Often times a Recognized Environmental Condition (REC) is identified at a property during a Phase I site assessment.  A REC is defined as the presence, or likely presence, of conditions that indicate an existing release, a past release or a material threat of a release of a hazardous substance or petroleum product to structures or the environment. The term does not include de minimis conditions that generally do not present a material risk of harm to public health or the environment. Following this definition, identification of a REC is often followed by environmental sampling of soil or water often referred to as a Phase II investigation.

Standard practice is to use a direct push technology (DPT) to collect subsurface soil and groundwater samples. This sampling equipment is often referred to as a Geoprobe® since it was the company that initially perfected this technology. Geoprobe® is a relatively inexpensive technology and is effective in collecting subsurface samples with minimal disturbance to the surface materials. Groundwater samples can be collected from the DPT boring after the boring is advanced to beyond the depth of groundwater. After the desired depth is reached, a short screen, typically less than two feet in length, is exposed to the groundwater and the groundwater sample can then be collected using a pump or bailer.

This groundwater sampling technique is an inexpensive way of collecting groundwater samples without installing a groundwater monitoring well. Therefore, this technique is often used to screen RECs for the absence or presence of contamination and is a very good screening tool for this purpose. This method of sampling is referred to as “discrete sampling” because a groundwater sample is being collected from a very specific depth interval within the saturated zone.

Many factors influence the fate and transport of contaminants in groundwater. The two most important factors to consider are the geologic setting and the particular contaminant being evaluated. Although the potential contaminant can often be recognized during the assessment process, the site-specific geologic setting is often not well understood during a groundwater screening investigation. Contaminants will generally flow with groundwater and groundwater will follow the path of least resistance in the subsurface. Small changes in the geology can create pathways in a small area near a release that will impact local groundwater flow and contaminant migration. For this reason, discrete sampling may or may not be positioned to obtain a sample that yields results considered representative of groundwater conditions and the results can be misleading.  

We are often faced with the question of what to do in response to impacts identified during this type of discrete groundwater sampling. The answer is dependent upon the objectives of the investigation. However, we often recommend that the conditions be further evaluated utilizing properly constructed groundwater monitoring wells prior to reacting to discrete groundwater sampling data. Regulatory agencies will typically accept discrete groundwater data collected using DPT methods. However, they will typically require installation of properly constructed groundwater monitoring wells prior to granting closure in a situation where contaminants are identified using DPT. This is because groundwater monitoring wells are considered the best method of obtaining samples considered representative of the overall groundwater conditions in the area of a release.

You should be aware of the many advantages and limitations related to discrete groundwater sampling using DPT while evaluating data obtained using these methods. Discrete groundwater sampling using DPT is a very good screening tool but should be considered just that.