6.9 SURFACE WATER SAMPLE COLLECTION METHODS
Figure 6-18. Isokinetic Sampler Constructed of Teflon
[Source: USGS, 2008b]
Environmental investigation sites featuring adjoining surface water bodies may require
the collection of surface water samples to assess environmental impacts. Select
the sampling approach and sampling equipment based on the type of surface water
(flowing versus still), the sampling platform and the contaminant characteristics.
6.9.1 Sampling Flowing and Still Surface Waters
In general, an isokinetic water sampler is used to collect a sample from water flowing
faster than 1.5 to 2 feet per second. Non-isokinetic water samplers are used for
all other surface water sampling applications.
In flowing water, always position the sampler upstream of any disturbance caused
by sampling activities. In standing water deploy the sampler away from any disturbance
caused by sampling. Avoid contact of sampling equipment with the bottom sediment.
Deploy the samplers in a fashion that minimizes disturbance and suspension of sediments.
6.9.1.1 Isokinetic Samplers
An isokinetic sampler is constructed of a cap with a nozzle and a bottle or bag
for sample collection. Fins are attached to the downstream end of the sampler, i.e.
away from the nozzle, to keep the sampler aligned with the flow direction in the
stream. Select the nozzle, cap and bottle material to be compatible with the contaminants
of concern. Consult the manufacturer's specifications for limitations on the use
of each sampler. Refer to United States Geological Survey (USGS) document entitled
"Handbooks for Water-Resources Investigations - National Field Manual for the Collection
of Water-Quality Data" for more information on isokinetic samplers (USGS,
2002).
Use an isokinetic sampler to collect a depth integrated, representative water sample
continuously and isokinetically (that is, stream water approaching and entering
the sampler intake does not change in velocity) from a vertical section of a stream
while transiting the vertical at a uniform rate. An example of this type of sampler
is illustrated in Figure 6-18.
Isokinetic samplers are rated for their maximum allowable transit rate based on
the stream velocity. Refer to the manufacturer's rating to calculate the maximum
allowable transit velocity. Use a lower than maximum transit velocity to ensure
that a representative velocity-weighted sample is collected. Do not overfill the
sampler bottle (USGS, 2002).
6.9.1.2 Non-Isokinetic Samplers
Laboratory Cleaned Bottles
The most widely used method for collection of surface water samples is simple immersion
of the laboratory cleaned sample bottle below the surface of a surface water body.
This method eliminates the need for other equipment and reduces the risk of introducing
other variables into a sampling event.
Immerse the open bottle by hand into surface water and allow water to slowly run
into the bottle minimizing turbulence. Collect samples for volatile organics analysis
first to prevent loss of volatiles due to disturbance of the water. Do not disturb
the sediment, especially when analytes (such as metals) could be impacted by turbidity.
Pond Sampler
The pond sampler may also be commonly referred to as a "Dipper". The pond sampler
consists of an arm or handle with a clamp to attach a sampling beaker. The construction
materials vary and are selected to be compatible with the site contaminants. Pond
samplers can be assembled from equipment found in swimming pool supply stores and
laboratory supply stores (NJDEP, 2005).
Slowly submerge and retrieve the sampling beaker with minimal surface disturbance.
Transfer the sample slowly into a laboratory supplied sample bottle, allowing the
water to flow gently down the inside of the bottle. Avoid turbulence in the sample
stream. Always collect samples for volatile and semi-volatile analyses first.
Weighted Bottle Sampler
The weighted bottle sampler consists of a bottle and an attached weight, which maintains
the upright bottle orientation during sample collection, and a stopper (when needed).
The construction materials for weighted bottle samplers vary and are selected to
be compatible with the analytes and/or site contaminants (if known).
Lower the weighted bottle sampler to the predetermined depth. Pull out the bottle
stopper with a sharp jerk of the sampler line and allow the bottle to fill completely.
When the bottle is filled, there should be no more bubbles rising to the surface.
Retrieve sampler and transfer the sample slowly into a laboratory supplied sample
bottle, allowing the water to flow gently down the inside of the bottle. Avoid turbulence
in the sample stream. Always collect samples for volatile and semi-volatile analyses
first.
Wheaton Dip Sampler
The Wheaton Dip Sampler consists of a glass bottle mounted at the end of a metal
pole of fixed length. The bottle lid is rigidly attached to a second metal pole,
which is loosely attached to the main pole. The second pole is used to unscrew the
bottle cap at the required sampling depth.
Use the Wheaton Dip Sampler to collect samples in shallow surface water. With the
bottle cap closed, lower the sampler to the required depth and unscrew the bottle
cap. Once the bottle is filled, (i.e. when no more bubbles reach the water surface)
screw the bottle cap back on and retrieve the bottle.
Transfer the sample slowly into a laboratory supplied sample bottle, allowing the
water to flow gently down the inside of the bottle. Avoid turbulence in the sample
stream. Always collect samples for volatile and semi-volatile analyses first.
Figure 6-19. Niskin Bottle Sampler
Sampler shown with the end stoppers open (left) prior to immersion in the surface
water body and closed (right) following sample collection.
[Source: General Oceanics, 2008]
Figure 6-20. VOC sampler
Sampler contains 40-mL glass septum vials.
[Source: Rickly, 2008]
Figure 6-21. Churn Splitters
Plastic Churn Splitter (top) and Fluoropolymer Churn Splitter (bottom).
[Source: USGS, 2002; Figure 2-8]
VanDorn Sampler & Niskin Bottle Sampler
The Van Dorn sampler and Niskin bottle sampler are cylindrical samplers closed with
water-tight stoppers on both ends. The stoppers are connected through an elastic
band that runs through the inside of the sample collection cylinder. The stoppers
can be pulled out and locked to the outside of the cylinder, leaving both pipe openings
unobstructed, which allows for water to enter the cylinder. After the sampler has
been placed at the pre-determined sampling depth, the lock on the stoppers can be
triggered to release, causing the stoppers to close. The elastic band pulls the
stoppers into their seat and maintains the closed position to create a water-tight
seal. A valve at the bottom of the cylinder together with a vent at the top are
used to drain the samplers while the stoppers remain in the closed position.
These samplers are commonly deployed from a boat. The Van Dorn sampler must be suspended
on a dedicated line, while the Niskin bottles may be attached in series on a line
and the closing mechanism triggered with auxiliary messengers.
Open the samplers and suspend them on a line to the sampling depth. Trigger the
closing mechanism and retrieve the sampler. Transfer the sample slowly from the
sampler drain valve into a laboratory supplied sample bottle, allowing the water
to flow gently down the inside of the bottle. Avoid turbulence in the sample stream.
Always collect samples for volatile and semi-volatile analyses first. An example
Niskin bottler sampler is illustrated in Figure 6-19.
VOC Sampler
The VOC sampler has been manufactured for the USGS and is used to collect open water
samples for volatile organic compound analysis. The device has been tested in the
laboratory and field for analyte loss, reproducibility and cross contamination.
The sampler is constructed of stainless steel and copper and consists of a cylinder
that holds four 40-milliliter volatile organic analysis vials. Filling tubes extend
from the sampler lid into the bottom of the vials (Figure 6-20).
When the sampler is lowered into water, the vials start to fill. The vials overflow
into the inside of the cylinder, which has sufficient volume to let the vials overflow
by seven times their volume. This allows sufficient time (i.e. 3 to 4 minutes) to
lower the sampler to the required sampling depth. A cover over the inlet ports prevents
contamination by surface oil and debris (NJDEP, 2005).
It is important to evacuate air and other gases from the sampler prior to sample
collection. Close and remove the vials from the sampler immediately upon retrieval.
Double Check Valve Bailers
A double check valve bailer is a cylinder that is equipped with a check valve on
both ends. Both check valves are designed to open as the bailer is lowered into
water and to close when it is retrieved.
Dedicate the bailer to one sample location, when feasible. Suspend the bailer on
rope to the selected sampling depth and then retrieve. Do not use the bailer for
sampling air sensitive parameters (NJDEP, 2005). Transfer
the sample slowly from the sampler drain valve into a laboratory supplied sample
bottle, allowing the water to flow gently down the inside of the bottle. Avoid turbulence
in the sample stream. Always collect samples for volatile and semi-volatile analyses
first. Use a bottom emptying device with flow control when the bailer is used to
collect water for volatile analysis.
6.9.2 Composite Sampling
Use composite sampling to represent a cross section of a water body. Composite samples
vertically over the depth of a water body in one location or horizontally along
a specific water depth.
Use an isokinetic sampler to collect a depth integrated, representative water sample
continuously and isokinetically (that is, stream water approaching and entering
the sampler intake does not change in velocity) from a vertical section of a stream
while transiting the vertical at a uniform rate. Alternatively, collect discrete
samples from varying depths along a vertical section and composite in a churn sampler,
as described below.
For horizontal cross sections, collect discrete samples along one water depth traversing
a body of water.
Determine the total water volume needed to fill all the sample bottles and add at
least 10% for filter losses etc. Select the number of sub-samples based on the length
of the vertical or horizontal cross section. Keep sample spacing at more than one
foot. In a narrow or shallow stream or pond, sample the verticals or horizontals
as often as required to collect a sufficient water volume. Ensure that all verticals
or horizontals are sampled the same number of times.
Do not composite samples for volatile analysis. Do not composite samples for organic
analysis, organic carbon, pesticides, herbicides and bacteria using a churn splitter.
Do not collect these samples in any plastic device because of the potential for
contamination. Use glass samplers for these analytes. Collect bacteriological samples
in auto-claved plastic containers (NJDEP, 2005).
Churn Splitter
Composite the sub-samples using a churn splitter (Figure 6-21). Compositing the
verticals and horizontals in the churn splitter creates single cross-sectional representations
of the stream. Place the composited sample into the necessary sample containers
for various analyses.
A churn splitter is an 8 or 14-liter plastic container equipped with a churning
paddle and a drain valve. Use a churn splitter to composite surface water samples.
Always collect a minimum of 3 liters of water volume for a composite sample when
using the churn splitter. Note that a churn splitter does not reliably produce representative
composited water samples when it contains less than 2 liters.
Prior to use, decontaminate the churn splitter as appropriate for the site contaminants.
Prior to sampling, rinse the churn three times with 1 liter of sample water each.
Let the sample water drain from the drain valve each time. This will allow the container
to chemically equilibrate with the sample water.
Collect the required number of sub-samples and add to the churn. Keep the churn
closed at all times except when adding sub-samples. Composite the water samples
by moving the paddle up and down at least 10 times achieving a churning rate of
9 inches per second before withdrawing water (NJDEP, 2005).
Faster or slower churning rates can cause maximum errors of 45% to 65%. The same
rate of churning should be sustained throughout sample withdrawal.
Increase the round trip frequency as the water volume in the splitter decreases
so that the churning disc velocity is constant. The disc should touch bottom, and
every stroke length should be as long as possible without breaking the water surface.
Increase of the stroke length and/or disc velocity beyond the recommended rate will
lead to a sudden change in sound and churning effort. This is accompanied by the
introduction of excessive air into the mixture. This is undesirable because excessive
air may tend to change the dissolved gases, bicarbonate, pH and other characteristics
(NJDEP, 2005). However, inadequate stirring may result
in non-representative sample splits.
Withdraw filtered samples last, directly from the mixing tank using a peristaltic
pump or other device. Once all sample bottles have been filled, rinse the churn
thoroughly with deionized water, allowing the rinse water to flow through the drain
valve. Keep the churn lid closed at all times to avoid contamination with airborne
particles.
Double bag the churn sampler during transport and storage to avoid contamination
by airborne particles.
6.9.3 Grab Sampling
Collect grab samples when:
- Natural stream conditions make compositing unnecessary. That is when the flow rate
is high and mixing in the stream is uniform.
- Contaminant characteristics preclude compositing.
- Discrete samples are required.
Collect discrete samples using the appropriate sampler. Deploy the sampler away
or upstream of any disturbance caused by sampling activities. Avoid touching the
sediment with the sampler.
6.9.4 Point Sampling
A point sample is collected at a specific sampling location from a specific depth.
To obtain such samples use a double check valve bailer, a Van Dorn sampler, a Niskin
bottle sampler, a VOC or other sampler. Lower the sampler to the target depth and
trigger the closing mechanism. In shallow water collect the sample by submerging
the closed sample containers by hand to the desired depth. Open the lid, let the
container fill and replace the top, while the container remains at the sampling
depth. Once the lid is in place remove the container from the water.
6.9.5 Lake/Standing Water Sampling
Collect water from lakes and other standing water as point samples. Either composite
the point samples or keep them as discrete samples as required in the SAP. Collect
surface water samples at a depth of one meter. If the water is shallower than one
meter, collect the sample from just below the water surface or at mid-depth. Sampling
events should also be conducted during subsequent seasons, as there may be seasonal
fluctuation.
If the temperature profile of the lake indicates stratification, collect discrete
samples/point samples in the observed layers. These samples may be composited or
analyzed discretely. Take care not to disturb the stratification with sampling activities
such as a boat or wading or sampler deployment. Calm waters will possibly help keep
a lake's stratification more observable and make sampling each observable layer
easier. The stability of a lake's stratification depends on the lake's depth, shape,
size, orientation of the wind, and inflow/outflow, so these considerations may need
to be accounted for in the SAP.
6.9.6 Estuarine Marine Sampling
Sample estuaries using the same methods employed for stream and lake sampling. Determine
stratification in an estuary using conductivity and salinity measurements in addition
to temperature. Both varying salinity and varying temperature can cause density
variations that in turn cause stratification.
During the design of the SAP take the tidal stages and currents into account. Collect
samples from a boat as far from the stern as practicable, and only after the turbulence
of the wake has subsided. Approach any sampling site from downstream, and sample
upstream of the boat.
6.9.7 Trace Element Sampling
Sampling for trace elements requires a more rigorous sampling procedure. Follow
the procedure recommended by the USEPA in its publication entitled: "Sampling Ambient
Water for Trace Metals at EPA Water Quality Criteria Levels" (USEPA,
1996d).
6.9.8 Filtration
For surface water filtration follow the same procedures outlined for groundwater
filtration of groundwater samples (Subsection 6.6, Filtration).
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