Groundwater sampling at environmental cleanup sites has been going on for decades, and sampling methods have continued to evolve and improve. The simplest of these methods sends a glorified bucket (bailer) down a hole to pull up a water sample. For many sites this method has given way to low-flow stability sampling, which is often considered the gold standard for sampling today. In low-flow sampling, one of a variety of pumps is used to slowly remove groundwater from a well at a rate equal to or less than the rate at which the aquifer can replenish it. The measurement of several physical parameters for the withdrawn water during pumping indicates when the water parameters are stable (hence the name) and representative of the aquifer. Groundwater is then sampled. Alternative sampling methods include passive diffusion bags (PDBs) that equilibrate with the aquifer water through osmosis, a method for which IDEM recently issued guidance (184.108.40.206/~wilcoxe1/blog/2015/06/19/customized-vapor-intrusion-pdb/reduce-costs-for-groundwater-monitoring-new-idem-guidance/). While all of these methods have their place in the sampling world, none of them were designed with vapor intrusion (VI) in mind.
The realm of VI is growing at break neck speeds, and what used to be uncommon sampling is quickly becoming routine. The EPA, which just recently updated their VI guidance (http://www.epa.gov/oswer/vaporintrusion/), utilizes groundwater concentrations of volatile contaminants to determine if and where vapor intrusion investigations should occur. But is the same sampling method used to characterize groundwater the bestmethod for VI? Not always.
As contaminated groundwater moves from a source, it disperses and mixes with uncontaminated water in the aquifer. The uppermost part of the aquifer becomes a source of vapor that can intrude into overlying structures. In many environments, however, water infiltrating into the ground mixes with and dilutes contaminated groundwater in the upper part of the aquifer, creating a zone of relatively unimpacted water that prevents vapor migration from the underlying contaminated water. Most groundwater sampling methods are not designed to characterize this zone of diluted contamination.
The Wilcox Vapor Intrusion Practice Group has developed an alternative, innovative sampling method designed to specifically target the groundwater interface and limit the extent of vapor intrusion investigations. By deploying specially designed PDBs, Wilcox is able to reliably and cost effectively characterize groundwater impacts in this VI critical zone. Field testing of the method identified interface concentrations four to six times lower than those resulting from low-flow sampling.
Engaging third parties about potential impacts to the air they breathe is a sensitive matter, and the decision to knock on that door should be made using the best data possible. Contact Wilcox Environmental Engineering today to find out how better data can reduce your project costs and liability exposure.