A detailed field-scale investigation of processes controlling the architecture, persistence and
dissolution of a 20 to 45 year old trichloroethene (TCE) dense non-aqueous phase liquid (DNAPL)
source zone located within a heterogeneous sand/gravel aquifer at a UK industrial site is
presented. The source zone was partially enclosed by a 3-sided cell that allowed detailed
longitudinal/fence transect monitoring along/across a controlled streamtube of flow induced by
an extraction well positioned at the cell closed end. Integrated analysis of high-resolution DNAPL
saturation (S
n) (from cores), dissolved-phase plume concentration (from multilevel samplers),
tracer test and permeability datasets was undertaken. DNAPL architecture was determined from
soil concentration data using partitioning calculations. DNAPL threshold soil concentrations and
low S
n values calculated were sensitive to sorption assumptions. An outcome of this was the
uncertainty in demarcation of secondary source zone diffused and sorbed mass that is distinct
from trace amounts of low S
n
DNAPL mass. The majority of source mass occurred within discrete
lenses or pools of DNAPL associated with low permeability geological units. High residual
saturation (S
n N 10–20%) and pools (S
n N 20%) together accounted for almost 40% of the DNAPL
mass, but only 3% of the sampled source volume. High-saturation DNAPL lenses/pools were
supported by lower permeability layers, but with DNAPL still primarily present within slightly
more permeable overlying units. These lenses/pools exhibited approximately linearly decliningS
n
profiles with increasing elevation ascribed to preferential dissolution of the uppermost
DNAPL. Bi-component partitioning calculations on soil samples confirmed that the
dechlorination product cDCE (cis-dichloroethene) was accumulating in the TCE DNAPL.
Estimated cDCE mole fractions in the DNAPL increased towards the DNAPL interface with the
uppermost mole fraction of 0.04 comparable to literature laboratory data. DNAPL dissolution
yielded heterogeneous dissolved-phase plumes of TCE and its dechlorination products that
exhibited orders of magnitude local concentration variation. TCE solubility concentrations
were relatively localised, but coincident with high saturation DNAPL lens source areas. Biotic
dechlorination in the source zone area, however, caused cDCE to be the dominant dissolvedphase plume. The conservative tracer test usefully confirmed the continuity of a permeable
gravel unit at depth through the source zone. Although this unit offered significant
opportunity for DNAPL bypassing and decreased timeframes for dechlorination, it still
transmitted a significant proportion of the contaminant flux. This was attributed to
dissolution of DNAPL–mudstone aquitard associated sources at the base of the continuous
gravel as well as contaminated groundwater from surrounding less permeable sand and
gravel horizons draining into this permeable conduit. The cell extraction well provided an
A detailed field-scale investigation of processes controlling the architecture, persistence and
dissolution of a 20 to 45 year old trichloroethene (TCE) dense non-aqueous phase liquid (DNAPL)
source zone located within a heterogeneous sand/gravel aquifer at a UK industrial site is
presented. The source zone was partially enclosed by a 3-sided cell that allowed detailed
longitudinal/fence transect monitoring along/across a controlled streamtube of flow induced by
an extraction well positioned at the cell closed end. Integrated analysis of high-resolution DNAPL
saturation (S
n) (from cores), dissolved-phase plume concentration (from multilevel samplers),
tracer test and permeability datasets was undertaken. DNAPL architecture was determined from
soil concentration data using partitioning calculations. DNAPL threshold soil concentrations and
low S
n values calculated were sensitive to sorption assumptions. An outcome of this was the
uncertainty in demarcation of secondary source zone diffused and sorbed mass that is distinct
from trace amounts of low S
n
DNAPL mass. The majority of source mass occurred within discrete
lenses or pools of DNAPL associated with low permeability geological units. High residual
saturation (S
n N 10–20%) and pools (S
n N 20%) together accounted for almost 40% of the DNAPL
mass, but only 3% of the sampled source volume. High-saturation DNAPL lenses/pools were
supported by lower permeability layers, but with DNAPL still primarily present within slightly
more permeable overlying units. These lenses/pools exhibited approximately linearly decliningS
n
profiles with increasing elevation ascribed to preferential dissolution of the uppermost
DNAPL. Bi-component partitioning calculations on soil samples confirmed that the
dechlorination product cDCE (cis-dichloroethene) was accumulating in the TCE DNAPL.
Estimated cDCE mole fractions in the DNAPL increased towards the DNAPL interface with the
uppermost mole fraction of 0.04 comparable to literature laboratory data. DNAPL dissolution
yielded heterogeneous dissolved-phase plumes of TCE and its dechlorination products that
exhibited orders of magnitude local concentration variation. TCE solubility concentrations
were relatively localised, but coincident with high saturation DNAPL lens source areas. Biotic
dechlorination in the source zone area, however, caused cDCE to be the dominant dissolvedphase plume. The conservative tracer test usefully confirmed the continuity of a permeable
gravel unit at depth through the source zone. Although this unit offered significant
opportunity for DNAPL bypassing and decreased timeframes for dechlorination, it still
transmitted a significant proportion of the contaminant flux. This was attributed to
dissolution of DNAPL–mudstone aquitard associated sources at the base of the continuous
gravel as well as contaminated groundwater from surrounding less permeable sand and
gravel horizons draining into this permeable conduit. The cell extraction well provided an
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