Integral pumping test analyses of linearly sorbed groundwater contaminants using multiple wells: Inferring mass flows and natural attenuation rates

This paper addresses a field-scale application of the integral pumping test (IPT) method, which is a method for quantitative characterization of contaminant plumes (e.g., benzene, toluene, ethylbenzene, and xylenes (BTEX), chlorinated hydrocarbons, and polyaromatic hydrocarbons) in groundwater. We extend the associated analytical framework by formal consideration of linear, instantaneous sorption/retardation and provide novel nonrecursive analytical solutions for estimation of field-scale contaminant mass flows. We find that estimates of the average concentration obtained through IPTs are independent of the retardation factor under equilibrium conditions, although linear retardation due to sorption decreases the investigation volume in comparison with the nonretarded case. Furthermore, whereas previous studies were based on single-well analyses, we present in this paper developments relevant for multiple-well analyses by introducing a simple, analytical multiple-well framework for evaluation of IPTs considering overall water budgets. Although single-well analyses provide consistent results in isolation, their applicability on multiwell systems suffer from the fact that the theory does not include mass balance considerations for the gallery of wells that constitutes a control plane. Through application of the developed multiple-well methods to measurement data from the field we show that the estimated total mass flows across well galleries can differ considerably from corresponding estimates using single-well methods, which to some extent, also influences natural attenuation (NA) rate estimates. We believe that the developments presented here can provide a useful basis for biogeochemical studies of field-scale NA processes, including assessments of changes in isotope ratios.