ENV-07-110 LNAPL Remidiation - When is Enough, Enough?

Description:

Since the mid-1980s, petroleum refineries have been addressing the remediation of light nonaqueous phase liquids (i.e., LNAPLs: petroleum-based stocks, products, and wastes) which have found their way to underlying groundwater. As many of these remediation systems “mature” in their removal of LNAPLs the question is increasingly being asked “what is clean and when do we reach an end-point?”. With increasing scientific advances in the understanding of how LNAPLs move and interact in shallow aquifers coupled with the growing regulatory acceptance of these processes, this question has evolved into “how much LNAPL can I safely leave behind without it being a significant threat to human health and the environment?” The purpose of this paper is to explore when is enough, enough….in other words, when do I start the process of determining that my remediation system has reached an end-point and start the laborious process of collecting data and proving to the regulatory authorities that I have reached that end-point? So when is clean “clean”? Traditionally the answer has been the nebulous “when the LNAPL is removed to the extent practicable”. Practicality is, however, an undefined term. The true answer is “when the LNAPL plume is no longer a threat”. This end point or lack of mobility for both the phase-separated hydrocarbon (PSH) and the dissolved-phase hydrocarbon (DPH) components of the LNAPL plume can be theoretically demonstrated through modeling and, more importantly, shown to occur by utilizing the actual field data collected during both the recovery effort and a brief non-recovery evaluation period. Effective evaluation of the plume(s) under historical stressed (removal) conditions and natural (static) conditions, the changes of the PSH plume and the reaction at the fringes of the DPH component plume. In this paper we will look at examples illustrating after years of effective utilization of PSH and DPH removal methods and the reduction of the mobile portion of the LNAPL plume footprint how the decision is reached that the plume has reached stasis.