title: Analysis of groundwater contamination using concentration-time series recorded during an integral pumping test: bias introduced by strong concentration gradients within the plume.
abstract: When only few monitoring wells are available to assess the extent and level of groundwater contamination, inversion of concentration breakthrough curves acquired during an integral pumping test can be used as an alternative quantification method. The idea is to use concentration-time series recorded during integral pumping tests through an inversion technique to estimate contaminant mass fluxes crossing a control plane. In this paper, we examine how a longitudinal concentration gradient along a contaminant plume length scale affects the estimated inversed-concentration distribution and its associated mass flux. The analytically inversed-concentration distribution at the imaginary control plane (ICP) is compared to a numerically generated concentration distribution, treating the latter one as a "real contaminant plume" characterized by the presence of a longitudinal concentration gradient. It is found that the analytically inversed-concentration can lead to overestimation or underestimation of concentration distribution values depending on the transport time period and dispersivity values. At lower dispersivity values, with shorter transport time periods, the analytically inversed-concentration distribution overestimates the "real" concentration distribution. A better fit of the estimated concentration distribution to the "real" one is observed when the transport time period increases, i.e. when the advective front has already crossed the ICP. However, for higher dispersivity values, underestimation of the real concentration distribution is observed. Deviation of the inversed-concentration distribution from the "real" one is assessed for a site-specific concentration gradient term. A concentration gradient adjusted contaminant mass flux is thus formulated to evaluate groundwater contamination levels at a given time period through an ICP. This concentration gradient ratio can indicate whether the ICP is well positioned to evaluate accurately contaminant mass fluxes which are representative of groundwater contamination levels.
authors.affiliation: Institut de Mécanique des Fluides et Solides de Strasbourg, Institut Franco-Allemand de Recherche sur l'Environnement (IFARE), UMR 7507 ULP-CNRS, 23 rue du Loess, B.P. 20, F-67037 Strasbourg Cedex, France.
authors.forename: Allelign
authors.initials: A
authors.lastname: Zeru
authors.forename: Gerhard
authors.initials: G
authors.lastname: Schäfer
mesh_terms: D015982:Bias; D004058:Diffusion; D004784:Environmental Monitoring / Q000706:statistics & numerical data*; D005618:Fresh Water / Q000032:analysis*; D008962:Models, Theoretical; D014872:Water Movements; D014873:Water Pollutants / Q000032:analysis*
chemical_list: D014873:Water Pollutants