Environmental Engineering Reference
In-Depth Information
15
12
9
6
1,4-dioxane, 15 ppm
1,4-dioxane, 3 ppm
TCA, 100 ppm
1,1-dichloroethane
Chloroethane
3
0
0
250
500
700
1000
1250
1500
1750
2000
2250
2500
Distance (feet)
FIGURE 3.11 BIOCHLOR-modeled distance along plume centerline at which contaminant concentration
exceeds regulatory levels for increasing release durations (source TCA concentration = 100 mg/L).
conductivity or where hydraulic gradients are insignii cant, molecular diffusion through the clay
pore l uid is the primary mode of contaminant transport. Predicting contaminant transport through
i ne-grained strata and engineered clay liners involves modeling diffusive transport (Barone et al.,
1992; Hoffman et al., 1998; Young and Ball, 1998; Ball, 2000).
Diffusion-driven contamination is inherently slow. It is controlled by concentration gradients,
solubility, and temperature. Limitations on diffusion-driven contamination include the occurrence
and heterogeneity of i ne-grained media within and adjacent to an aquifer and rates of adsorp-
tion, biodegradation, and abiotic degradation. Over long periods of time, contaminants with high
concentrations in coarse-grained media will move into adjacent low-permeability zones, including
aquitards, i ne-grained lenses within an aquifer, and the micropores within individual grains of the
aquifer matrix. The process of contaminant diffusion into i ne-grained sediments is best articu-
lated by Mackay and Cherry's classic 1989 article “Groundwater Contamination: Pump and Treat
Remediation,” paraphrased as follows: *
As dissolved contaminants spread through aquifers, they rapidly migrate through more permeable
zones while slowly invading less permeable materials by diffusion. Over many years, diffusion can
cause dissolved contaminants to occupy large volumes of low-permeability material. To obtain clean
water from wells, the lower-permeability components of the aquifer system must be remediated as well
as the higher-permeability zones.
Consider a clay lens in the middle of a predominantly sandy hydrostratigraphic unit through which
contaminants have migrated for decades. The porosity of the clay lens is often larger than that of the
adjacent aquifer, promoting diffusion into the clay. Dissolved contaminants permeate the clay lens by
molecular diffusion, which leads to delayed diffusive release. Also, the capacity of a clay lens to sorb
contaminants is generally much greater per unit volume than the sorption capacity of the aquifer. When
the aquifer is l ushed by clean water, the only signii cant process for release of the contaminant from
the clay will be a reversal of the diffusion direction. The relatively slow rate of release of contaminants
from the clay by diffusion and the potentially appreciable contaminant mass contained in dissolved and
sorbed form in the clay causes a long-term “bleed” or “back-diffusion” of contaminants into the aquifer
during remediation. In many aquifers, there are numerous thin beds of silt and clay.
Contaminant concentrations in water extracted for remediation or benei cial uses often remain
higher than low regulatory cleanup thresholds for very long periods of time, greatly increasing the
* See Mackay and Cherry (1989).
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