• ASTM Standard Guide D7929-14

  • ​Peer-Reviewed Paper in ES&T

The ASTM Standard is widely accepted as one of the most rigorous credentials a method can have and we believe it to be an important distinguishing factor to consider in your monitoring method decisions.


ASTM Standards don’t name specific commercial products, but the Snap Sampler method is described to a “T” and Snap Sampler research documents are cited in the Standard at least a half-dozen times.  This is the ASTM Standard that applies directly to the Snap Sampler. 


More information on the ASTM Standard here:


Importantly, the guide is for passive sampling in particular, not all no-purge sampling.  It requires that the sampler equilibrate at a fixed position and contain the sample prior to closure/collection.   As such, it applies to diffusion samplers and the Snap Sampler, but does NOT apply to bailers or other grab samplers that require the device to be moved to go collect the sample within the well.  These other devices not covered in the ASTM Standard and should not be considered equivalent to the Snap Sampler because they fundamentally operate differently and have not been validated through the rigorous ASTM Standard process.


Peer-Reviewed Paper in ES&T

This is the definitive paper on Snap Sampling in the scientific literature.

Six field studies are examined in this Environmental Science and Technology Paper.

World known researchers Beth Parker and John Cherry collaborated with Sandy Britt to develop this wide ranging study.

The Snap Sampler is shown to be equivalent to low flow and other sampling methods, while highlighting data quality advantages of sealing samples in the in situ well environment.


Abstract and paper available here:


Expanded Technical Information

The Snap Sampler Groundwater Sampling Method

The Snap Sampler is one of group of passive sampling technologies developed over the last 15 years that address the problem of waste generation during well purging and also significantly reduce the time required to collect a sample.  Numerous studies demonstrated early on that multiple-volume purging of groundwater monitoring wells was not necessary to collect samples that were similarly representative of groundwater conditions (Powell and Puls 1993; Varljen 1994; Puls and Barcelona, 1996).  These studies, and those like them, set the stage for the acceptance of low flow purging as a viable replacement for multiple volume purging.  Powell and Puls (1993) went further and suggested that no purging was needed at all, as long as the sampling device is set in the screen zone of the well, and there was sufficient hydraulic gradient in the aquifer to cause water exchange.  Later, investigators such as Don Vroblesky of the USGS successfully experimented with devices that were deployed in the well screen, equilibrated downhole and then collected directly without any purging (Vroblesky, 1997).  

Since these early experiments, several devices have been commercialized to take advantage of the passive sampling approach for groundwater, including the Snap Sampler—which was developed in 2004 and first tested at the “McClellan” study conducted by the Air Force at McClellan Air Force Base in Sacramento, California.  The Snap Sampler yielded results that were most similar to the volume purging results of the study (Parsons, 2005).  It also correlated best of the all of the methods with low flow purging for multiple analytes.  Later, the Snap Sampler went through a rigorous demonstration/validation study conducted by the US Army Corps of Engineers through the ESTCP program (Environmental Security Technology Certification Program).  The Principal Investigator described the Snap Sampler data comparison as “passing with flying colors” and explained further that projected cost savings in their study ranged from 46% to 68% compared to the low flow purging approach.  Separate from the very thorough ESTCP project, a journal paper in Environmental Science & Technology was put together outlining 6 separate field studies that illustrate the capabilities of the Snap Sampler method (Britt, Sanford L., Beth L. Parker, and John A. Cherry, 2010, A Downhole Passive Sampling System to Avoid Bias and Error from Groundwater Sample Handling.  Environ. Sci. Technol. 2010, 44, 4917–4923 ).

Over the last decade, the Snap Sampler has undergone several refinements that have improved function and utility.  The original pull trigger system was supplemented first with an electric trigger system and then a pneumatic system.  These systems now allow the Snap Sampler to be used to depths of at least 2000 feet.  Snap Sampler bottles started with just VOA vials, then expanded to include a 125ml, then 350ml HDPE containers.  The number of Snap Samplers that can be placed on a “string” of Snap Samplers also has increased from 3 to 6, meaning that multiple different bottles can be collected as needed to get fairly extensive analyte lists.  Sample volume is still the main limitation for Snap Sampling (well diameter greater than 2inch / 50mm is also a requirement).  In 2” wells, 750ml is the maximum that can be collected in a single “string” of Snap Samplers; in 4” wells, the maximum increases to 2.1 liters.  Many, many sampling suites can be accommodated with these sample volumes, but not all.  Often times, laboratories can accommodate smaller sample volumes as techniques and equipment continually improve.  This is always a consideration when choosing any passive sampling method.

The specific function of the Snap Sampler can be described like this:  the Snap is a dedicated passive sampling system that consists of a series of individual “samplers” that each hold one Snap Sampler bottle.  The bottles have “snap caps” on both ends of each bottle, with a Teflon-coated spring that connects the two caps through the inside of the bottle.  The caps are set in an “open” position during deployment and are “snapped” closed by the user just prior to retrieval.  The Snap Samplers themselves hold the bottles, and have a release pin system that holds the bottles open and provide a mechanism for the user to trigger the caps to close.  The triggering mechanism consists of a manual pull trigger or an electrically or pneumatically activated system.  At surface, the user activates the Snap Sampler bottles to close, then retrieves the whole system for removal and preparation of the bottles for direct laboratory submittal.  The “Sealed In Situ” aspect of the Snap Sampler is unique.  Bottles closed downhole do not need to be transferred to separate laboratory containers.  This avoids exposure to the atmosphere, which can cause VOC loss from volatilization, field contamination, or simply changes to chemistry from exposure to oxygen.  Pouring is also a source of error and variability that can be different from one event to the next.  Outside factors such as personnel or weather cannot contribute to variability if the samples are sealed downhole from a fixed and repeatable position every time, and samples are never poured.  This is a main feature of the Snap Sample method. For information on the magnitude of effects due to sample pouring, see:  Parker, LV and Britt, SL, 2012, The Effect of Bottle Fill Rate and Pour Technique on the Recovery of Volatile Organics, GWMR 32, Fall p. 78-86.

Snap Samplers can be stacked in series in any combination depending on the bottles required for collection.  The “string” of Snap samplers may consist of just 2 VOA samplers for example, or a combination of 2 VOAs and 4 125ml Polys.  Any combination up to 6 is possible.  Snap Sampler bottles are removable from the sampler.  The bottles are loaded in the samplers, then deployed downhole for equilibration.  Most typically, the dedicated Snap Samplers are left downhole between sampling events.  The only time the field sampler handles the device is to retrieve it from the well, remove and replace bottles, then redeploy.  This mode of operation stores the equipment in the well and no special procedure is needed for handing or decontamination.  The system is dedicated.

More detail of the operation is included in the Snap Sampler Standard Operating Procedure (ProHydro 2014), including detailed explanation and picture instruction cards.  In short, the procedure includes a bottle loading step, a cap opening step, connection of each of the Snap Samplers to each other, and deployment downhole.  Collection procedure included triggering the bottles to close, retrieval, removal of bottles, and reloading for redeployment.  It is a fairly simple procedure that takes approximately 10 minutes plus the time to “trip” out of the hole.

The materials of the Snap Sampler and Snap Sampler bottles include acetal (Delrin), PFA (Teflon), glass, HDPE, FKM (Viton) fluoroelastomer, and stainless steel for connecting cables and pneumatic assembly screws.  The plastics are more chemically inert than the PVC in the well, and have been deployed for many many years in wells with no adverse effect other than discoloration.  For particularly challenging geochemistries, there is a system where the connecting cables are fully plastic materials too.  We anticipate that dedicated systems deployed now will last upwards of 10 years.  ProHydro warrantees Snap Sampler for 10 years.

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