A plume of chloride (100 to > 1000 mg/L) was found to extend up to 40
m laterally from the edge of a landfill located on a thick sequence of
clay-rich glacial deposits. The plume is mainly confined to the seaso
nally saturated highly weathered and fractured zone (upper 3 to 4 m) o
f the deposits. in the high concentration region of the plume there wa
s good agreement between chloride concentrations determined from piezo
meter water samples, which were expected to sample fracture pore water
, and pore water extractions from continuous core samples, which were
expected to be dominated by pore water from the fine-grained matrix. T
he pore water extractions provided much better lateral and vertical re
solution of the plume than was obtained from the piezometer water samp
les. The length of the plume based on the data from the piezometer wat
er samples was 12 m less than the length determined from the core samp
les, apparently because the piezometers were located above or below th
e relatively thin plume, or were above the water table at the time of
sampling. Chloride concentrations in many of the deeper piezometers we
re higher than measured in core samples. This was likely due to inflow
from the shallow highly contaminated zone during drilling, as indicat
ed by declining concentration values measured in many of these piezome
ters over a 10-month period. The core samples were protected by a line
r and were not significantly influenced by drilling-induced contaminat
ion. Downhole electrical conductivity profiles measured,vith a Geonics
EM-39 correlated well with chloride concentration profiles determined
from the core samples. Surface measurements of electrical conductivit
y with a Geonics EM-31 were effective at detecting the edge of the was
te (which was buried under 2 m of clay), but were less effective at de
tecting the lateral extent of the chloride plume. Based on this and pr
evious investigations, it is expected that an equivalent porous medium
(EPM) approach will be appropriate for simulating lateral contaminant
transport in the highly weathered and fractured zone. However, predic
tive modeling of the behavior of the chloride plume will be complicate
d by the presence of multiple possible sources of chloride and uncerta
inty concerning the hydraulic conductivity distribution, as well as by
difficulty in predicting the influence of large seasonal variations i
n the elevation and slope of the water table.