Effects of grain-scale mass transfer on the transport of volatile organics through sediments .1. Model development

In the first paper of this two-paper series, we present a new model that attributes nonequilibrium sorption of moderately hydrophobic, volatile organic compounds to intragranular diffusion. The model differs from those of previous researchers in that for the first time, it combines the following elements: (1) We account for two distinct intragranular rate-limiting diffusion processes, occurring in series and at widely different timescales; (2) we describe the slower of the two processes with a gamma distribution of diffusion rates; and (3) we use the disparity of timescales of the two processes to approximate a boundary condition for the distributed diffusion equation, allowing it to be solved analytically. The slower diffusion process is attributed to activated diffusion through very small pores, called micropores. In paper 2 [Werth et al., this issue] we evaluate the capabilities of the model and use it to interpret experimental results.