Characterization and Modeling Approach for Matrix Diffusion and Remedial Alternatives Evaluation in Fractured Sedimentary Rock

Detailed site investigations and numerical modeling was conducted to evaluate transport and fate of chlorinated solvent contamination in a fractured sedimentary bedrock aquifer at a Superfund site in New Jersey. Investigations followed the Discrete Fracture Network (DFN) Approach including transmissivity profiling, borehole geophysics, hydrogeophysical logging and intra-borehole flow testing, continuous coring with detailed rock matrix sampling, installation of multilevel wells providing head profiles and depth-discrete groundwater data, and integrated pumping tests to evaluate contaminant mass discharge.  This information provided a framework for understanding fracture network characteristics, transport pathways and matrix diffusion and reaction processes that strongly influence contaminant distribution and fate, which is critical to assessing rates of plume front migration, risks to downgradient receptors, including  municipal well fields, and assessment of remedial alternatives.  The field data provided the basis for numerical modeling which was conducted in two stages: 1) an equivalent porous media (EPM) flow model (MODFLOW-2005) was used to evaluate the bulk groundwater flow system and contaminant transport paths under different historic aquifer stress conditions; then 2) results of the EPM model informed a discrete fracture network (DFN) model (FRACTRAN) for rigorous assessment of flow and transport including effects of matrix diffusion and other processes such as sorption and degradation that control plume transport and fate in fractured sedimentary rock.  The combined model results were used to further develop the conceptual site model and evaluate remedial strategies, and showed contaminant transport occurs in a well-interconnected network of fractures and that matrix diffusion and other processes cause strong plume attenuation such that future risks to municipal well fields should be negligible. Modeling also showed futility of further source remediation efforts, which would have minimal impact on the plume over practical timeframes.  Results of the study were used in support of a Technical Impracticability (TI) waiver for the site.