Coupled multicomponent NAPL dissolution and transport in the subsurface: analytic solutions and forensic aspects
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Dissolving multicomponent NAPL as a source of contamination in subsurface water is considered. In particular, two processes are analyzed with regard to how they alter inter-species concentration ratios at remote monitoring locations relative to inter-species molar ratios in the NAPL: nonlinear dissolution governed by Raoult’s Law and differential sorption during subsurface transport. An analytic solution for Raoult’s Law-governed dissolution is presented. Separately, it is shown how a variety of 1D analytic transport models for simple boundary conditions may be adapted to use arbitrary time-varying concentrations by use of some properties of Laguerre series. This is combined with the analytic solution so that Raoult’s Law-governed multicomponent NAPL dissolution may be employed as the boundary condition for analytic transport models. A new computer model implementing this technique in an environment of discrete, parallel fractures is presented, and its accuracy verified for specific conditions against an existing code. The new code is applied to a parametric study on the plume separation of PAH and phenolic compounds from the dissolution of creosote. Narrow fracture spacing as well as significant values of matrix organic carbon are seen as particularly conducive to separation of these types of plumes, which in some circumstances may be entirely disjoint. Concentration ratios downgradient are shown largely unrelated to concentration ratios at the source. Finally, a study of PCB speciation is undertaken in fractured rock with known parameters, for which a rigorous, least squares speciation approach is developed. Even at distances of 5 m from the source, given perfect information about the subsurface, it is found not possible to chemically fingerprint a source PCB mixture from a list of three absent a model of the weathering of the NAPL. Both the PCB and creosote studies demonstrate that forensic inference of source compositions from field data is unreliable and the need for coupled dissolution and transport models like the one developed here.