Last week’s post described what peepers are used for and what this week will involve. After a team meeting Monday, we decided we were ready to start that morning! We decided to finish five peepers that day, which would mean getting pore water samples into about 80 vials for various measurements; each peeper needs about 16 vials: pH, Iron, and sulfide/sulfate vials for each of the four peeper wells, two composite vials, and two for surface water measurements. The pH is measured immediately and the rest are analyzed at the lab.
Before getting a peeper for sample collection, we began by pipetting the appropriate proportions of acids and reagents into the vials so everything would be ready for the pore water samples. It is important to do everything we can in advance, since this is a time-sensitive process and the pore water samples can be contaminated by the higher concentration of oxygen in the air. We weigh the vials before and after adding sample.
The process of getting pore water from a peeper involves multiple moving parts. Samples are drawn into a syringe in a low-oxygen environment created using a vacuum bag (typically used for storing linens in a smaller space) and stream of nitrogen gas. We start with the most reduced well, number four, that was deepest in the water-saturated ground and work up to the first well. This is because the wells will be more exposed to oxygen closer to the surface and we do not want to risk any contamination. We put appropriate measurements of sample into each prepared vial for separate peeper wells, test the pH, gather surface water samples, and make the composite samples. To do this efficiently we had about five people with their own task to speed the process along.
After each day of sample gathering, the completed samples are analyzed at the lab. Michelle and I helped measure the absorbance of sulfide and Iron on the spectrophotometer. This is done using a tall cuvette that we zero with deionized water then fill with sample and quickly get the absorbance back as a decimal.