Sampling Tips For CFCs and SF6
It has become apparent from sampling results that unless extreme care is taken, there can be some problems with using the simplified bottle method for CFC and SF6 sampling. Although this method is extremely simple in comparison to the old flame sealed ampoule method, this simplicity can be deceiving and makes it very easy to contaminate the samples either during sampling, shipping, or while in the sample processing queue. Specific problems that have been identified include:
- Always use fresh water from the well being sampled to fill the bucket or container in which the sample bottle is submerged for sample collection.
- Be sure to place the discharge end of the sample line all the way to the bottom of the sample bottle and fill from the bottom up, displacing the water already in the bottle.
- Make sure you allow a minimum of 2 liters of water to overflow the CFC bottle before capping it - measure this by collecting the overflow in a beaker, DO NOT GUESS! One way to do this is to place the sample bottle in an empty 2 liter beaker, then allow the sample bottle to fill and overflow until the beaker itself fills and overflows.
- Submerge the bottle cap in the bucket or beaker with the foil liner facing up. Make sure that all the air bubbles are displaced from the cap before sealing the sample bottle.
- Tightly cap the bottle while it is completely submerged. Dry the bottles and re-tighten the caps. Invert the bottle and look for air bubbles. A small pea-sized bubble may occur due to degassing but there should not be any bubbles of significant size. This is a subjective judgment call, but remember it will only take a few minutes to collect another sample whereas it may be very costly or impossible to resample the well at a later date if the sample is contaminated. Neither the CFC nor the SF6 bottles require that the caps be loosened and retightened as the samples warm. This was recommended previously with SF6 samples when the larger bottles were being used but is not necessary now with the smaller one liter bottles.
- Tightly wrap the cap with at least 3 revolutions of electrical tape. Make sure you wrap clockwise in the same direction as the threads, therefore forcing the cap on tighter. This will prevent the cap from working loose during shipping and also helps prevent the CFC bottle cap liner from drying out.
- Store and ship the CFC bottles upside down, and ship them weekly to the CFC lab in Reston, VA; holding times for the CFC and gas samples is limited and they need to get into the sample processing queue. Do not ice these samples for storage and shipping. Do not store or ship SF6 bottles upside down.
- Do not refrigerate the CFC or SF6 bottles.
There are a few other potential problems that are not directly related to the new method.
- Make sure you use the appropriate pump and tubing for sample collection. Plastic and Teflon components can sorb and then desorb CFCs therefore resulting in potential contamination of samples. The sample tubing for CFCs should be refrigeration grade copper. If a peristaltic pump is used for sampling shallow wells or minipiezometers, a 6" length of Viton tubing must be used in the pumphead only with copper tubing used as the inflow and outflow tubing.
- Inadequate purging of wells may result in high CFC and SF6 concentrations and therefore anomalously young ground-water ages. This can be avoided by purging completely and collecting the CFC, SF6, and dissolved gas samples after all other samples.
- In urban areas, CFC and SF6 atmospheric concentrations are often higher than the air curves we use to date groundwaters. If you suspect local anomalies, corrections may be possible but they require collecting air samples. Contact the lab if you have questions about this or to arrange for air sampling.