Permeation of hydrocarbons through polyvinyl chloride (PVC) and polyethylene (PE) pipes and pipe gaskets
Plastic pipes have been used increasingly as a means for the conveyance of drinking water in water distribution systems. Although there are research and case studies documenting the permeation of organic compounds through plastic pipes, there is still a lack of understanding on the performance of PE and PVC pipe materials in hydrocarbon-contaminated soils commonly encountered under field conditions.;A microscopic visualization technique was developed to investigate the permeation of organic solvents through PVC pipes by observing the formation and propagation of organic fronts in pipe materials with a light microscope. The threshold concentrations of toluene where a moving front was formed were 25% (v/v) of toluene in polyethylene glycol and 40% (v/v) of toluene in NIST reference fuel. For a combination of BTEX compounds in NIST reference fuel, a BTEX concentration of 40% (v/v) or higher was required to initiate a moving front and potentially cause permeation. This implies that new PVC pipe materials are an effective barrier to resist the permeation of typical commercial gasoline.;Permeation of benzene, toluene and trichloroethylene (TCE) through 1-inch diameter PVC pipes from vapor and aqueous phases was further investigated by using pipe-bottle tests, gravimetric sorption tests and microscopic visualization tests. Saturated organic vapors penetrated through 1-inch PVC pipes within 30 days. Organic compounds in saturated aqueous solutions also permeated through PVC pipes but the breakthrough times were significantly delayed. The breakthrough times of saturated aqueous solutions of TCE and benzene were found to range from 60 days to 240 days, depending on the experimental mixing conditions. Insignificant sorption and no moving front were detected when exposed to the organic vapors that were in equilibrium with ≤40% (v/v) benzene or toluene in NIST reference fuel. Insignificant sorption and no moving front were detected when exposed to water that is ≤60% of the aqueous solubility of benzene and toluene. Based on the experiments conducted, new PVC pipe materials are an effective barrier against the permeation of BTEX in either gasoline vapors or gasoline-contaminated groundwater.;Permeation of petroleum-based hydrocarbons through PVC pipes equipped with Rieber gasket systems was examined by conducting pipe-drum tests as well as model simulation. Under premium gasoline-exposure conditions, the steady-state permeation rates of benzene were estimated to be 0.73+/-0.29 mg/joint/day and 0.19+/-0.18 mg/joint/day for 2-inch SBR and NBR gaskets, respectively. The corresponding diffusion coefficients of benzene in SBR and NBR gaskets were determined to be 1.1x10-7 cm2/s and 6.0x10-8 cm2/s, respectively. The results of model simulations demonstrated that small size gasketed pipes were more vulnerable to permeation than large size gasketed pipes, and pressurized pipes joint systems potentially posed much higher permeation risk than non-pressurized joint systems.;Permeation of BTEX compounds through 1-inch diameter SIDR 9 high density polyethylene (PE) pipe was investigated under simulated field conditions of subsurface gasoline spills, gasoline-contaminated groundwater and unsaturated soil with varied levels of contamination. Using the time-lag method, the concentration-dependent diffusion coefficients of BTEX compounds in PE pipe were estimated to be in the order of 10-8 cm2/s when exposed to free product gasoline and in the order of 10-9 cm2/s when exposed to gasoline-contaminated water solutions or unsaturated contaminated soil. This study demonstrated that small size pipes were more vulnerable to permeation than large size pipes, and pipes with water stagnation periods posed a much higher risk of exceeding the MCL of benzene than pipes with continuous water flow. Under otherwise identical conditions, a PE pipe buried in soil of high organic matter was found to permeate to a lesser extent than a pipe buried in a soil of low organic matter.;Permeation parameters of benzene in 1-inch SIDR 9 PE pipes were estimated by fitting the measured data to a permeation model based on a combination of equilibrium partitioning and Fick's diffusion. For bulk concentrations between 6.0 to 67.5 mg/L in soil pore water, the concentration-dependent diffusion coefficients of benzene were found to range from 2.0x10-9 cm2/s to 2.8x10-9 cm2/s while the solubility parameter was determined to be 23.7. PE pipes exposed to an instantaneous plume exhibit distinguishable permeation characteristics from those exposed to a continuous source with a constant input. The properties of aquifer such as dispersion coefficients (DL) also influence the permeation behavior of organic contaminants through PE pipes.