The effect of Langmuir sorption on pump-and-treat remediation of a stratified aquifer

An analytical solution is derived for the mass flux during remediation of a perfectly stratified aquifer by pumping. The idealized plume and the flow field are defined in a circular-cylinder geometry, where the aquifer is assumed to display spatial variability in the hydraulic conductivity, K, in the vertical direction only. The interaction between the solute in the mobile and immobile phases is described by the nonlinear Langmuir equation, which for a remediation case introduces additional dispersion to that caused by the heterogeneous flow conditions. Calculations for the case of variable K and constant sorption parameters show that the parameters expressing heterogeneity (σY2) and sorption capacity (N0) may cause changes of one order of magnitude, or more, in the time periods needed to fulfill the goals of remediation operations. For a given concentration, increased values of σY2 and/or N0 lead to prolonged cleanup times. Furthermore, the effects of different cleanup goals, expressed as the mass fraction needed to be recovered, and the concentration-dependent effect, i.e. the variation in cleanup time with concentration for given natural conditions, are found to be important. Simultaneous spatial variability in K and N0 is modeled assuming different degrees of negative correlation between these parameters. Calculation results based on very limited field data that spatial variability in N0 may have relatively small impact on cleanup times. The degree of negative correlation is found to be an important factor for determining whether spatial variability in N0 needs to be included in the analysis.