Assimilation and value of injection response data for enhanced contaminated site characterization

Abstract Collecting detailed hydrogeological data before, during, and after remediation campaigns is essential for effective management and monitoring of contaminated sites. As in-situ remediation injection treatment becomes more popular, recording the hydraulic response during these events offers an opportunity to collect detailed data on hydrogeological parameters. Often, multiple injections are performed in a grid pattern over an area targeted for treatment, essentially serving as spatially distributed and high-frequency inverse pumping tests. This information, which is often overlooked, can be assimilated into numerical models for enhanced site characterization and improved forecasts of management actions. However, the high pressures during injections often result in hydraulic fracturing. These events can represent temporary or permanent changes in aquifer properties and should be considered when assimilating injection response data into a model to reduce the risk of introducing parameter bias. Four groundwater models were developed for a contaminated site in Sweden, treated with in-situ injections in 2017. The models were designed to evaluate strategies for assimilating injection response data, including alternative weighting schemes and the use of time-varying parameters around injection points to account for hydraulic fracturing during injections. Results show that assigning a higher weight to observations recorded during injections, compared to records of ambient head, combined with the use of time-varying parameters, yielded the best results with the lowest risk of underestimating predictive uncertainty. A subsequent data worth analysis revealed that injection response data provided detailed insights into aquifer properties in the injection zone that were not available from ambient head data alone.