Desorption and release of polycyclic aromatic hydrocarbons (PAHs) from contaminated aquifer materials

Abstract

Contaminations of aquifer material in the subsurface by PAHs are a widespread source of groundwater contaminations at practically all former manufactured gas plants and coking plants. Possible risks that arise for the groundwater quality in the vicinity of such sites depend largely on the release rates of contaminants from the site into the adjacent groundwater body. Recent field studies and laboratory experiments indicate that the emission of contaminants appears to be controlled by two major processes: dissolution of PAHs from residual coal tar and the desorption of PAHs from soil solids. Interaction of dissolved organic species with aquifer- or soil solids e.g. sorption and desorption, depend on the physical-chemical parameters of the contaminant as well as of those of the aquifer material. As the transfer of contaminants into the aqueous phase (from sorption sites in the intraparticle domain or out of residual phase) may be facilitated by molecular diffusion, the time scale that has to be considered for remediation actions is controlled by the effective transfer coefficients. The scope of this study was to measure the pollutant release rates in benchscale column experiments. The samples consisted of various aquifer materials either from aged contamination sites or aquifer materials that were contaminated under controlled laboratory conditions and left to interact for extended periods of time (hundreds of days). Desorption of PAHs from the contaminated aquifer sands appears to be completely reversible. The derived diffusion rate constants are in agreement with those achieved in sorption experiments. Increasing hydrophobicity of the PAHs is reflected by decreasing diffusion rate constants and is in accordance with the retarded pore diffusion model. Monitoring over extended periods of time may expose changing desorption regimes. An initial period of high release rates is followed by an extended period of slowly declining fluxes. Derived long-term rate constants facilitate predictions as to future release rates and times for contaminant removal. In any case the predicted release rates are a conservative estimate and represent maximum values. Diffusion rate constants are temperature dependent. Activation energies derived from temperature controlled experiments yield values that indicate hindrances other than retarded pore diffusion in an aqueous environment, e.g rate limitation due to intrasorbent diffusion or constrictivity effects. The use of surfactants to increase release rates seems to have only limited effect on PAHs sorbed in the intraparticle domain. Samples containing residual tar phase show a correlation between release rates and flow velocities. Effluent concentrations can display maximum solubilities for the respective PAHs. The saturation distance needed to reach maximum saturation in the effluent was found to be on the order of decimeters for PAHs of a Kow higher than 5.2, indicating that even small contaminated domains may cause max. effluent concentrations downgradient. PAH release rates from residual phase at nonequilibrium conditions depend on flow rates. PAH fluxes increase as flow velocities increase. In contrast to equilibrium release the increase is nonlinear.