Gravity gets a lot of request to support sediment sampling programs and often the subject turns to the question – “what tool is right for the job”. We thought a brief overview of sediment sampling equipment and its correct application may be helpful to company’s interested in using our services. This first posting will review surface sediment sampling equipment with more posts later on subsurface etc.
*Note this is our opinion based on 14+ years of sampling in many different environments (not all mind you)…..and it should be taken with a grain of salt! Email us or comment to the posts with corrections or additions!
Surface sediment sampling – 0 to 35 centimeters
Sampling of surface sediments sounds pretty straight forward – after all its just that top little scuff of sediment we are interested in right? Well sometimes that answer is yes…..but most often it is not. On last count there are over 30 sampling devices for surface sampling. This is justified considering that the surface sediments are most likely the most important layer of substrate where most biological activity, sediment transport, contaminant fate, etc occurs. Also because it is very important to collect this layer as undisturbed as possible certain devices work better than others depending on substrate. Below are some of the main sampling devices:
Ekman Grab – great little sampler designed for soft muds and higher water content substrates. Because it spring closes with about 15 lbs of force it can not be used where any sands, rocks, wood debris etc may come in contact. This unit is nice because it is small and light and can be connected to a pole for shallow water. Grab is closed with a messenger Sample volume is another negative as it is only about .025 cubic meters so plan on many drops to collect a practical volume for analytical testing.
Petite Ponar – another easy to handle grab that can be used in deeper water off of a dingy or kayak. As the name suggests it is small, but it does have much more closing force than an Ekman by utilizing lifting bars that close the jaws. Gravity has custom extended a number of ponars lifting bars for more leverage in courser waters. This unit is great for rivers and shallow waters. Low volume issue still comes into effect like the Ekman for analytical.
Van Veen Grab – this is the industry standard tool for getting that top 15 cm of material. This grab weighs about 90 lbs and has the same leveraged bars for lifting as the ponar. It has good closing force and works good for sampling in areas with sedimentation present. The Van Veen is popular for contaminant programs because you can get plenty of volume for sediment analytical characterizations. A double Van Veen set-up is also available by many companys (not Gravity) for collecting extra sediments for bioassay testing. The Van Veen does have a few negatives including a funky release chain mechanism that does not always work and sometimes pinches fingers. It also does not work well on slopes and fails completely if cobble or wood debris are present.
Smith McIntyre grab – The Smith Mac is a good deep ocean grab sampler that uses a very powerful spring to close its jaws. This device is great in consistent bottom substrates but does not work well if it encounters cobbles, wood debris or other material because it depends on a heavy spring only to close. Many of the Smith Macs do have a bottom frame which helps with taking samples on slopes.
Power Grab Samplers – we are of course bias to the power grab sampler because we have designed and built numerous versions of these……but their is good justification for their use. The power grab sampler is typically the same jaw closure as a Van Veen although often much larger. We have one Power Grab that is triple the size of a standard Van Veen with 35 cm penetration. The power grab uses a bottom frame and cylinder powered by either air or hydraulic pressure to close the grab. The power grab system is good because the bottom frame keeps it level on the bottom and the sample is taken by forcing the jaws down while not lifting (the Van Veen lifts as it closes). This often gives you the most undisturbed sample and the highest sample penetration success. Power grabs having closing forces from 150 to 1500 ft/lbs. Many of the power grabs out there where built in the NW states because of all the wood debris sampling required – where only the power grab will work. We have also used the power grabs to collect deeper than standard sediment samples and to collect grabs in rocky environments. Keep in mind though we don’t have a power grab that can crush rock and the lab has no way to analyze rocks…..so if your sampling site is all 3″ cobble consider negotiating your way out of sampling or send us out to take pictures or video instead! Downside to the power grabs – they are heavy and need at minimum a 300 lbs davit to deploy and retrieve.
Other Surface characterization equipment
Sediment Profile Imaging (SPI) camera – this is a great way to review surface sediments without having to get dirty! The SPI camera uses a prism and camera to penetrate the sediment surface and collect an image for later analysis. Germano and Associates are the true experts at this and are awesome to work with more info at remots.com.
Peepers, fat bags and other in-situ devices – if you want to collect a sample at equilibrium with the sediment, biota and pore water these devices are the correct tool. Peepers are usually diver deployed into the sediments and left in location for a month. Peepers should not be deployed in fast moving waters – we have had lots of problems in the past with this. The other criticism of peepers is the unknown variable – that is what actually happened over that month is was in the water? Were events typical for the site? Was their atypical seasonal variation or other events that may have biased your results?