Porewater samplers in wetland soils

Porewater in wetland soils has a tremendous information. Scientists can use porewater to know more about geochemical properties of soils, such as pH, salinity, conductivity, nutrients, etc.

But how do we collect porewater? Although there are many different types of samplers, I particularly like my design because it does not disturb soil environments at all when we collect samples.

Let's see how I make.

Things and materials used (for making one sampler):
1. PVC pipe
2. Two PVC pipe connectors (fitting)
3. One PVC pipe cap
4. Tubing
5. Two three-way stopcocks
6. Porex porous membrane pipe (nomial pore size: 20-40 um): I purchased Porex pipe from Interstate Inc.
7. One rubber stopper (cap)
8. PVC cutter, PVC cement, cork borer (or drill to make a hole on rubber stopper)

I can make any length of sampler to collect water from a different depth. But for this instruction, I will pretend to make a sample which has a target collecting depth at 10 cm below soil surface.

Cutting (see my note below):
1. Cut PVC pipe at a length of 35 (top) and 20 (bottom) cm
2. Cut porous membrane pipe at a length of 9 cm.

Top left to right: a sampler designed to collect porewater at 10, 25, and 50 cm below soil surface (Olivia De Meo note)

Assembling the pipe:
1. Using PVC cement and PVC connector, I connect the 35-cm PVC pipe to porex membrane pipe. Use enough PVC cement and even apply silicon gel when the cement is dry. The outer diameter of PVC and porex pipe is slightly (less than 1mm) different.
2. Attach another side of porex pipe to the 20-cm PVC pipe using the connector and PVC cement.
3. Put the bottom PVC cap on the another side of 20-cm pipe. I don't permanently attach the bottom piece because I want to retrieve a sampler out of soil to clean periodically.

Preparing a tubing:
1. Prepare a 80-cm and 25 cm long tubing
2. Using either a drill or cork borer, making two holes on a rubber stopper.
3. Put the two tubing through the stopper and leave 20-cm tubing above the stopper. (use hot water to ease the process)
4. Insert three-way stopcock (use hot water to ease the process)
5. Mark a stopcock with electrical tape to show which one is longer (sampling port) and short (air port)

To collect porewater, we need following items:
1. Nitrogen gas bag
2. Syringe
3. Sample tube

Picture showing porewater samplers installed in our experimental plots

How to collect sample:
1. Connect a syring and nitrogen bag into a long and short tubing stopcock, respectively.
2. Remove old water stored in a sampler by pulling water out using a syringe. This action will replace water volume with nitrogen gas. This is very important step because we don't want to inject oxygen into anoxic porewater sample. If we replace water with air (not nitrogen gas), it will spontaneously oxidize some ions (such as reduced sulfur, manganese, iron).
4. Disconnect both the syringe and the gas bag.
5. Wait about 30 min.
6. Do the step 1 again.
7. Collect sample.

Happy sampling!