| INTRODUCTION Soil
electrical conductivity (EC) measurements are uniquely useful
during geo-environmental exploration of brine contaminated
groundwater. Both surface and downhole methods can be used
to monitor increases in EC resulting from the migration and
diffusion of brine through groundwater. Surface methods have
the utility of rapidly covering large areas to identify where
brine intrusions might have occurred. Downhole methods are
very useful in pinpointing the vertical extent of contamination
as well as in estimation of brine concentrations. Downhole
measurements can be obtained in cased boreholes, or through
use of cone penetrometer soundings which include EC measurements.
Soil electrical conductivity is controlled
by the conductance of the soil particles and the conductance
of the fluid occupying the soil pore spaces. The ratio between
pore fluid and combined soil-pore fluid electrical conductivity
is termed the formation factor (Archie, 1942). Clay particles
can be electrically conductive due to adsorbed water and ionic
electrical charges on the clay platelets, so clay electrical
conductance depends on mineralogy, porosity and pore fluid
characteristics. Sand grains are typically nonconductive,
so sand conductance depends primarily on pore fluid characteristics
and porosity. The addition of brine to groundwater greatly
increases soil electrical conductance.
A brine groundwater contamination exploration
method was developed based on several technologies. First,
surface electromagnetic (EM) surveys can be used to rapidly
map affected areas, and give reasonably good indications of
the degree of impact of brine releases on groundwater, to
depths as great as 60 m (200 ft). However, surface EM methods
lack the ability to adequately quantify brine concentrations,
cannot provide detailed definition of the depth intervals
affected by brine intrusions, and cannot easily differentiate
between natural electrically conductive clayey soils and zones
of elevated chlorides.
Downhole geophysical induction logging
can be performed to great depths in suitably cased boreholes.
Data quality is high, allows vertical delineation of affected
intervals, and allows estimation of brine concentrations.
However, drilling and casing boreholes, and the disposal of
large volumes of exploration derived wastes from drilling
activities, are expensive and time consuming. Further, borehole
stability can be a significant problem where thick sequences
of saturated sands are encountered.
A high capacity, truck mounted, geo-environmental
cone penetrometer system, including downhole EC logging and
penetrometergroundwater sampling, can be used to vertically
profile subsurface conditions and obtain direct groundwater
samples. The penetrometer exploration method is fast and accurate,
and provides high resolution lithologic and electrical conductivity
data. The direct push penetrometer exploration method requires
no borehole, so no soil cuttings are generated, and borehole
stability is of no concern.
GEOPHYSICAL METHODS
The electromagnetic (EM) geophysical method
determines electrical properties of earth materials by inducing
electromagnetic currents in the ground and measuring the secondary
magnetic field produced by these currents. An alternating
current is generated in a wire loop or coil above the ground's
surface. Both the primary magnetic field (produced by the
transmitter coil in the instrument) and the secondary field
(produced by currents in the earth) induce a corresponding
alternating current in the receiver coil of the instrument.
The coils are kept at a fixed distance and orientation relative
to the ground to simplify data analysis.
After compensating for the primary field,
both the magnitude and relative phase (in-phase and quadrature)
of the secondary field are measured. The quadrature-phase
component, using simplifying assumptions of homogeneous and
isotropic conditions, is converted to a value of apparent
soil electrical conductivity (EC). This value represents an
estimate of the local average soil EC. The depth of measurement
is dependent on the instrument's coil spacing, orientation,
and operating frequency, and the actual subsurface EC variations.
Averaging limits discrimination of thin, high concentration
brine intrusions from broader, more diffuse plumes. Multiple
profiles using differing coil spacing can be performed to
bracket approximate depths of brine affected groundwater.
Data quality may be degraded by cultural interference as caused
by utility lines, steel fences or other large metallic objects.
Surface electromagnetic measurements were
taken by Geosphere with Geonics EM38, EM31 and EM34 systems,
using coil separations of 1 m (EM38) , 3.7 m (EM31), and 10
m or 20 m (EM34). The nominal explored depth is proportional
to coil spacing. For the shallow, near field EM38 instrument,
the explored depth is about 1.5 m; for the intermediate EM31
it is about 6 m; and for the deep, far field EM34 instrument,
the explored depth is about 15 m with a 10 m coil spacing,
30 m with a 20 m spacing, and 60 m with a 40 m spacing. Instruments
were carried manually, and measurements were digitally logged
during profiling at the chloride brine release site. Shallow
and intermediate readings in the source area were typically
taken at 15 m (50 ft) intervals along lines 15 m apart using
the EM38 and EM31 instruments. EM34 (deep far field) coverage
was conducted on lines spaced 60 m (200 ft) apart in the western
portion of the plume, where the chloride brine had migrated
to much greater depths.
Permanent geophysical monitoring stations
were installed using hollow stem auger drilling techniques.
A water filled, PVC casing was grouted into place to provide
an access conduit for lowering downhole geophysical logging
tools. Screened intervals are not necessary (or even desired)
as there is no need for direct communication with the surrounding
groundwater/soil system. In induction logging, the field is
induced in the surrounding soil through the casing itself.
Electrically conductive (i.e., steel) casing materials cannot
be used.
Periodic downhole logging of the geophysical
monitoring stations was performed at the chloride brine release
site using a Geonics EM39 induction system. The induction
technique has been used for decades by the petroleum industry
for formation characterization in oil and gas wells. The Geosphere
application of these methods to shallow groundwater exploration
includes increased resolution due to slim, compact logging
tools (3.6x130 cm or 1.4x50 in) and slower logging rates.
This permits the detection and resolution of formation conductivity
changes across vertical intervals of less than 0.3 m (1 ft).
The EM39 induction logger is capable of measuring with an
accuracy of 2 mS(milli-Siemen)/m in the range of 0 to 1000
mS/m.
The geophysical monitoring station provides
significant advantages over traditional screened monitoring
wells at brine affected sites. A continuous EC profile is
obtained from the surface to the bottom of the cased hole,
where traditional monitoring wells only provide data across
the screened interval. This is significant as brine plumes
typically sink with time, retreating from screened intervals,
which can lead to misleading results. The geophysical monitoring
station avoids the costs of installing monitoring well clusters,
and eliminates data gaps which occur where monitoring wells
are screened in different portions of the aquifer. Monitor
well purging and analytical testing are also eliminated, leading
to additional cost savings.
Like surface EM and penetrometer CPTU-EC
methods, the borehole EM39 system measures the bulk (soil
and pore fluid) EC. With suitable assumptions of formation
factors, a value of groundwater conductivity, and thus approximate
concentration of a predominant ion such as chloride, can be
estimated from downhole EC measurements.
CONE PENETROMETER SYSTEM
The STRATIGRAPHICS cone penetrometer system
is used during geo-environmental and geotechnical exploration
programs in difficult soil conditions. The heavy (240 kN and
300 kN, or 24 and 30 ton) truck mounted rigs are fully self-contained,
and include data acquisition systems, dry and wet work areas,
water tanks, steam cleaners, decontamination and grouting
systems, separate rodstrings for sounding and sampling, optional
dynamic rod driving, and heavy duty downhole equipment for
use in glaciated terrains (Figure 1).
Cone penetrometer (direct push) systems
require no borehole to advance probes and samplers and result
in very little exploration derived waste. Downhole equipment
is decontaminated during retrieval using an automatic rodwasher,
and the open hole is pressure grouted. Most exploration activities
are performed inside an enclosed portion of the rig, providing
all-weather capability and a low visual profile during operations.
Truck mounted penetrometer systems can be very productive,
with as much as 400 m (1300 ft) of stratigraphic logging per
day, with depth capacity exceeding 60 m (200 ft). As many
as 18 groundwater, or up to 30 soil or soil gas samples, can
be acquired in a day.
Soil resistance to penetration acting
on the tip and soil friction on the sides of the penetrometer
are separately measured during CPT. These measurements are
accurate and repeatable, and have been used for the evaluation
of stratigraphy and geotechnical parameters for decades. Performance
of geo-environmental CPT is specified by ASTM Standards D5778,
D6067 and Guide D6001.
The CPT tip resistance increases exponentially
with soil grain size. Tip resistance in a sand aquifer is
typically one to two orders of magnitude greater than in a
clay aquitard. For example, the CPT tip resistance varies
from about 10 to 40 MPa (100 to 400 tons per square ft (tsf))
in a dense sand aquifer, while tip resistance in a stiff clay
aquitard ranges from about 0.5 to 1.5 MPa (5 to 15 tsf). The
friction ratio (proportion of friction to tip resistance)
is also used as an indicator of soil type. The friction ratio
allows discrimination between loose sands and hard silts and
clays, where tip resistances can be similar. The friction
ratio ranges from about 1% in a sand to greater than about
3% in a clay. Silts have intermediate friction ratios.
High resolution, continuous soil profiling
(sounding) for geo-environmental exploration is most often
performed by STRATIGRAPHICS using the Piezometric Cone Penetration
Test with soil Electrical Conductivity measurements (CPTU-EC).
The cone tip and friction resistance measurements (CPTU-EC)
are evaluated for soil types and geotechnical parameters (Douglas
and Olsen, 1981). The piezometric measurement (CPTU-EC) allows
evaluation of soil saturation, hydraulic conductivity, potentiometric
surfaces, and soil types (Saines et al, 1989, Robertson et
al, 1986). The soil Electrical Conductivity measurement (CPTU-EC)
provides information on soil moisture in vadose zone soils
and indications of groundwater quality in saturated soils.
The EC measurement has proved very useful in exploration for
inorganic (metal, brines and landfill leachate) contamination
and somewhat useful in non-aqueous phase liquid (NAPL) exploration.
Soil EC is measured by STRATIGRAPHICS
using a rugged two electrode array mounted on the tip of the
penetrometer (Strutynsky, et al, 1991). A 3 kHz AC voltage
is applied to the array to control polarization and contact
resistance effects. EC is computed based on in-phase currents
induced across the array and a reference resistor. The EC
measurement has a resolution of about 1-3 cm.
The STRATIGRAPHICS Penetrometer Sampler
can be deployed in groundwater, soil or soil gas sampling
modes by using interchangeable components. The groundwater
sampler is a heavy wall, shielded wellpoint sampler. The shield
prevents cross contamination and clogging of the sampler screen.
The shield is retracted to allow groundwater to flow through
a 0.5 m (20 inch) long screen into the barrel. Sample can
be decanted from the barrel or can be pumped to the surface
using an inertial pump. A small diameter pressure transducer
can be lowered into the sampler to log the rate of groundwater
inflow. Inflow results can be analyzed to estimate soil hydraulic
conductivity.
Penetrometer exploration can be used at
sites where predominant soil grain sizes are less than about
½ the diameter of the downhole tools, i.e. less than
about 3-4 cm (medium to coarse gravel). A small fraction of
larger particles can be tolerated, as long as the coarser
particles are within a matrix of much finer soil grains, as
is common in many glacial till units. Deep penetration can
be limited by excess friction on the rod string where soft,
squeezing clays are encountered. Rod friction can also be
a problem where thick sequences of hardpan silts or very fine,
saturated sands are encountered. Thick sequences of very soft
soils, such as peat layers, can limit deep penetration as
little lateral support is provided to the slender penetrometer
rod string in the weak layer, while attempting to push through
dense soils at depth. Extremely dense (SPT blowcounts greater
than about 100) can be difficult to penetrate. Dynamic rod
driving may help in this situation.
SITE CHARACTERIZATION
Chloride brine production (40% chloride
salt solution) from well fields in Michigan is transported
to a processing facility through pipelines buried at shallow
depths. At one site, galvanic corrosion at a welded pipeline
joint led to a release of about 40,000 gallons of chloride
brine. The initial response removed over 100,000 gallons of
surface water and brine, but a portion of the released brine
permeated into a thick sand aquifer, and some seeped into
a nearby creek. Chloride brine migration was primarily controlled
by differences in density between the heavy brine and lighter
fresh groundwater, and by the generally westward regional
groundwater flow.
Soils in the release area consist of about
180 m (600 ft) of sands, silts and clays, which overlie shale
bedrock. These soils represent a sequence of lakebed and shoreline
deposits in a paleo environment when water levels in the Great
Lakes were much higher than at present. The sand aquifer is
probably the result of dune and beach activity. The aquifer
ranges in thickness from about 6 m (20 ft) near the brine
release point, to over 40 m (130 ft) at a distance 600 m (2000
ft) west of the release point. An aquitard consisting of interlayered
clays, silts, and silty sands underlies the sand aquifer.
The suspected release point and surrounding
area were initially profiled to depths of about 6 m using
near field EM38 and intermediate EM31 surface geophysical
instruments. Within a week, the shallow plume was mapped to
an area of about 35 acres. Purge and monitoring wells were
installed to control the migration of the chloride brine through
the aquifer. Additional geophysical profiling, and installation
of more purge and monitoring wells, was performed in several
phases over a period of years, as it became apparent that
westward plume migration was continuing.
Due to the dense chloride brine sinking
to the top of the westward dipping aquitard layer and the
westward regional flow of groundwater, a far field EM34 surface
geophysical instrument had to be used for profiling the central
portion of the plume. At a distance of about 600 m (2000 ft)
west of the release point, the plume had migrated deeper than
the layer resolving capabilities of the EM34 instrument. Further
plume characterization was performed with the CPTU-EC cone
penetrometer method.
A series of 33 CPTU-EC soundings, to depths
as great as 50 m (165 ft) and totaling 1061 m (3481 ft) of
data, and 27 penetrometer groundwater samples, were completed
to characterize the deep, westward extension of the plume.
A thin zone of groundwater with slightly elevated chloride
concentrations was eventually found to extend along the top
of the confining aquitard unit to as far as 1200 m (4000 ft)
west of the initial release area.
The indirect CPTU-EC data indicated that
the sand aquifer was relatively homogeneous, with very few,
thin, apparently discontinuous clay or silt seams or layers.
In contrast, the aquitard was found to be much more heterogeneous
than the aquifer, with interlayers of clays, silts, and silty
sands. EC data (both CPTU-EC and geophysical) indicated groundwater
unaffected by the chloride brine is low in electrical conductivity,
which provides a very clear contrast to groundwater with elevated
chloride content. Piezometric testing during CPTU-EC indicated
aquifer hydraulic conductivities to range from about 1E-3
to over 1E-2 cm/sec. Aquitard hydraulic conductivities ranged
from about 1E-4 to less than 1E-8 cm/sec, depending on whether
the testing was in silty sand or silty clay layers. The indirect
penetrometer evaluations of stratigraphy, hydraulic conductivity
and EC agreed very well with indirect geophysical, direct
sampled borehole, and pump test data collected at the spill
site.
The CPTU-EC data were evaluated to delineate
the extent of the sand aquifer and clay aquitard units. Depth
intervals within aquifer soils, where peak EC measurements
were encountered, were selected for direct penetrometer groundwater
sampling. Sample targeting using the continuous CPTU-EC sounding
logs significantly reduced the number of samples and analytical
testing required to adequately characterize groundwater chloride
concentrations. Near the plume source, where CPTU-EC measurements
were greater than about 400 mS/m (4mS/cm), direct penetrometer
groundwater sampling was typically not performed, as high
chloride concentrations were not in question for these conditions.
A comparison between CPTU-EC penetrometer
data and a drilled, sampled and geophysically logged monitoring
station is shown in Figure 2. The two exploration points were
within about a 9 m (30 ft) distance of each other, with the
CPTU-EC sounding slightly closer to the plume axis. Very good
correspondence is observed between the soil types evaluated
from the CPTU-EC measurements and visual classification of
obtained samples. The continuous CPTU-EC logs provide data
across unsampled intervals in the discontinuously sampled
borehole, and greatly increased resolution of thin layering.
Comparison between the CPTU-EC and downhole
EM39 data also shows very good correspondence. The CPTU-EC
data provide higher resolution (2-3 cm versus 15-30 cm for
EM39) of layering. In many instances, this can be advantageous
(for example, during other projects when trying to detect
thin LNAPL or DNAPL layers). A disadvantage to the high CPTU-EC
resolution is that in gravelly soils where particle size is
comparable to EC sensor resolution, a significant amount of
noise can be exhibited in the EC data.
The CPTU-EC piezometric data in the example
sounding log indicate that the bottom of the aquifer unit
is slightly less permeable than at shallow depths. This fact,
not apparent from visual descriptions of drilled samples,
may explain the somewhat unusual shapes of the EM39 geophysical
and CPTU-EC logs, and thus the distribution of chloride concentrations
at the bottom of the aquifer.
The peak value of EC (and thus, highest
chloride brine concentration) might be expected to fall at
the interface between the sand aquifer and the underlying
aquitard at 25.5 m, if the aquifer was homogeneous in hydraulic
conductivity. Instead, the peak EM39 and CPTU-EC values were
measured at a depth of about 21 m. This is the depth at which
aquifer hydraulic conductivities slightly decrease, as indicated
by CPTU-EC piezometric measurements. EC measurements slowly
decrease with depth from this point, and somewhat abruptly
decrease at about
23 m. The CPTU-EC piezometric data indicate
a thin (<10 cm) silt seam at this depth, not detected during
drilled sampling. A very abrupt decrease in EC finally occurs
at the aquifer/aquitard interface, at 25.5 m.
A comparison between peak EC data measured
in the CPTU-EC soundings and average EC data from surface
geophysical surveys is presented in Figure 3. Generally good
correspondence can be seen between the two exploration methods.
As expected, the CPTU-EC peak data are much higher than the
average data from the surface EM surveys. The deep, low concentration
chloride affected groundwater in the western portion of the
site was only detected using CPTU-EC data. The surface EM
geophysical survey was of great benefit where plume depths
did not exceed about 20-30 m.
Correlations were developed from comparing
CPTU-EC and downhole EM39 data to laboratory chloride concentrations
obtained from groundwater samples, in order to allow estimation
of chloride concentrations. For the CPTU-EC data, and for
chloride concentrations less than about 1000 ppm (parts per
million), the following relationship was derived: chloride
concentration, in ppm = (0.8 * EC (in uS/cm))-171. For chloride
concentrations up to 50,000 ppm, an alternate relationship
was developed: chloride concentration, in ppm = (1.9 * EC
(in uS/cm))-1029. For a regulatory maximum chloride concentration
in groundwater of 250 ppm, the corresponding CPTU-EC value
had to be less than about 500 to 550 uS/cm.
It is important to note that these relationships
were developed at a site where background groundwater was
very low in dissolved minerals, and thus of low electrical
conductance. Application of these correlations to other sites
is limited - site specific correlations should be performed
at each project site.
CONCLUSION
An exploration technique for groundwater
affected by chloride brine was developed based on surface
and downhole geophysical, and direct push cone penetrometer
methods. Indirect and direct measurements using these methods
were obtained and compared to results of traditional drilled
and sampled boreholes and monitoring wells. The new technique
provided good technical and economic justification for extensive
use at the described site. Use of the combined geophysical
and cone penetrometer exploration technique was then adopted
for use at other brine release sites in the area. These programs
met with very good success and acceptance by regulatory agencies.
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