Environmental Fate and Transport of Solvent-Stabilizer Compounds - Environmental Investigation and Remediation - page 174

Miscible organic contaminant mass distribution
Sorbed mass
Migrating aqueous-phase mass
Static aqueous-mass
Total aqueous-phase mass-g/m 3
Total aquifer mass-g/m 3
FIGURE 3.12 Distribution of mass of hydrophilic compounds as mass versus distance from site or time at
a location down-gradient from the point of release. (From Payne, F.C., et al., 2003, Developing in situ reac-
tive zone strategies for 1,4-dioxane. Presented at 1,4-Dioxane and Other Solvent-Stabilizer Compounds in the
Environment. December 8, 2003, San Jose, CA, Groundwater Resources Association of California. http://
www.grac.org . With permission.)
duration and cost of cleanup. Most investigations fail to delineate site geology in sufi cient detail to
predict the effects of delayed diffusive release.
Diffusion into i ne-grained media is driven by a concentration gradient, away from sustained
high concentrations in the higher-mobility aquifer zone supplied by a continuous source of contami-
nation. When that source of contamination is terminated or remediated, the concentration gradient
reverses, leading to back-diffusion out of the i ne-grained media. The back-diffusion concept is
often explained in the context of a sorption-desorption system; however, in the case of high-solubil-
ity or miscible hydrophilic compounds with low potential for sorption such as 1,4-dioxane, sorption
may not play an active role in the process. Instead, diffusion can lead to long-term storage of
1,4-dioxane in i ne-grained media near the source of the release, followed by long-term slow release
(Suthersan, 2002; Payne et al., 2003, 2008; Nguyen et al., 2005). Remediation projects commonly
cease extraction when the plot of mass removal over time becomes asymptotic; however, a subse-
quent rebound in concentrations is often observed. Figure 3.12 shows the relative distribution of
hydrophilic contaminants in aquifer media.
The diffusion coefi cient for a contaminant in water ( D w ) can be predicted by using a variety of
QSAR equations. A commonly used approach is the Wilke-Chang equation, which incorporates
the molar volume of the solute, the viscosity and temperature of water, and several empirical con-
stants (Wilke and Chang, 1955). For many organic compounds, D w is in the range of 10 −6 -10 −4 m 2 /
day. Through the use of the Wilke-Chang method, the calculated diffusion coefi cient for
1,4-dioxane in pure water at 22°C is 10.6 × 10 −6 cm 2 /s (Barone et al., 1992).
A laboratory study to measure diffusion coefi cients and adsorption coefi cients for 1,4-dioxane
and four other compounds in samples of a clayey soil was conducted by Barone et al. (1992) at the
University of Western Ontario.