The Methodologies for collecting sediment samples

September 24, 2014

To collect sediment samples there are several methodologies that can be used depending on your site and type of sediment.

Methodologies:

Standard Surface Grab Collection with Scoops and Grabs

Scoops and spoons are inexpensive, widely available, non-mechanical, very portable, able to sample nearly every sediment type and easy to use. Scoops are used to collect sediment samples primarily from shallower waters. Attaching the scoop to telescoping poles allows for collection of sediments in deeper waters.

Care should be taken when the sampler is raised through the water column or is passed through a river current during retrieval to minimise the loss of extremely fine material.

With very little experience, a sampler can “feel” the substrate with the grab attached to a pole and quickly find appropriate material for sample collection.

Some disadvantages to using a scoop or spoon include: limited sample volume; possible loss of very fine material during retrieval; not useable in deeper water.

Standard Surface Grab Collection with Dredges/Grabs

Surface sediment samplers (dredges and grabs) are relatively inexpensive, are widely used, are standard for some sampling purposes (benthos), often don’t need expensive equipment to operate and come in a wide variety of sizes.

A sampler should be “set” according to the manufacturer’s instructions and lowered through the water column. Grabs, like our Van Veen Grab, should never be allowed to free fall into the substrate. The sampler should be carefully lowered the last few feet to minimize dispersal of fine material due to a sampler induced shock wave.

Extension handles can be attached to Eckman Grabs for sampling in shallow waters to plunge the sampler into the sediment. These handles can minimize some of the limitations of the grab.

The sampler is then tripped. The sampler should be slowly raised through the water column and placed in an appropriate container. If an insufficient or improper sample is collected, additional weights should be added (if appropriate) to the sampler to allow deeper penetration into the sediment.

If additional weights do not help in the collection of a sample, then the sampling equipment and techniques should be re-evaluated for the type of sediment encountered. For compositing, a minimum of three to five grab samples (as near the same volume as possible) from a site should be taken and thoroughly mixed. An aliquot of that composite should be collected and submitted as the sample for the site.

Some disadvantages to the use of surface sediment samplers (dredges and grabs) include: shallow depth of penetration; possible shock wave and loss of very fine grained surface deposits; potential for water column contamination and nearby down-current sediment re-deposition; loss of depth profile; not appropriate for waters with current (sampler drifts in current, “lies down” and can’t be triggered); larger materials such as twigs and stones can prevent jaw closure; probable loss of some water soluble and volatile organic compounds; and it is possible to dilute the toxic pore water with relatively clean surface water (which is important in conducting sediment bioassays).

Standard Core Collection

Sediment corers like the Stitz Corer, Beeker Sampler, Multisampler or Peat or Russian Corer are usually simple cost effective sampling devices, manufactured in a variety of materials, can collect samples at depth, can maintain a more representative vertical profile of the sediment stratigraphy, create less disturbance by shock waves and can collect more highly consolidated deposits.

Sediment corers are slowly lowered to the substrate (gravity corers are released at the water surface and allowed to free fall) and simply allowed to penetrate the sediment under the samplers own weight or pushed or vibrated (vibro-core) into the sediments. Corers can be as simple as homemade tubes of steel, plastic or glass.

More specialist corers often contain core catcher inserts and one-way valves that allow the sample to enter the tube, but not exit and to hold it in place. Inserts should not be reused between sample locations unless decontaminated.

Inserts made of plastic should not be used when collecting samples for organic analysis. Upon retrieval, the corer can be disassembled and the sample laid in a container or a prepared decontaminated surface for further processing.

Cores from simple tubes and most other corers often drop out or can be pushed out with a clean rod. Plastic or thin walled metal corers (or core liners) can be cut, the ends capped, secured with tape and the entire segment sent to the lab. This process and the split spoon sampler reduces contamination from one segment to another in vertically stratified samples.

Some disadvantages to the use of sediment corers include: they do not work well with sandy sediments; they collect limited sample volume and very small surface area; they sometimes require expensive and bulky equipment to work in deeper waters and sediments.

Other Types of Collection

In some cases, sediment can be collected directly from the substrate by a diver using SCUBA gear or supplied air.

The sediment can be collected directly into the sample container or placed into the container by the diver with a scoop and sealed and composited at the surface.

The diver should be downstream of the sample site and should use caution not to disturb the fine grained sediment at the substrate surface.

Coffer dams can be used in very small streams. Coffer dams are temporary barriers that allow a small segment of stream to be isolated from the main water body and the isolated stream segment de-watered. After de-watering, the sediment inside the coffer dam can be collected with a scoop similar to a soil sample.

The coffer dam can be made by placing a 6″ diameter or larger pipe on the stream bottom parallel to the stream current. This reduces eddy currents and possible scour of the sediment when installing the pipe as a coffer dam.

Quickly tilt the pipe vertically so the top of the pipe is above the water surface. Care should be taken to avoid washing fines from the sediment surface during installation of the pipe. And once in place, the pipe should be pushed into the substrate with a circular back and forth motion.

Water inside the pipe is removed by a pump or by bailing. The sediment inside the pipe can then be sampled with a simple scoop.

Sieve samples for special circumstances measure the volume sieved and the specific gravity of un-sieved sample to calculate concentrations.

To help you select the right sampler click here or visit www.vanwalt.com