Data worth in models of groundwater contaminant fractionation

In hydrogeology and in the geosciences in general, quantification of uncertainty is just as crucial as parameter estimation when using models to make assessments and decisions. Moreover, understanding the relative value of individual measurements and of measurement types enables more effective and targeted decision-making when planning acquisition of additional field data. This analysis of data worth is even more impactful when one considers novel types of measurements or novel problems that involve multiple coupled physical or chemical processes. Here, we introduce and employ a novel software interface COMPEST to investigate data worth and uncertainty related to groundwater contaminant fate. COMPEST is a compact and open-source program that functions as a user-friendly interface between the multiphysics modelling package, COMSOL, and the parameter estimation package, PEST. This modelling framework allows for the numerical treatment of an extremely broad range of inverse problems involving coupled processes The phenomenon of isotopic fractionation of chlorohydrocarbons (e.g., PCE, TCE), present as groundwater contaminants, is exemplary of the type of problem for which this modelling approach is ideally suited. Isotopic enrichment and depletion due to degradation and diffusion can be measured using compound-specific isotope analysis (CSIA). To date, research has not yet established the data worth of these measurements in informing model parameters and reducing the uncertainty of predictions. We thus seek to evaluate the worth of CSIA data in constraining models of contaminant transport and degradation. As the phenomenon of back-diffusion, wherein the contaminant diffuses from a lowpermeability zone to a higher-permeability zone, causing plume persistence, is a current challenge, we focus on the aquifer-aquitard interface. Our work also includes development and implementation of novel isotopic fractionation models that take into account primary and secondary isotope effects. Our results show the utility of both the developed modelling approach and the usefulness of CSIA measurements in investigating contaminant fate