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Home • TCE
Monitoring System • Groundwater Sampling System • Monitoring
Chloroform • Chromium (VI) Mointoring System • HJ-100 • HJ-300
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Trichloroethene Optrode Detection System Case Studies
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TOPIC A
DESIGN AND CAPABILITIES OF THE SAMPLING SYSTEM
The patented ground-water monitoring sampling system was specifically designed for deployment of sensors in the field. The system may be configured for use with multiple sensors. The sensor(s) may be located within the monitoring well, adjacent to the monitoring well, or both (Figure 1). The entire system is powered by 12 volts (solar cell) for deployment in the field.
The sampling system is designed for use in a
4-inch (or larger) diameter monitoring well.
The system is not a ground-water pump. The sampling system is a pneumatically controlled device
capable of transporting samples from selected depths within a monitoring well
to the location of a sensor. The
system has the capability of altering the physical and/or chemical
environment of the water sample. This
capability has several advantages for the deployment of sensors in the
monitoring well and for the transport of the analyte to sensors located
adjacent to the monitoring well. The
design of the sampling system incorporates a sample chamber. The sample chamber (typical volume of
chamber: 250 mL) is fitted with one or more water level sensors, one or more
inlet valves, and an air/vent valve (Figure 2). A water level sensor controls the
collection of a precise volume of water within the chamber and the creation
of a headspace (air space) over the water sample. Additional water level sensors control the amount of other
solutions (standards, reactants) introduced into the chamber. The inlet valves are used for two
purposes: 1) to introduce water samples into the chamber from different
sampling ports located in the monitoring well, and 2) to introduce solutions
or gases into the chamber.
The sampling/analytical options of the sampling
system are:
- Sensor mounted to directly analyze an analyte
or physical parameter in the water
- sample
- Sensor mounted to analyze a volatile analyte
in the headspace
- Sensor mounted adjacent to the monitoring
well and the sample transported to the
- Sensor (HJ-100 option) TOPIC B
- Any combination of the above options
- Water samples may be collected from several
levels in the well
- Water samples may be transported to the
sensor located in the well
- Water samples may be transported to sensors
located adjacent to the well. Analyte volatilized in the chamber then
transported to sensors located adjacent to the well
Analytical Options
Direct
analysis of the analyte in the water sample
Molecular
Spectroscopy (absorption, florescence)
TOPIC C
Electrochemical
Partitioning
of volatile analyte into the headspace (volatile organic hydrocarbons)
TCE
Optrode TOPIC C
Photo-ionization
Detector
Reaction of water with
reagent creating a volatile product for headspace analysis
(Mercury, hydrides)
Molecular Spectroscopy (absorption)
Atomic Spectroscopy (Mercury)
Ability
to construct a three-step calibration curve
TOPIC B
Ability
to perform standard additions
Ability
to perform a mid-point calibration check
Ability
to perform duplicates and spike duplicates
Ability
to create and analyze blank samples
Ability
to clean the entire system periodically with blank water and
air
to remove non-volatile and volatile contamination from the system
Sampling pumps and other commercially available sampling
methodologies cannot perform the sampling and analytical options made
possible by the sample chamber. The fundamental design criteria of the
sampling system are:
1.
It is easier to transport a sample to a properly located sensor than
to place the sensor in a location difficult to maintain and operate.
2.
Sensors must be located in enclosed chambers that protect the sensors
from the environment and allow the sensor to be interrogated by standards and
blanks prior to analysis.
3.
The entire system must be capable of operating on 12 volts supplied
by a solar cell (for deployment in remote locations).
If long-term monitoring is a goal of any program,
the design and operation of a system intended for field deployment must
adhere to the same procedures required of a fixed-based laboratory. In other words, you are only as good as
your standard. It is inconceivable
that any chemical sensor would be capable of providing reliable data for
months or years without a periodic check of the “goodness” of the signals being
produced by the sensor. The cost to
the user in terms of money and credibility of responding to false values
produced by sensors without the benefit of calibration would exhaust even the
most ardent proponent of field long-term monitoring. The solution is to design a system with
the capability of producing reliable data by interrogating the sensors on a
continuing basis. The Burge sampling
system was designed to be a “laboratory in the field without the chemist” by
automating all the typical analytical protocols. A simplified comparison of
the sampling/analytical system (sensor mounted adjacent to the well) versus
an analytical design of GC/MS is presented on Figure 3. The system was designed to incorporate
many types of sensors to overcome problems associated with deployment of
sensors in the field.
OPERATION OF THE SAMPLING SYSTEMS
The operation of the sampling system is divided
into four sections to explain each of the following capabilities:
Sampling
of the water in the monitoring well for direct analysis of the water
sample
Sampling of the water in
the monitoring well for headspace analysis of a volatile analyte
Transporting
the water to a sensor deployed adjacent to the monitoring well
Transporting
a volatile analyte to a sensor deployed adjacent to the monitoring
well
The following section will only discuss
sampling. The analysis and
calibration are discussed in other sections: calibration and analysis. TOPIC B
This sampling option assumes that the
chemical/physical sensor is located inside the sample chamber of the sampler
and the chemical/physical sensor is in direct contact with the water (Figure 4). The sampler is placed approximately 2 to 3
feet below the static water level.
The sampling system activates one of the four sampling valves (1)
allowing the water sample to be conducted by hydrostatic pressure from the
desired depth in the monitoring well in the sample chamber. The air in the chamber is vented through
the air/vent port to the atmosphere.
The water fills the sample chamber until the water level sensor is
satisfied (Step One: Figure
4).
The water may be analyzed or purged from the
chamber (Step Two: Figure 4)
allowing the sample lines and sample chamber to be rinsed to insure the
sample collected is free of contamination from previous samples. The water is purged from the chamber after
the water level sensor is satisfied by closing sampling valve (1) and the
air/vent valve is activated pressurizing the headspace of the sample chamber
causing the water to be transported through the water tube and up to the
surface (Step 2: Figure 4).
After the sample chamber is filled with a precise
volume of water, the physical/chemical sensor is allowed to measure the
analyte concentration in the water.
Alternatively, chemical reagents may be introduced into the sample
chamber allowing the chemical nature of the analyte to be altered for
analysis using molecular absorption or florescence. A schematic of such an analytical system for the measurement of
chromium (VI) is presented in the analytical topics section. TOPIC C
After the analysis is completed, the analyzed sample
may be returned to the monitoring well, or transported to the surface (Set
Two: Figure 4). At the termination of the analysis, the
sample chamber is purged of water to avoid contamination of the surfaces of
the sample chamber.
Sampling of the water in the monitoring well for headspace analysis of the water sample
This sampling option assumes that the chemical
sensor is located in the headspace of the sample chamber of the sampler
(above the water level sensor) (Figure
5). The sampling and elimination
of the analyzed sample is similar to the above option. The sampler is placed approximately 2 to 3
feet below the static water level.
The sampling system activates one of the four sampling valves (1)
allowing the water sample to be conducted by hydrostatic pressure from the
desired depth in the monitoring well in the sample chamber. The air in the chamber is vented through
the air/vent port to the atmosphere.
The water fills the sample chamber until the water level sensor is
satisfied (Step One: Figure 5).
The water may be analyzed or purged from the
chamber (Step Two: Figure 5)
allowing the sample lines and sample chamber to be rinsed to insure the
sample collected is free of contamination from previous samples. The water is purged from the chamber after
the water level sensor is satisfied by closing sampling valve (1) and the
air/vent valve is activated pressurizing the headspace of the sample chamber
causing the water to be transported through the water tube and up to the
surface (Step 2: Figure 6).
After the sample chamber is filled with a precise
volume of water, the analyte is allowed to partition into the headspace. Agitation, using a stirring motor, may be
used (not illustrated). The chemical
sensor is allowed to measure the analyte concentration in the headspace until
an equilibrium concentration is attained (constant signal, see analytical
topics). Alternatively, chemical
reagents may be introduced into the sample chamber changing the chemical
nature of the analyte by creating a volatile species. Examples of this would be reduction of
mercury ions to mercury vapor and arsenic to a hydride. The sensor would then measure the
concentration of the volatile species in the headspace.
After the analysis is completed, the analyzed
sample may be returned to the monitoring well or transported to the surface
(Set Two: Figure 5). At the termination of the analysis, the
sample chamber is purged of water to avoid contamination of the surfaces of
the sample chamber. The sample
chamber and sample tubes are subjected to air purging to remove volatile
species from the spaces and surfaces to reduce cross-contamination of the
samples or standards.
This sampling option assumes that the chemical/physical
sensors are located adjacent to the monitoring well and require a water
sample for analysis (Figure
1). (Note: A sensor may be mounted inside the sample chamber of the
sampler with this option). The
sampling is similar as the two options described in the above sections. The sampler is placed approximately 2 to 3
feet below the static water level.
The sampling system activates one of the four sampling valves (1)
allowing the water sample to be conducted by hydrostatic pressure from the
desired depth in the monitoring well in the sample chamber. The air in the chamber is vented through
the air/vent port to the atmosphere.
The water fills the sample chamber until the water level sensor is
satisfied (Step One: Figure
6).
After the water level sensor is satisfied, the
sampling valve (1) is closed and the air/vent valve is activated pressurizing
the headspace of the sample chamber causing the water to be transported
through the water tube and up to the surface (Step 2: Figure 6). The primary difference between the Burge
sampling system and a traditional ground-water sampling pump become apparent
in the next operation. However,
before discussing the sampling system it is important to compare the system
with a traditional ground-water pump.
A traditional ground-water pump fills the entire
sample tube with water from the pump to the surface. As the depth to the static water level
increases the energy required to lift the water also increases. The Burge sampling system does not fill
the entire sample tube with water (except in situations where the depth to
ground water is less than 25 feet bgs).
Instead, the sampling system transports exact volumes of water
(usually 150 mL) to the surface. The
water is delivered to the surface as discrete columns of water separated by
columns of air. The advantage of the
system is the low amount of energy required to raise the water from the
sampler to the surface. An example of
this advantage is the sampling of a well with a static water level of 100
feet bgs. If the Burge sampling
system were employed, a 150-mL sample transported in a 3/16” ID sample tube
would occupy approximately a 28-foot column of water in a sample tube of a
height of 100 feet. The amount of air
pressure required to lift the 28-foot column is approximately 12 psi
(assuming it requires .43 psi for each foot of water to raise the column).
The amount of air pressure required to lift the entire 100-foot column of
water is 43 psi. The advantage
becomes even more apparent when sampling a 200-foot deep monitoring
well. The pressure required to lift
the water column is approximately 86 psi while with the Burge sampling system
requires the same amount of pressure (12 psi) used in the 100-foot well to
lift the 28-foot column of water. It
was important to reduce the demands on the solar cell by decreasing the
pressure required to lift the sample because the Burge sampling system uses a
solar cell to produce air pressure to lift the sample. The solenoid valves used in the sampling
system are rated not to exceed 25 psi, therefore, the system would not
operate at higher pressures.
The problem of separating the water column from
the air column was solved by the air cell option located on the
analytical/calibration unit (HJ-100) designed to accept the water sample from
the monitoring well. The air cell
option strips the air from above and below the water column using the
following operation (Figure
6).
Step 1 illustrates the
filling of the sampler with water introduced into the sample chamber and air
vented from the chamber.
Step
2 illustrates the closing of the sampling valves and the pressurizing of
the sample chamber causing the water sample to rise up the sample tube. A column of air precedes the column of
water. The air column is introduced
into the entrance of HJ-100 through the normally open port of valve PC3 and
into the top of the air strip cell.
The air continues to be passed through the top of the cell through the
normally open valve of the air strip option.
After a time interval, water begins to enter the air strip cell and
starts to fill the air strip cell until the water level sensor at the top of
the air strip cell is satisfied (water level reaches top of cell). This action causes valve PC3 to be
activated diverting the water into the sample vessel of the HJ-100. The volume of the air strip cell is 10 mL,
therefore, the cycle strips off the air column and the first 10 mL of water
from the top of the sample (water column).
Step
3 illustrates the water sample diverted into the analytical unit of the
HJ-100 less the 10 mL that was trapped in the air strip cell. A timer is used to divert the sample into
the HJ-100. After a predetermined
number of seconds, the valve PC3 is deactivated and the remaining water
column and air pushing the water column is switched back to the air strip
cell.
Step
4 illustrates the final cycle of the air strip cycles. The remaining water in the sample tubing
is directed into the top of the air strip cell through the bottom port of the
air strip cell and through the normally closed port of the air strip
cell. After the final water is
removed from the sample tubing, air fills the air strip cell purging the
water from the cell. After a predetermined
amount of time, the cycle is terminated.
The sample chamber in the sampler, the sample tube and the air strip
cell are devoid of water (except for small droplets adhering to the side of
the tubes and chambers). The system
is then prepared for another sampling and transfer cycle.
The water sampling and transfer operation was
tested by the USEPA during an ETV program.
The program tested the sampling module using six volatile organic
compounds. The report is available from the EPA.
This option allows the water sample to be
introduced into the sample chamber of the sampler. The water is then purged of the volatile compound using
mechanical means (air stripping using bubbling gas through the water sample)
or chemical means (reduction of mercury to mercury metal, or arsenic to
hydride) and the volatile analyte is transported to a sensor mounted adjacent
to the well. The obvious advantage of
this type of sampling is the avoidance of the energy required to lift the
water sample to the surface. Additionally,
the partitioning of the analyte into the vapor phase eliminates the interferences
ions and compounds in the original water sample.
The water may be analyzed or purged from the
chamber (Step Two: Figure 4)
allowing the sample lines and sample chamber to be rinsed to insure the
sample collected is free of contamination from previous samples.
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