Temporal Evolution of Contaminant Mass Discharge: Effect of Source Remediation at Contaminated Sites

Contaminant mass discharge (CMD) is a key metric for evaluating remediation performance at contaminated sites posing a risk to groundwater. This study assesses temporal CMD trends and associated uncertainties using a geostatistical approach at a chlorinated solvent contaminated site following source zone remediation, supported by two decades of groundwater monitoring data. The site serves as a case to derive learnings and recommendations for long‐term monitoring at contaminated sites using the CMD approach. A significant reduction in mean CMD was observed from 2009 to 2023, 82% when using a reduced dataset of traditional monitoring wells and 73% when including data from five high‐resolution multilevel samplers, reducing CMD uncertainty from CV ~45 to 80% to ~35 to 40%. The timing and magnitude of CMD decline aligned with expected plume transport and reflected combined remediation effects. A temporary vinyl chloride CMD increase in 2015–2016 suggested enhanced dechlorination, highlighting the need to monitor both parent compounds and degradation products. The study emphasizes the value of CMD uncertainty quantification in long‐term monitoring, enabling robust interpretation of remediation effects, temporal trends, and risk. While the reduced dataset captured overall patterns, high‐resolution data yielded lower uncertainty and clearer insights into plume dynamics. A tiered monitoring approach—combining frequent lower‐resolution sampling with periodic high‐resolution campaigns—is recommended. Consistent monitoring of flow and attenuation processes downgradient can further improve CMD quantifications without excessive resource demand. Overall, this study shows that long‐term, consistent monitoring supports reliable evaluation of source zone remediation and interpretation of CMD response in downgradient groundwater.