Is sorption a problem using polyethylene tubing for low-flow sampling at gasoline hydrocarbon sites?
Is sorption a problem using polyethylene tubing for low-flow sampling at gasoline hydrocarbon sites?
The other factor to consider is changes in organic concentrations in groundwater over time. The initial concentrations within a well prior to purging and sampling may or may not match concentration in the groundwater near the well. As the well is purged at a low pumping rate (as in low-flow purging and sampling), water within the well screen should reach a flow-weighted average concentration of the water adjacent to the well screen. Even if this average concentration is somewhat higher or lower than the equilibrium concentration in the tubing, the residence time for the water within the tubing is typically very short, far too brief for sorption to have any measurable effect on samples. For example, if a pump is operated at 250 mL/minute and there is 500 to 1,000 mL of water in the tubing (50-100 feet of tubing), the residence time is 2-4 minutes; this is even shorter as the flow rate increases.
Several years ago, QED performed a series of leaching and sorption tests on our SamplePro polyethylene pump bladders to see if there was any potential for sorption of benzene or TCE into the plastic that would affect sample concentrations. The sorption testing was performed for 20 minutes, much longer than the residence time of groundwater in the discharge tubing during sampling. The results showed that there was no measurable loss in the samples. Based on this, the State of Florida DEP's Quality Assurance Section has approved the use of PE tubing and bladders for VOC sampling in their FS2200 SOP guidance document. (You can find this SOP on the FL-DEP's Website at http://www.dep.state.fl.us/labs/bars/sas/qa/sops.htm)
All plastic materials have some capacity for sorption of various organic compounds. The rates of sorption will vary based on the type of plastic, the organic chemistry, temperature, and other factors. This sorption could be significant (i.e., measurable) when the material is newly exposed to water with organics in solution, but the rate of sorption will decrease with time until the concentration of organics in the plastic reaches an equilibrium concentration with the water. At that point, sorption is no longer a factor affecting organic concentrations in samples. Thus, dedicated pumps and tubing are the best solution to avoiding any effect of sorption on groundwater samples – both the pump and tubing (along with the well casing) will have reached equilibrium long before any samples are taken.
The other factor to consider is changes in organic concentrations in groundwater over time. The initial concentrations within a well prior to purging and sampling may or may not match concentration in the groundwater near the well. As the well is purged at a low pumping rate (as in low-flow purging and sampling), water within the well screen should reach a flow-weighted average concentration of the water adjacent to the well screen. Even if this average concentration is somewhat higher or lower than the equilibrium concentration in the tubing, the residence time for the water within the tubing is typically very short, far too brief for sorption to have any measurable effect on samples. For example, if a pump is operated at 250 mL/minute and there is 500 to 1,000 mL of water in the tubing (50-100 feet of tubing), the residence time is 2-4 minutes; this is even shorter as the flow rate increases.
Several years ago, QED performed a series of leaching and sorption tests on our SamplePro polyethylene pump bladders to see if there was any potential for sorption of benzene or TCE into the plastic that would affect sample concentrations. The sorption testing was performed for 20 minutes, much longer than the residence time of groundwater in the discharge tubing during sampling. The results showed that there was no measurable loss in the samples. Based on this, the State of Florida DEP's Quality Assurance Section has approved the use of PE tubing and bladders for VOC sampling in their FS2200 SOP guidance document. (You can find this SOP on the FL-DEP's Website at http://www.dep.state.fl.us/labs/bars/sas/qa/sops.htm)