ENV-05-200 Are LNAPL Well Thickness Measurements Reliable?

Description:

Measuring the thickness of light non-aqueous phase liquids (LNAPL) in a monitoring well is a common practice at refinery sites. This measurement is commonly the primary information that is used to assess volume, mobility, and risk, which are critical parameters in evaluating the need for and approach to remediation. Although the measurements are made regularly, very rarely are the values evaluated for reliability, much less an understanding of the conditions within the surrounding formation that produced the product thickness in the well. As a result, large LNAPL thicknesses, which are an inaccurate representation of reality, are commonly reported and are perceived as an accurate portrayal of the LNAPL volume in the subsurface. These large thicknesses typically promote additional unnecessary characterization and costly remedial actions. Unreliable LNAPL thickness measurements may result from various natural conditions. Three hydrogeologic settings commonly produce non-representative LNAPL measurements, (1) confined conditions, (2) perched conditions, and (3) preferential flow conditions. These conditions can be determined by a comprehensive site characterization program that may incorporate the use of CPT - ROST technologies and soil cores. This paper documents these natural settings and discusses approaches to evaluate the reliability of LNAPL measurements taken within these settings.