Assessing natural attenuation at field scale by stochastic reactive transport modelling

Abstract

A combined integral measuring and modelling approach is presented and applied, to assess natural attenuation at the former gasworks site in Stuttgart, ´Testfeld Süd´. The approach copes with subsurface heterogeneity by two different means. The integral measuring approach provides mean concentrations and mass flow rates at control planes, that are obtained by enlarging the sample volume and thus constitute values integrated over the heterogeneous subsurface. Within the modelling approach, heterogeneity is accounted for stochastically by a Monte Carlo approach. To describe the heterogeneity at the ´Testfeld Süd´, a geostatistical analysis of the categorical variable ´facies´, that hydraulic and geochemical parameters are attributed to, was performed. Equally likely aquifer realizations were generated, that served as input for the subsequent flow and reactive transport modelling of acenaphthene and o-xylene. Results of the combined integral measuring and modelling approach were compared to results obtained from an integrally applied compound-specific isotope analysis (CSIA). Therefore, two additional integral mass flow rate measurements were performed to determine both contaminant mass flow rates and mean isotope ratios at two control planes. Results of the integrally applied CSIA method by using the Rayleigh equation revealed that the complete mass flow rate reduction observed for o-xylene is due to biodegradation, which was confirmed by the stochastic transport modelling for o-xylene.