Van Walt Environmental Connect Issue Two

VANWALT Monitoring your needs

The Evolution of Groundwater Sampling

Not just any Water Level Logger...

E Q U I P M E N T R E Q U I R E D

* Pumps * Purge Drums * Generator * Heavy Lifting * Equipment Rental * Significant time on site * Turbidity

* Pumps * Drum(s) * Power or compressed air * Equipment Rental Meter calibration * Less time on site * Lower turbidity * Small volume water removal

* Equipment

pre-installed

* Sample

immediately * Less time on site * Lower turbidity * No waste * No handling

* Large Volume water removal

Low Flow

Purge

Passive

Time on site

flowing groundwater will replicate samples collected through the rigorous purging process. Isn’t this counter- intuitive? Haven’t we been told to remove the “stagnant” water from the well? Yes, we’ve been told that, but it’s not really the case if you think about the mechanics. The screen zone itself is simply a large pore space installed into the subsurface. If there is water flow in the surrounding aquifer, water will flow through this space. In fact, the well filter pack and screen is likely to be more permeable than the surrounding formation, so water will tend to flow through the screen a little faster than in the surrounding formation. What may be “stagnant” in the well is the overlying water column located above the screen. In the old days, pumping or bailing required that you get past that stagnant water column. As a result, guidance recommended purging lots of water to make sure you have removed that “stagnant” water prior to sampling with a bailer, for example. The reason that “low-flow” sampling avoids that large volume requirement was in part because low flow pumps were installed in the “live” screen zone, avoiding the overlying water column. Flow rates were low to avoid drawdown—avoiding entrainment of that overlying water column. Passive sampling goes one step further. It relies on the natural flow through and

mixing effects present in the screen zone to produce the flow-weighting effects that we used to force by purging. The simple, sustainable thing to do is rely on the well to do the work for you—and it does! Multiple large scale studies show that passive samples are not statistically different from low flow and traditional purge sampling methods. What does that mean for you? You can avoid what environmental practitioners have suffered for many years: cumbersome purging ahead of simple sample collection. Now you can collect samples right away from pre-deployed passive samplers. The only practical limitation of the technique is sample volume available. This can impact your analyte capability, but in this case your lab can be your friend. Many labs have improved their capabilities to accommodate low volumes. Van Walt can help you with common minimum sample volume requirements and determine if the Snap Sampler is appropriate for your application. Further, Van Walt may be able to assist in finding a lab that can accommodate your need.

LevelSCOUT: Van Walt’s Level Logger of Choice

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Got a story you want to share? email tracey@vanwalt.com to be featured in our next magazine

Sandy Britt, PG, ProHydro, Inc., developer of the Snap Sampler

38 vanwalt.com

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