| Discrete
interval sampling can be used to profile an open
borehole or screened well, to collect samples
from distinct levels or points of inflow, to obtain
representative water samples from below floating
product layers (LNAPL), and to obtain samples
of sinking product layers (DNAPL). There is negligible
disturbance, as can be caused by pumping and purging.
Mixing of water from different levels in the well
is minimized.
Purging
and disposal of purged water can be avoided.
Sampling directly from a specific depth results
in the collection of the most recent water that
has flowed into the well at that depth.
The
Model 425-T Discrete Interval Sampler offers
a zero-headspace sampler that can be transported
directly to the laboratory in down-hole condition.
Solinst
also has a stainless steel Point-Source Bailer
with a second check valve at the top of the
bailer, to prevent water entering the sampler
on the way out of the well
The
Solinst Model 425 is a stainless steel sampler,
with LDPE tubing mounted on the convenient Solinst
reel. The reel has a pressure attachment for
the high pressure hand pump, and a pressure/vent
switch to apply and release the pressure on
the sampler. A sample release device is also
included.
The
sampler is pressurized using a high pressure
hand pump before being lowered into the well.
This prevents water flowing through the sampler
on the way down the well. Once the desired depth
is reached, the pressure is released and hydrostatic
pressure allows the sampler and tubing to fill
with water directly from the sampling zone.
When
the sampler is filled, it is repressurized and
raised to the surface and the sample is decanted
using the sample release device provided, to
avoid degassing of the sample.
The
sampler is ideal for groundwater sampling from
below an oil/product layer on the surface of
the water, as it allows a sample to be obtained
which is untouched by the oil.
The
sampler is easily disassembled for decontamination,
without the need for tools.
APPLICATIONS
Obtaining a representative groundwater sampler
from below oil/product layers
Discrete interval sampling
Chemical profiling of wells
Sampling at points of inflow to well
DNAPL sampling
VOC SAMPLING
Discrete interval samplers are excellent for
VOC sampling. There is no mixing with water
from different levels in the well. The sample
does not travel through a long length of tubing,
risking loss of volatile organics. The sample
has minimal contact with air.
For
the most accurate VOA results the Model 425T
Transportable DIS can be used. It retains the
volatiles at down-hole conditions with zero
headspace during retrieval and transport to
the laboratory.
ADVANTAGES
Sample not pumped through tubing
No mixing of water from different levels
Avoids purging and disposal of purged water
No-tools disassembly for decontamination
METHOD OF OPERATION
The Discrete Interval Sampler is connected to
the tubing. The high pressure hand pump, or
a compressed air source, is attached to the
reel.
The
operating pressure required is determined using
the chart at right. For other depths the formulas
may be used. Pressure is applied, which closes
the check valve in the bottom of the sampler,
preventing any water from entering the sampler.
The
sampler is then lowered to the chosen sampling
depth while pressurized. At the chosen depth
the pressure is released. Hydrostatic pressure
fills the sampler and tubing, as the water rises
to hydrostatic level.
The
sampler is again pressurized to close the lower
check valve, and is raised to the surface. It
is removed from the borehole and the sample
is transferred to a sample bottle using the
sample release device for controlled flow.
MATERIALS
The samplers are constructed entirely of stainless
steel with Viton o-rings.
The
tubing most commonly used is low density polyethylene
(LDPE), however, Teflon or Teflon-lined polyethylene
tubing is also available.
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