.  Many benefits have resulted by focusing on an alternative way to utilize the bulky waste, as opposed to how to dispose of it.  One of the alternatives is utilizing waste tires to control streambank erosion and streambed grade.  In addition to saving valuable public and private land or property threatened by erosion, scrap tire reuse has other benefits.  It provides a use for problem tires, has financial benefits, increases public awareness in the area of natural resource conservation, and improve solid waste management. 
  The widespread availability and durability of tires has led to their use in the marine environment for breakwaters/coastal defense structures and as artificial reefs for promoting fisheries. Tires have a low density and have been used in floating breakwaters.  Shorelines can be protected and strengthened by tire structures. The void space in tires facilitates the construction of artificial reefs to attract fish.  Scrap tires can be utilized to drain surface and subsurface water, including storm sewers, cross road culverts, low water stream crossings, and subsurface drainage pipes.  They can also be used for trapping rainfall and irrigation water in golf courses and lawns.  
  Scrap tire applications in engineering can be in the forms of whole tires and processed tires.  Reuses of scrap tires in hydraulic engineering applications in the form of whole tires perhaps have better economic and technical feasibilities than processed tires.  Hydraulic structures in surface water and groundwater projects can be constructed by using whole scrap tires for erosion control, drainage, habitat enhancement (artificial reefs) and rain collection (rain trap system).  Applications in erosion control include small overfall dams for stream grade control; tire mats, retaining walls, and tire-concrete units for bank protection; and breakwaters and revetments for shore and harbor protection.   Scrap tire pipes or culverts are used for drainage, including subsurface drainage, storm water sewers, roadway cross drainage, and low water stream crossings.    Fastening systems for connecting the tires and fill materials to provide structure weight are needed in whole scrap tire applications.  Whole scrap tires have broader applications in hydraulic engineering than processed tires because whole tires meet project needs while do not require processing costs. 

  3     Technical Feasibility—Design and Construction

  Overfalls for Stream Grade Control
  Small overfall dams for stream grade control can be made of scrap tires.  Channel degradation is a natural event that can occur in any stream or river, especially acute in erodible deep loess region.  As streams erode deeper and wider, the structural safety of numerous roads, bridges and other infrastructures is affected.  The widening of the channel also causes the loss of valuable farmland, which can never be reclaimed once it occurs [11].   Grade control structures are overfalls that raise the streambed elevation, which decreases the slope of the flow line for a given reach upstream, and thus reduces the streamflow velocity within that reach and the rate of channel degradation [12].   As a low cost alternative to concrete and natural rock in stream channel stabilization, small overfall dams using whole scrap tires for stream grade control are developed by Gu et al. [3].   In this application, the entire mass of a stream grade structure (overfall) is composed of whole scrap tires that are tied together and filled with materials to provide adequate resistance to erosion, uplifting, and overturning due to flowing water.  The tire structures recycle a hazardous waste material that has become problematic and at the same time provide a relatively inexpensive method of stabilizing degrading streams. 
  A low drop grade control structure of whole scrap tires (Fig. 1) may fail externally by sliding on a plane within the tires above the foundation or along the contact between the tires and soil foundation and by uplift due to overflowing water and internally by breaking the fastening system or tearing the tire rubber itself.  Sliding along the foundation of the structure may be resisted by the shear resistance between the soil surface and the bottom layer of tires.  Because the damage to fastening systems and the pullout strength of the connecting materials from tires may also cause structure failure, the fastening systems and connection to the tires should be strong enough to resist all potential hydraulic forces.  Uplift due to overflowing water can be overcome by properly designed weight of the structure, i.e. the required unit weight.
  Theoretical studies were performed to analyze stream flows and forces acting on the grade control structures [3].  The calculated horizontal and vertical forces are used to determine the required stability (Fig. 1).   The pullout strength of a binding system or tires and the unit weight of the structure (tires and fill materials) must be adequately designed to resist uplifting, internal breaking-down and sliding.  Drag and lift forces on the structure due to flow can be expressed, respectively, as:
  where FD is the drag force which can cause sliding of the structure, CD is the drag coefficient, which is equal to 1.0 for turbulent flow, r is the density of water, g is gravitational acceleration, Q is flow rate, W is stream width or dam length, FL is the lift force, CL is the lift coefficient, which is equal to 0.25, d is the total depth of flow upstream the dam and y is the depth of flow overtopping the dam, which is determined by
  The safety factor for overcoming uplift is computed from the vertical forces and the weight of the structure, i.e. the ratio of the weight to the vertical forces [3]. In order to determine the required unit weight of a grade control structure and to compute the factor of safety against uplift, the basic rule of statics is applied:   
  From Equation (4), the required structure weight is determined by subtracting the uplift and lift forces from the downward components of water and sediments weights, which gives a safety factor of 1.0 against uplift and lift.   The uplift force is due to the upward pore water pressure beneath the structure, while the lift force is due to the flow on the structure (Equation (1)).  A safety factor of greater than 1.0 may be necessary, which is achieved by using a lager required structure weight than that determined by Equation (4).
  The factor of safety against sliding is determined by comparing the river bottom shear stress and the effective horizontal pushing force.  The effective pushing force includes drag force due to flow on the top of the dam and the difference between upstream and downstream in horizontal components of water and sediment weights.  The shear force between the dam structure and riverbed soils beneath it serves as a resisting force against sliding.  To compute the factor of safety against sliding, the basic rule of statics is also applied:
   Where C is the cohesion between the structure and river bottom, f is the friction angle of river bottom soils, H is the height of the structure, and g is the dry unit weight of the dam. 
  The safety factor of the fastening system is the ratio of the strength of the system to the hydraulic force applied to the structure.  To connect tires into a coherent structure, bindings between and within layers are needed. This binding system is built to resist shear forces between riverbed soil and structure bottom and between tire layers and hydraulic forces generated by flowing water.  Three fastening methods were identified through laboratory measurements of tensile and shear strengths [3] to provide sufficient resistance strength to a tire grade control structure.  They are nylon or steel cable, bolts and nuts (combination of machine bolts and nuts), and lag screws and washers.
  After tires are placed and fastened in designated construction area, various types of materials could be used to fill up voids in the structure for providing the required weight to overcome uplifting.  Bases on laboratory direct shear strength tests and hydraulic resistance experiments [4], three types of fillings are recommended: construction rubble, sand and gravel, and flowable mixture of loess, cement and sand.
  To increase stability and reduce sliding of the whole structure, a key-in structure may be used.  It is suggested that key-in is used to insert the tire structure into the riverbed and walls of the channel.  The dimension of key-in portion is about two tires wide and three tires deep.  These tires are fastened to the bottom and two ends of the proposed structure.  In case actual in-stream flows exceed the design flow, the tire grade control structure may not provide enough unit weight and resistance to hydraulic forces.   Anchoring of the tire structure to the riverbed could help to hold it from floating and sliding.  Steel rod with screw and plate is one of commonly used earth anchors.  For secure gripping and holding power, anchors are screwed into riverbed.  Steel piles may also be used for anchoring purpose.

  Drainage Systems

  There is an increasing need for replacement of many old, unsafe bridges on low volume roads.  Many communities have bridges that are no longer adequate, and are faced with large capital expenditure for replacement structures of the same size.  In this regard, Low Water Stream Crossings (LWSCs) can provide an acceptable, low cost alternative to bridges and culverts on low volume and reduced maintenance level roads.   There are two common types of LWSCs: unvented ford and vented ford with pipes.   Unvented fords are similar to overfalls used in stream grade control.  Reinforced concrete, crushed rocks are most widely used materials for unvented fords.  The application of scrap tires in LWSCs is similar to that in stream grade control structures. 
  A vented ford is one of LWSCs that has pipe(s) under the crossing that accommodate low flows without overtopping the road.  High water will periodically flow over the crossing.  The pipe(s) or culverts may be embedded in earth fill, aggregate, riprap, or Portland cement concrete.  Corrugated metal, plastic and precast concrete are commonly used materials for pipes in vented fords (LWSCs).  Scrap tires may also be used to replace these more expensive pipe materials if they adequately designed.  A vented ford for LWSCs is a type of roadway cross culverts.  In addition to the ford, the pipes in the vented ford can be made of scrap tires as that in the roadway cross drainage.
  The design of surface water and groundwater drainage systems, including roadway cross culverts, low water stream crossings—vented fords, subsurface drainage pipes, and stormwater sewer pipes consists of two parts: hydraulic design and structural design.  Open channel flow principles, culvert hydraulics, and the theory for pressured flow in pipes are used in hydraulic analyses of the drainage systems.  Procedures were developed by Gupta [13], Yang [14] and Normann et al. [15], applying conservation laws (including continuity, energy and momentum equations), Manning’s equation, critical flow equation, frictional head loss, and local or minor energy losses.  The theory, principles, and procedures can be used in the analysis and design of scarp tire drainage systems.  A design flow capacity and a length of the culvert or pipe are selected first.   As the size of scrap tire is known, the slope and number of pipes are adjusted so that the pipes are adequate for conveying the design flow.
  A conduit built with whole scrap tires by steel bandings is suitable for low or moderate water flowrates.  The roughness coefficient of the tire culvert is an important parameter for hydraulic design.  A value of 0.05 was estimated by Yang [14].   The tire culvert is designed for partial flow only due to the high roughness and to avoid buoyancy effects associated with air trapped in the top of the tires.  The maximum water depth inside the pipe is limited to 75% of the pipe diameter.  For underground pipes, the range of soil depth above the tire pipes is limited by allowable pipe deflection, which is determined by the type and degree of compaction of the backfill soil and the surface load.   Yang [14] recommended 0.3-1.2 m for minimum soil covers and 6-45 m for maximum soil covers.
    Cross roadway drainage pipe can also be made of scrap tire beads and sidewall (Fig. 2), which are usually used as culverts at small streams and ditches on county or rural subdivision roads.   The pipe material is produced by cutting the bead and adjacent sidewall from a scrap tire.  The pipes are made using a hydraulic press to compress about 80 tire bead-sidewalls to a length of approximately 2.4 m.  Four #3 rebars are then wrapped lengthwise around the pipe walls, 90 degrees apart, and welded.  Rebars are steel rods, commonly used in reinforcing concrete.  Field connections between two end-to-end tire pipe sections can be made with a flexible belt, which is wrapped around the two abutting pipe ends and cinched in place with a strap.  The pipes can be used as gravity flow drainage conduits where soil provides support to the pipe walls.

  4     Environmental Impact Assessment

  Beneficial Effects
  Successful hydraulic engineering applications can, in addition to construction material cost saving, create beneficial impacts of scrap tires in hydraulic structures on the environment, which is primarily waste reduction, relieving the nation's horrendous, ever-mounting, scrap tire problem.  A hydraulic structure may consume thousands of scrap tires.  The rain trap system uses 1.2 million tires during the construction of an average golf course.  Moreover, scrap tires are proven to be benign in the environment, for instance, in rain trap system, one of the hydraulic applications.  
  The rain trap system installed in golf courses can help in meeting requirements for groundwater protection from maintenance chemicals because the tires act as a barrier between fertilizers and the groundwater below.   The rain trap system will not only save irrigation water and protect groundwater but also reduce the amount and frequency of fertilizer application.  In the rain trap application, tires protect the environment by trapping and reusing the fertilizers and other additives, which are applied to turfgrass. This reuse allows lighter applications and temporary storage so the compounds can further break down. The next substantial rainfall will flush out the tires and the cycle repeats itself. 

  Potential Problems
  Applications of scrap tires in hydraulic engineering benefits the environment through waste reduction, yet people naturally ask, for example, whether scrap tires leach compounds that may adversely affect the environment.  Practical issues and potential problems related to environmental impact of scrap tires in hydraulic structures include chemical leaching, toxicity, soil contamination, water pollution, structural integrity, and aesthetical feature.
  Use of recycled materials such as scrap tires in engineering projects should not pose a problem with respect to environmental impact and human health.  Scrap tires used in hydraulic structures may have the potential of chemical or metal leaching into the surface water or groundwater.  The water may then serve as a pathway to transport these chemical contaminants to human and environmental receptors.   When determining and conducting beneficial uses of scrap tires in hydraulic engineering applications, one must assess risk to human health and the environment.
  In surface water applications, the issue of physical integrity and aesthetics of hydraulic structures must be addressed as scrap tires are widely accepted as a suitable construction material.  Poor deployment and construction practices may, however, lead to tires washing off after storms and result in environmental problems.   The stability and integrity of a scrap tire structure must be maintained to prevent structural failure.  This problem can be solved by improved design and construction with sufficient fastening and anchoring.
  Another concern over the use of scrap tires in hydraulic structures is that the aesthetic characteristics of protective structures using scrap tires may be less than desirable to some observers.  Nevertheless, the tradeoff with cost and ease of construction will determine the relative importance of aesthetic attitudes. 

  Chemical Leaching to Seawater
  A review of the scientific literature has yielded some information on the environmental impact of tires and in particular, the leaching of heavy metals and organic compounds from tires into sea water.  Preliminary results of tire and seawater leaching studies are presented by Collins et al. [16].  These identify zinc as the major leachate (totaling 10 mg/tire after 3 months).  Diluted leachates have not shown significant effects of the growth of the phytoplankton phaeodactylum and isocrysis [16].  The work by Stone et al. [17] to characterize the sea water leaching of tire compounds demonstrated that tire exposure had no detrimental effects on two species of marine fish, pinfish and black sea bass.  
  An investigation of toxicity of scrap tires leachates in estuarine salinities was conducted by Hartwell et al. [18] to determine if tires are acceptable for artificial reefs.  It was found that tire leachates are toxic to some marine species; and their toxicity varied inversely with salinity.  Toxic effects were not apparent at 2.5% salinity.  This may be due to differential leachability of toxic chemicals, differential interaction of salts and toxicants, and an effect of salinity on tolerance of the organism, or some combination of these factors.  Toxicity diminished substantially with sequential extraction and quickly, rather than gradually and steadily over several weeks.  Similar results were observed in freshwater experiments [19].  The toxicity values do not suggest a substantial threat by tire reefs to water quality.   The use of tires in higher salinity environments appears to pose little direct toxicological risk to resident organisms.
  Chemical analyses revealed no specific components as the cause of observed toxicity.  Antagonism between sea salt and toxic chemicals is hypothesized to cause differential toxicity at varying salinities, as opposed to differential solubility of the toxicants.  Extrapolation of laboratory results indicates that proposed tire reefs should not pose a serious threat to water quality in Chesapeake Bay.  No observed effects concentrations (NOEC) were an order of magnitude or greater above field concentrations calculated from simplified methods.  Toxic substances appear to leach from the surface of the tires not from the tire matrix.  The use of tires in higher salinity environments appears to pose little direct toxicological risk to resident organisms.

  Leachates to Fresh (Surface) Water

  In all situations where scrap materials come into contact with surface water, there will be a concern regarding the potential for contamination of water by the material.  Nozaka et al. [20] found no harmful substances leached from tire material soaked in fresh water while the results of Kellough’s [21] freshwater tests suggested that some factor in tire leachate was toxic to rainbow trout.  Evidence [22] suggested that tires are unlikely to cause danger to trout and other aquatic life primarily because the rates of water flow can provide sufficient dilution to prevent the effect.  Barris [23] studies a 32-acre pond half filled with 15 million tires.  All metal and organic compounds tested were either below detection limits or below regulatory limits except Iron.  While toxicity caused by zinc was observed in laboratory tests by Nelson et al. [14], it is unlikely that zinc concentrations leached from the tires used in artificial reefs in canals (freshwater) would ever cause acute or even chronic toxicity.   Tests with whole tires showed that zinc concentrations declined over time.  Chemistry tests for organic compounds also indicated that these chemicals would not be a problem.  Therefore, the use of tires in water would not result in adverse changes in water quality.
  The use of tire reefs in aquatic environments which have relatively small volumes (e.g. canals) with small dilution capability may raise water quality concerns.  Nelson et al. [14] conducted three laboratory tests using plugs cut from tires and whole tires to identify tire leachate and performed a risk assessment of water quality effects.    It was believed that the use of tires in artificial reefs in water would not result in deleterious changes in water quality [14].
  A laboratory study was conducted by Day et al. [19] to determine if automobile tires immersed in fresh water leach chemicals, which are toxic to aquatic biota.  No toxicity to cladocerans (Daphnia magna; 48-h exposure) or fathead minnows (Pimephales promelas; 96-h exposure to leachate from 20 and 40 days only) was observed with these same leachates.  Tires from a floating tire breakwater, which had been installed for several years, did not release chemicals.  In separate experiments, concentrated leachate from tires immersed for 25 days in water inhibited bioluminescence in the marine bacterium.  Several other screening tests (e.g., nematode lethality/mutagenicity and bacterium motility inhibition test) were not sensitive to tire leachates.

  Impact on Groundwater and Soils
  When scrap materials are placed in contact with groundwater, they may potentially cause groundwater pollution and soil contamination.  Spagnoli et al. [24] found that the long-term immersion of tires in water creates two main groundwater contaminants, iron and manganese.  However, in most cases, these contaminants are considered secondary issues that could affect the color and taste of drinking water.  Opinions appeared divided over whether the two contaminants would cause a significant groundwater problem in septic system leach fields.  
  Park et al. [25] presented measured data, using USEPA toxicity characterization leaching procedure (TCLP), of metal and organic compounds that were below detection limits or regulatory levels.    It was concluded by Edil et al. [26] that scrap tires leached very small amounts of substances and have little or no effect on groundwater.   Bosshner et al. [27] also concluded that leachate from tire chips is not likely to have adverse effects on groundwater.
  For applications in the rain trap system, the results of the EPA's toxicity characterization leaching procedure (TCLP) indicate that none of the tire products tested exceeds proposed TCLP regulatory levels.  Most compounds detected are found at trace levels, (near method detection limits), from 10 to 100 times less than TCLP regulatory limits and U.S. EPA drinking water standard maximum contaminant level (MCL) values.  The Florida Department of Environmental Regulation released its final report on tire leachability in potential usage environments. The study, which evaluates the leachability of shredded tires in different aquatic environments, finds that scrap tires pose no harmful effects when used in applications that are above the water table.
  The study of water quality effects of tire chip fills placed above the groundwater table by Humphrey et al. [28] showed no evidence that tire chips increased the concentrations of metals that have primary drinking water standard.  They did not increase aluminum, zinc, chloride, or sulfate which have secondary (aesthetic) drinking water standards.    Downs et al. [29] recommended that tire chips be used above the groundwater table or where increased levels of iron and manganese can be accepted. 
  Some reduction of leachate from scrap tires to surface and ground water may be achieved by soils.  A way to reduce the general leaching of heavy metals from shredded tires to acceptable levels can be achieved by mixing shredded tire with a clayed soil as clays have some ability to adsorb heavy metals.  In the scrap tire applications to river grade control structures and low water stream crossings, loess in fill materials and river bottom soils (clay) may assist to reduce leaching of heavy metals to stream water by adsorbing heavy metals if any leach out of scrap tires.  In other applications, such as bank protection, drainage pipes, and culverts, soils in contact with tires may serve as a buffer for attenuating possible leaching of heavy metals to water through adsorption before they enter stream water.

  5     Conclusions and Recommendations

  A large number of used tires are disposed every year.  Existing reuses of scrap tires in hydraulic practices, such as bank and shore protection and artificial reefs for habitat enhancement, were reviewed and their potentials, engineering and economic feasibilities, and environmental impact were analyzed.  New areas of applications have been identified and discussed, including stream grade control, low water stream crossings, surface and sub surface drainage, and rain trapment.  Studies and analyses presented in this chapter indicated that hydraulic structures built with scrap tires are technically adequate, economically feasible, and environmentally desirable for their waste reduction potentials. 
  In grade control and drainage applications, considering their sizes and aesthetics, scrap tires are most suitable for small-scale projects on small streams and in small towns and communities.   They include overfalls, low water stream crossings, subsurface drainage, storm-water runoff drainage, and roadway cross drainage.
  One of the benefits of scrap tire reuse in hydraulic engineering is lower material cost than conventionally used materials in surface and groundwater structures.  The availability of scrap tires is so great that they can be readily and inexpensively obtained with only transportation cost.  The total cost of using scrap tires in hydraulic structures, including tire processing costs, can be ranked according to reuse methods as (from low to high): whole scrap tires, split tires, cutting beads/sidewalls, shredded, and bale. 
  The investigations and results to date have examined the potential environmental impact of scrap tires on marine environment (salty water), surface (fresh) water, groundwater, and soils.  The methods used in previous studies included laboratory tests and measurements and field monitoring with various testing procedures.  Leachability tended to depend on influent solution pH.  Toxicity varied with salinity and time.  Previous studies and assessments demonstrated that use of scrap tires in surface water and groundwater is safe to the environment in most cases.   However, previous studies also indicated some chemical leachates from scrap tires in surface and ground waters.   Therefore, applications of scrap tires in some extreme conditions should be avoided, such as in extreme pH values, small water volumes for assimilative capacity, low velocity or dilution capability, and where iron, zinc, and manganese not acceptable.  Tire applications should not be made in extremely acidic conditions.  It is preferred that scrap tires be used above the groundwater table over below the table, if possible. 

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