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).