Sampling Lake Bottom Sediments

The sediments at the bottom of lakes accumulate over a long time. These sediments contain thousands of years of fossil remains.

In 2003 the community around Cowley Lake asked us to explore the reason the lake level had dropped almost a meter over a three-year period.

We decided to examine core sediments taken from shallow reaches of the lake to see if there were layers of terrestrial vegetation indicating the lake had been lower in the past.

We needed to find a way we could take a number of sediment samples. We wanted to extract a core from 4 to 5 meters of sediment to be able to study the lake history since the last ice age. We needed to develop a process that allowed us to take such a sample. We decided to sample from the lake when it was frozen.

This instructable describes the tools we made that permitted more than 4 meters of sediment sampling.

• three sections of 2 inch galvanized EMT
• 2 inch male and female black pipe couplings
• three 1 inch by 6 foot black pipe
• one 6 inch section of 3 inch black pipe
• three 12 inch sections of 1 1/2 inch black pipe
• three 8 inch sections of 5/16 round bar
• three 2" ratchet straps,
• three 40 inch sections of 3/8 static line for anchors
• three sections 60 inches of 1/2 inch tubular webbing for prussic knots

Weld the female 2" coupling on the ends of two of the EMT pipes. Weld the male 2" couplings on one pie with the female coupling and the other on the pipe with no coupling.

We needed a way to extract the core sampler once it had been driven into the lake sediment.

Make a tripod head using the 4" pipe and weld, on at about 30 degrees, the 1 1/2 pipe sections. Drill and tap the 1 1/2 pipes near one end for 5/16 bolts, cut the ends of these pipes at about 60 degrees. The 1' black pipe should slide into the 1 1/2 pipe and be held in place by the 5/16 bolts.

We needed to be able to apply 3 to 4 thousand pounds of pressure to push the 2" EMT pipe into the sediment. To do this we set Abalakov ice anchors and used 2" cinch straps with prusik knots on the pipe to pull it into the lake sediment once it had been pushed as far as we could by hand.

The whole assembly was designed to be assembled and disassembled to be moved to the sample site

In order to pull the pipe out of the sediment, we needed a tripod that we could pull against. We use the same prusik knots and anchored the ratchet straps on the tripod. This allowed us to pull the pipe up.

Once we pulled the pipe from the lake bottom, we pushed a plunger pipe through the 2" EMT using the ratchet straps. The core was extruded into a PVC pipe cut in half along the long axis. This core was then cut in half along the full length by pushing down along the core. Care was taken to avoid dragging the sediment from one part of the core into another. Samples were taken at regular intervals along the core and each sample was examined for diatom populations.