Practical Modeling Applications to Support In Situ Remediation System Design and Assessment

In in-situ groundwater remediation pracitices, specific reagents are often delivered to the aquifer to create in-situ reactive zones (IRZs), where contaminant treatment is achieved through biotic and/or abiotic processes. Based on ARCADIS experiences, overall treatment success is  more dependent on effective reagent delivery than on the kinetics of contaminant reactions. Therefore, successful in-situ remediation system design and performance assessment rely heavily on an accurate understanding of the regent delivery and distribution in the subsurface.  In many cases, numerical modeling provides a uniquely effective method for interpreting field data and understanding flow systems and critical solute transport processes. This paper presents two case studies on the practical applications of numerical modeling to support in-situ remediation system design. In both cases, a finite model was developed and calibrated to the hydraulic and tracer/reagent data. The numerical model was used to evaluate and simulate the flow  pattern and to interpret tracer, reagent, and remediation performance data.

The first case presents an extensive data set combined with detailed modeling evaluation of paired dual-screened groundwater recirculation wells used for in situ bioremediation. A key understanding developed from the evaluation is that there is a very strong component of vertical flow beween the shallow and deep screen intervals.The modeling evaluation confirmed that the lower-than-expected vertical anisotropy caused the degree of vertical flow observed. Further, the modeling provided a clear understanding of potential stagnation zones and supported appropriate expectations of reagent distribution under continued operation. In the second case, the numerical transport model, based on the dual-domain concept, was used to interpretate tracer data in a pilot-scale IRZ for metal remediation. The results of the tracer test and modeling supported the the remediation system design/implementation and played a key role in the IRZ remedy success.