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GROUNDWATER REMEDIATION - FIRST PRINCIPLES

An On-Line Version of a Column First Published in:
The National Environmental Journal Mar./Apr. 1993 Vol. 3 No. 2 Pages 18-19

by: David B. Vance

Groundwater is a topic that stimulates a wide spectrum of responses from scientists and engineers, economic decision makers, and socially concerned citizens. When the specific issue is the degradation of groundwater resources caused by subsurface contamination the response typically involves much controversy and often little understanding. The technical and social issues associated with groundwater contamination are all pertinent and deserve examination. The primary concern of this column is to improve understanding of the diverse aspects of groundwater issues and provide a coherent analysis of all the facets of those issues.

The start of that analysis demands a thorough understanding of "first principles", without which subsequent points can not be adequately addressed. These principles lie in understanding the physical, chemical and biological processes governing the behavior of groundwater in the subsurface. All the technical and social issues must ultimately focus on these processes. If one technical element were to be termed key, it would be that of mass transport. For example:

  • how do contaminants migrate into the subsurface and through the groundwater table;
  • how do contaminants react once there;
  • what is the contaminant mobility, is there the potential for to create a risk to public health;
  • what is an acceptable level of risk for any given site situation; and
  • given a risk based clean up goal how may the impacted groundwater be remediated; and
  • lastly, what are the required expectations of the stake holders (the public, business and regulatory communities) and what is practical for the restoration of the aquifer?

 These questions are not only scientific or technical; each has a social component, especially the last four.

There are two dominant facts that must be remembered with regards to mass transport in the subsurface:

  • First, the soil and/or rock in the subsurface exhibits physical differences in a spatial sense, it is most typically heterogeneous not homogenous.(1) (2)
  • Second, contaminants interact physicall and chemically with soil and rocks as well as the indiginous bacteria in the geologic matrix.

 The remainder of this column will address the physical impact of heterogeneity, chemical behavior (such as adsorption) and biological processes such as degradation(1)(2)(3) are issues that bears further discussion at a future date.

Soil and rock are the mediums through which groundwater flow occurs. Soil and rock varies in composition and texture in both the horizontal and vertical dimensions of the subsurface. The in-situ physical and chemical behavior of groundwater and impacting contaminants changes in response to these differences in the host medium. The commonly illustrated vision of a homogenous subsurface that uniformly responds to pumping is seriously flawed. That vision has done much to generate misunderstanding in the technical, business and regulatory arenas.

The physical heterogeneity of the subsurface is expressed by many phenomena, of particular concern are zones of varying permeability within the aquifer. Typically a single groundwater recovery well will be screened through the depth of the impacted saturated zone. There also may be multiple wells across the site screened in a similar manner to provide horizontal coverage of the plume. Within the recovery zone of a pumping well there are likely to be horizons or zones in the subsurface with differing permeabilities. Groundwater flow induced by a recovery well is preferentially stimulated in the higher permeability zones. Little to no advective flow of groundwater will occur in the less permeable zones. These are of course simplifying statements, the reality is much more complex. For example:

  • In a water table aquifer, within the cone of depression, there may be less permeable horizons that will have gravity driven groundwater flow vertically through them, as the once saturated soils dewater.
  • The scale or size of the heterogeneity can vary dramatically. Interlayers can vary in thickness from less than an inch to tens of feet. Use of nested wells with screens selectively installed within discrete permeability zones can partially address this problem, if the scale of the interlayering is large enough (on the order of feet.) However, this solution would be at significant additional costs.
  • The degree of heterogeneity, which is reflected by contrast between the adjacent soil units, can range from the extreme of sand seams in glacial till, to moderate as seen in a uniformly sandy horizon with occasional thin layers of poorly sorted sand with interstitial fines or silt.
  • In rare geographically limited cases subsurface conditions are uniquely homogenous.

 The heterogeneity of contaminated aquifers results in a crucial technical and social fact; pump and treat remediation systems can not be designed to yield the desired results based on the displacement of a single pore volume from an aquifer. The mass transport stimulated by groundwater flow largely occurs in the most permeable zones accessed by the screened interval of the recovery well. The contrasting less permeable zones are not exposed to a groundwater flow of equal magnitude.

Therefore, the less permeable zones of the formation are not rapidly purged of the contaminant load by advective transport. Contaminants in these low permeability zones are unfortunately not completely isolated from the advective groundwater system. Although slowly, contaminant transport does occur via a totally different mechanism. This mechanism is diffusion.

Diffusion is not driven by a pressure differential, diffusion is driven by a concentration gradient. Advective flow in the permeable portion of the aquifer replaces groundwater carrying dissolved contamination with relatively clean groundwater. Meanwhile the adjacent, less permeable units, will still contain high levels of dissolved (or adsorbed) contamination. This differential concentration gradient will act as the driving force for the contaminant to diffuse from the low permeability zone into the advective transport zone. Pumping is essential to set up the concentration gradient that drives the diffusional transport system.

The actual mechanism of subsurface remediation via pump and treat methods is two fold:

  • Groundwater is effectively removed, along with its dissolved contaminant load, from the more permeable zones of the subsurface.
  • Once native groundwater, without a significant load of dissolved contaminants, is present in the permeable zones, diffusional transport of contaminants will begin from the static zones to the "clean" groundwater provided by the pumping system.

 An important point is to realize that once the initial permeable pore volume has been swept clean, the recovery system needs to only be pumped at a rate sufficient to maintain the diffusion driving concentration gradient. Given that the radius of hydraulic capture is maintained, pumping at higher rates will not yield additional benefit during the diffusional stage of the remediation.

The crux of the issue with regards to the overall clean-up is the fact that mass transport via diffusion is "orders of magnitude" slower than advective transport. The process of advective and diffusional flow from within the saturated formation significantly extends the time required to accomplish the desired level of contaminant removal. This mechanism (along with adsorption/desorption) accounts for the requilibration reflected by the often observed increase of contaminant concentrations in static groundwater at sites that have had pumping halted because the recovered groundwater had tested clean.

Pump and treat is a valuable tool in the remediation and control of contamination in groundwater systems. It is of particular value for controlling the spread of a contaminant plume. Hydraulic capture of a contaminated groundwater plume is not affected by the mechanisms described above. Pump and treat can and does prevent the spread of dissolved contamination. But, with regards to portions of the groundwater system already contaminated, success is not as easily forthcoming. Consultants, engineers, those in industry, regulators and the public need to understand the processes described above. False expectations of performance lead to additional costs, increased liability and public controversy.

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Copyright 2008 David B. Vance
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