Sampling groundwater from deep wells is a challenge on its own, but add to that the need for accurate dissolved gas analysis, and the task gets that much harder.
Low Flow, Deep Groundwater Sampling
HydroTerra, a leading Australian company, and exclusive distributor of Solinst products, provides environmental monitoring solutions. They have developed a method of low flow sampling that uses the Solinst Model 408 1.66″ diameter Double Valve Pump, to depths of up to 1,200 meters (3,900 feet).
In order for Double Valve Pumps to reach these extreme depths, HydroTerra use specially designed weights that fit to the bottom of the standard pumps. These weights counteract the buoyancy caused by the very long drive and sample tubing lengths. In addition, HydroTerra have designed custom-built wellheads to ensure an organized and secure setup for various diameter wells.
How do Double Valve Pumps work? When the pump is lowered to depth in the well, water fills the pump and both sample and drive lines to static level. Gas is supplied from a pump controller, which pushes down on the water column and closes the valve at the base of the pump. This forces water to cycle through the pump and up the sample line. When the pressure is released, the top valve closes preventing water from draining down the sample line. To see how the Model 408 Double Valve Pump works, Solinst has a document showing its operating principles.
As part of their methodology, HydroTerra recommends dedicating one Double Valve Pump to each well, especially when sampling to these depths. They suggest dedication for the following reasons:
- Saves time setting-up and lowering the pump for future sampling events, and prevents tubing or pump from getting stuck
- Ensures representative, repeatable sample collection
- No decontamination required between sampling rounds
Dissolved Gas Sampling
In addition to using the Double Valve Pumps for deep groundwater sampling, HydroTerra, in conjunction with their project partners, have combined them to create a customized equipment setup that allows the collection of dissolved gases. Their water/gas separator chamber eliminates the problem of off-gassing as the groundwater samples depressurize on the way to the surface.
HydroTerra and their project partners, have developed a standard operating procedure (SOP) that uses the water/gas separation vessel to collect the groundwater sample obtained using a Double Valve Pump. The water/gas separator fills to a known volume; it is then sparged with a burst of nitrogen gas. The nitrogen strips the sample water of dissolved gasses, which are collected in a Summa canister, and sent to a laboratory for analysis.
HydroTerra recently helped complete an extensive deep groundwater sampling project. Using the unique methodology and custom-designed equipment, they successfully conducted numerous groundwater-sampling rounds to depths in excess of 1 kilometer.
The project has allowed HydroTerra to fine-tune their SOPs and set them apart as a leader in the collection of representative deep groundwater and dissolved gas samples. In addition, using the Solinst Double Valve Pumps in their work has helped HydroTerra maintain their goal of providing high quality field data in a cost effective way.