Numerical Modeling to Investigate the Uncertainty of Mass Discharge Measurements from Point and Pumping Based Methods under Heterogeneous Conditions

Abstract:

A Monte Carlo simulation to investigate the uncertainty of mass discharge measurements from point and pumping based measurements was conducted. Mass flux (M_F) and mass discharge (M_D) have been recognized as important metrics for contaminated site management. They utilize both groundwater flux and contaminant concentration information to estimate the contaminant mass per unit time across control planes at contaminated sites. Two different approaches are generally applied to measure M_F: point measurement methods and pumping based methods. Both methods inherently contain uncertainty that should be identified and quantified to utilize M_D information appropriately. This uncertainty results primarily from incomplete knowledge about subsurface complexity and an inability to fully characterize such complexity due to cost and resource limitations. The focus of this study is the evaluation of uncertainty resulting from aquifer heterogeneity, measurement methods, and plume conditions. The result showed that the common conception that pumping based methods are generally superior to point measurement methods for estimating the M_D is not valid for all circumstances. If there are no extreme hot spots in the domain, the point-based methods showed comparable uncertainty to pumping based methods, even under highly heterogeneous conditions. It was also noted, as in previous studies, that the hydraulic conductivity and hydraulic gradient are determining factors for the uncertainty of M_D measurements. The uncertainty varied significantly depending on how these parameters were estimated.