Chemical heterogeneity and exposure time accounting in subsurface fate and transport.

摘要

Geochemical properties of the natural subsurface play an important role in the fate and transport of contaminants. Characterization of (Eulerian) aquifer properties in general is difficult, whereas the Lagrangian approach may be more useful particularly in that it reports “global” variables (e.g. travel time). A novel Lagrangian modified stochastic convective reactive method (mSCR) is developed based on the conventional streamtube method to predict solute fate and transport in physically and chemically heterogeneous porous media. The method introduces a new global parameter “cumulative reactivity”, or in the more generic term “exposure time”, which is defined as the available reaction time to reactive zones endured by the solution as it undergoes transport through the aquifer. The approach characterizes the geochemical heterogeneity in the form of joint distributions of flux over travel time and cumulative reactivity. The joint distributions are obtained via a series of reactive tracer experiments and the technique is called the inverse modified stochastic convective reactive method (ImSCR). The joint distributions of flux over travel time and cumulative reactivity define the fractions of the flux that experience various travel times and exposure times to individual reactive regions. Via the distributions, the combined effects of physical and chemical heterogeneities on reactive solute transport are quantitatively established without detailed knowledge of spatially distributed porous media properties. The joint distributions can subsequently be applied to obtain breakthrough curves of other solutes undergoing general first order reactions in similar geochemically heterogeneous configuration via the forward mSCR. In addition, multiple scales of chemical heterogeneity, which typically naturally occur in subsurface porous media, are also taken into account in the mSCR and in a newly developed generalized deterministic mass transfer model. The deterministic model is a generalized form of the conventional distributed reactivity models, which are applicable only in porous media that are microscopically chemically heterogeneous but macroscopically homogeneous. In conclusion, the mSCR in conjunction with the ImSCR is an alternative technique in modeling fate and transport of solutes in physically and chemically heterogeneous porous media particularly where spatially variable properties of the porous media are not characterized in detail.