ABSTRACT: And Now the Bad News...That Hydrocarbon Thickness You Just Measured in the Monitoring Well is Probably Wrong!

J. P. Hughes, C. R. Sullivan, R. E. Zinner

The hydrocarbon thickness measured in a monitoring well often exceeds--sometimes by a large amount--the true thickness of the mobile hydrocarbon in the formation. In addition, a change in the monitoring well thickness may actually reflect changes in the local water table elevation (due to tidal influences or wet season/dry season effects) rather than true changes in the amount of hydrocarbon in the aquifer.

Errors of this nature can result in a series of problems when the monitoring well data is used to make decisions, such as where to locate recovery wells how great the volume of hydrocarbons beneath the project site. Recovery systems can be very expensive and therefore must be located efficiently. Recently, regulatory agencies have required responsible parties to show evidence of a substantial reduction in hydrocarbon volume by a certain date. It is easy to understand the difficulty you would have meeting this requirement if you were not even sure how much hydrocarbon you had to begin with.

The search for an accurate thickness estimate therefore becomes more than simply an academic exercise. The estimate can also have a direct impact on the overall cost of the project and on one's ability to meet the cleanup requirements. This presentation will provide a working knowledge of the theory behind exaggerated thickness measurements as well as some practical examples of the ways that using these measurements can create problems during assessment and remediation activities at a contaminated site.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990