2 The 4 Technology Solutions


An On-Line Version of a Column First Published in the:
National Environmental Journal May/June 1995 Vol. 5 No. 6

By: David B. Vance  dbv7@mindspring.com

The perspective of the regulatory community towards contaminated groundwater is undergoing a dramatic shift. Changing from enforcement driven by arbitrary clean-up standards, to action (or no action) driven by health based risk assessments in conjunction with a cost/benefit analysis. The current debate includes the application of this concept at the highest level for the promulgation of new regulations or modification of existing regulations.

However, the more powerful application of the new regulatory paradigm is on a site specific basis. The purpose of this column is to examine the components of the site specific risk\benefit\cost process. The following discussion is predicated on two basic assumptions.

  • First, if a release of contaminants has occurred at a site, the source of that release will be abated.
  • Secondly, free product hydrocarbons are not allowed to remain on or near the water table.

 These actions quickly limit the potential size of a contaminant plume, minimize further migration of the core of that plume, and recover contaminants as free product versus dissolved phase. All of which offer significant cost savings in the remediation effort.

A health based risk assessment determines what concentration of contaminant represents an acceptable risk level. The cost side of the process is driven by the effort required to achieve that risk determined contaminant concentration. Health based risk assessment can be a complex process. However it has two key technical components. First, is the dose/response relationship of a chemical and secondly, the pathway through which that chemical can ultimately enter a human body. A third non-technical issue is how accepting the public is of the process, particularly those exposed to the risk.

Dose/response relationships are categorized into two broad areas, non-carcinogenic and carcinogenic.

  • Non-carcinogenic responses are generally characterized by a threshold dose. The threshold represents a level of exposure to a contaminant that can be tolerated by a person with no adverse effects until the protective mechanisms of the individual are overwhelmed. Since individual levels of tolerance vary, threshold determination can be controversial.
  • Carcinogenic responses are assumed to have no threshold. This assumption means that there is some finite cancer risk no matter how small the dose.

 The primary routes of exposure to groundwater are ingestion, inhalation of vapors during bathing or showering, and dermal exposure during bathing or showering. Direct ingestion is straight forward, inhalation and dermal adsorption are dependent upon physical properties of the contaminant such as vapor pressure and partition coefficients.

The public perception of risk is contradictory. Significant risk to which an individual will voluntarily expose themselves, such as driving, improper diet, or smoking are of concern, but deemed acceptable. In contrast much smaller risks associated with environmental impairment that is imposed, and not voluntarily engaged, often generate alarm at any level above zero risk. This is an issue that can only be resolved by education to each individuals aggregate risk during their life time from many sources with a view towards the benefits each receives from our industrial society. This should be an important part of the current risk versus cost and benefit debate.

In a practical sense the most effective method of controlling the risk from contaminated groundwater is simply to not use it, which in many situations may be the most viable alternative. However, our groundwater resources are finite and are diminishing in quality with time. One solution that is likely to become more common is surface treatment to remove contaminants. The most cost effective goals are to maintain as much of our groundwater as possible in pristine condition and the timely restoration of contaminated aquifers to acceptable conditions.

Cost effectiveness enters from establishing a health based contaminant concentration that is considered acceptable and expending the minimum effort sufficient to insure that level is maintained. When ever possible natural attenuation should be utilized for the final clean-up of the core of a plume and the clean-up of the dilute distal portions of the plume.

Groundwater recovery, or in-situ treatment should be designed with three goals in mind:

  • Control and treat the core of the plume in which the dissolved concentrations exceed the capacity of the aquifer to naturally attenuate in a timely fashion.
  • If required, recover groundwater at a rate just sufficient to achieve the above goal.
  • Design a system that takes into account the heterogeneity (1)(2) of the aquifer matrix.

 A fine grained matrix will in general take a significant amount of time to remediate because the overall mass transport rates the matrix will support are low and the amount of adsorbed contaminant high. However, groundwater recovery rates or in-situ remediation systems need only be sufficient to meet the low mass transfer requirements.

The worst case exists in aquifers that contain strata of fine grained material mixed with highly permeable continuous layers. Pumping rates must be high to obtain hydraulic control, but treatment durations are extended due to the slow leaching of adsorbed contaminants from fine grained material. In aquifers of this type with pronounced hydraulic gradients and subsequent high groundwater flow rates, it may be necessary to allow much of the contamination to spread through the aquifer such that concentrations will be at levels amenable to natural attenuation. A health based risk assessment will be key in this scenario, which in essence uses an extensive portion of the aquifer matrix as a chemical/biological reactor for contaminant destruction.

For permeable heterogenetic aquifers with small hydraulic gradients and low groundwater flow rates it will be more economical to run a groundwater recovery system or an in-situ treatment system for short durations on a periodic basis. The core of the process is the diffusion of contaminants from fine grained units into the more highly permeable portions of the aquifer. This contaminated pore volume is then treated to restore the contaminant concentrations to low levels providing a concentration gradient that drives the diffusion process. Initial capital costs must be met, but operation and maintenance costs will be significantly decreased over the life of the project.

This new regulatory approach will rationalize the process of environmental restoration. Regulators, industry and the public will benefit. For it to succeed we all must understand and accept reasonable levels of risk. Cost effectiveness will hinge on the understanding and exploitation of site specific aquifer characteristics, and acceptance of clean-up time frames that allow for the use of natural attenuation.

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Copyright 2008 David B. Vance
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If you have comments or suggestions, e-mail me at dbv7@mindspring.com