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The effects of aquifer heterogeneity on the natural attenuation rates of chlorinated solvents

Önkal, Başak
Monitored natural attenuation has been particularly used at sites where petroleum hydrocarbons and chlorinated solvents have contaminated soil and groundwater. One of the important aspects of the methodology that has been recognized recently is that the mass removal rates, the most important parameter to determine effectiveness of the methodology, is controlled by the groundwater flow regime and the aquifer heterogeneity. Considering this recognition, the primary objective of this study is to quantitatively describe the relationship between the natural attenuation rates and aquifer heterogeneity using numerical simulation techniques. To represent different levels of aquifer heterogeneity, the hydraulic conductivity distribution (ln K) is statistically simulated with the numerical algorithm, Turning Bands Random Field Generator, by changing the statistical parameters, Coefficient of Variation (CV) and correlation length (h) and Visual MODFLOW and RT3D software programs are used for the simulation of groundwater flow and chlorinated solvent transport. Simulation results showed that degradation rates and the shape of the contaminant plumes show variations for different heterogeneity levels. Increasing CV resulted in the decrease in the transport of the plume and shrinkage in the areal extend. On the other hand, أhؤ determined the shape and the size of the plume through its affect on mechanical dispersion. For a given أhؤ, degradation rates increased with increasing CV, but change in أhؤ did not show a regular trend. Such findings are expected to be beneficial when assessing the effectiveness of natural attenuation process for a selected site during the feasibility studies without need for detailed site characterization.