Modeling for Design and Testing of Treatment and Remediation Technologies for Aquifer Soils Contaminated with Organic Waste Chemicals

Principal Investigators
T.H. Illangasekare, University of Colorado

Abstract

Goals: Objectives include (1) investigation of NAPL entrapment; (2) mobilization of entrapped NAPL; (3) modeling of interphase mass transfer; (4) development of numerical models; (5) generation of data in soil flumes for model validation; and (6) field validation.

Rationale: Existing models of transport and entrapment behavior of nonaqueous phase organic chemicals based on traditional petroleum engineering formulations are inadequate for situations dealing with complex soils and chemical types found at wastes sites.

Approach: The proposed approach involves continuation of laboratory investigations with more realistic soil types and chemicals which are of relevance to waste problems in EPA Regions VII and VIII. Model development efforts will be continued, guided by the qualitative results of the fundamental processes and data generated in the laboratory investigations. Issues related to micro- and macro-scale entrapment, preferential flow, mobilization, and mass transfer between phases will be investigated further to develop realistic and accurate models. A radically new approach in which the core of the model becomes the available data will be utilized in the transport model development. Basic to this approach is that the models will be driven by the data rather than by a sophisticated numerical algorithm as in existing numerical models. The developed model will then be verified in the laboratory. The developed models will be used to conduct transport simulations to perform a retrospective performance analysis of a field treatment system. A waste site in Region VIII is being used to conduct this case study and field demonstration.

Status: Entrapment and mobilization experiments have been performed. Results of experiments on macro-scale entrapment in sandy soil demonstrated that preferential flow and macro-scale entrapment are not controlled by properties of a "mixing layer" at the coarse/fine interface. It is believed that these phenomena are dependent solely on the displacement pressure of the fine soil. NAPL mobilization experiments were conducted in a soil column where saturated zone entrapment was established using coarse sand layers embedded in a fine sand matrix. Possible mobilization by changing the water and NAPL fluid pressures which may result during water flooding and water table fluctuations were investigated. An experimental program was developed to investigate macro-scale retention of NAPL in the vadose zone. The experiment showed that macro-scale retention can be observed in a layered system and that it is semi-permanent (high saturations of NAPL found in the fine sand on top of the coarse sand did not decrease measurably in more than a month). Comparison with static pressure distribution profiles for capillary pressure and saturation show that the experimentally-found high saturations cannot be explained under assumption of a static pressure distribution. Thus it can be concluded that macro-scale retention is controlled by a dynamic process, or more specifically, by the minimum hydraulic conductivity in a soil profile. Modeling of flow and transport processes through fractures in soil formations was performed. Dissolution of trapped fluids has also been studied. Development and laboratory validation of numerical models are ongoing. The third year of this project has been completed.

Clients/Users: This research is partially supported through the Robert S. Kerr Environmental Research Laboratory. Other researchers, regulatory personnel, responsible parties, and private contractors have expressed interest in this research.

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