Gas-Phase Chlorinated Organic Solvent Plume Analysis Through Numerical Simulation

At subsurface waste disposal sites, degradation of containment materials can cause leaks of chlorinated volatile organic compounds (Cl-VOCs) in the vadose zone. Material Disposal Area L (MDA L) is a heavily monitored waste site at Los Alamos National Laboratory in Northern New Mexico where a sharp increase in contaminant concentrations was measured in February 2019. Subsequently, soil vapor extraction (SVE) was performed as part of an ongoing interim measure. Here, we demonstrate a new method to introduce possible leakage within an existing numerical framework to bound possible leakage related to concentration increases seen in site monitoring data. A previously calibrated three-dimensional (3-D) model for SVE at MDA L is used to simulate the three conceptual stages from June 2017 to July 2024. The three conceptual stages based on the observed events are: leakage, passive diffusion, and soil vapor extraction. We use a 3-D multiphase flow simulator to introduce a simulated leak and attempt to approximately match monitoring data collected in February 2019, May 2024, and July 2024. After approximately matching the observed leak, outputs from the 3-D simulations were used to quantify the simulated mass of Cl-VOC leaked. Simulated results for a leak on the order of 40 kg of Cl-VOC showed general agreement with the monitoring data. Although the solution is non-unique, this paper presents a proof-of-concept addition to an existing case study, to show that a suspected subsurface container failure could create a signal consistent with the measured data and sets the stage for further analysis of future potential leak signals at the site. The work can also be adapted at other sites where changing subsurface conditions can require innovative modeling techniques to answer regulatory questions.