Hydraulic and earth engineering – Soil remediation – In situ contaminant removal or stabilization
Patent
1994-10-20
1996-01-30
Novosad, Stephen J.
Hydraulic and earth engineering
Soil remediation
In situ contaminant removal or stabilization
405267, 405278, 588249, C02F 100, E02D 1918, E02D 506
Patent
active
054876222
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a system for treating contaminated groundwater.
BACKGROUND TO THE INVENTION
One of the difficulties in conventional systems for treating contaminated groundwater arises from the fact that taking water out of the ground for treatment is expensive. From the cost point of view, it is highly desirable that the groundwater be treated in-situ, ie while remaining in the ground.
It has been proposed to place treatment material in the ground, the material being inserted into a trench which has been excavated in the path of a plume of contaminant. The nature of the treatment material of course depends on the nature of the contaminant. An example is described in WO-91/08176, published 13 Jun. 91, in which the contaminant is an halogenated hydrocarbon, and the treatment material is iron. The iron is in the form of iron filings, which are placed in a trench excavated down into the material of the aquifer in the path of the velocity of the halogenated hydrocarbon contaminant plume.
When groundwater water is treated in-stiu or in-ground, many of the expenses of above-ground treatment, such as the expense of disposing of the treated water, can be avoided. Inexpensive though in-ground treatement usually is, however, still, in practice, it can be difficult and expensive actually to provide a trench of the required depth and extent in the material of the aquifer.
Aquifer materials often include large proportions of sand and gravel. One of the difficulties in providing a trench lies in the fact that such materials have little structural cohesion. In fact, sand/gravel material is especially unable to support itself when saturated with groundwater, ie below the water table, which is where an in-situ treatment system is particularly required to operate. It may be regarded that saturated sand/gravel aquifer material, from the standpoint of excavating a trench, has hardly any more ability to support itself than if it were a liquid.
One conventional manner for providing trenches in an aquifer material is to insert a pair of barriers, made of sheet metal, and inserted by pile-driving, down into the material of the aquifer. The two barriers are disposed in a spaced, parallel relationship. After the barriers are in place, the aquifer material is then extracted from between the barriers. Treatment material (eg iron filings) is inserted between the barriers, and replaces the sand/gravel that has been extracted. The two barriers are removed once the treatment material is in place.
The treatment material in the trench thus acts as a curtain through which the contaminated groundwater passes. The contaminant undergoes treatment--preferably chemical or microbiological breakdown--in passing through the treatment material. As such, of course the treatment material must be permeable.
The velocity with which groundwater travels in an aquifer of course varies a great deal from place to place, and from time to time, but velocities of the order of a few cm, or a few tens of cm, per day are typical. If the trench is say 1 meter wide, the groundwater has a residence time in the trench of perhaps a few hours or a few days. Generally, this is ample time for the chemical breakdown of the contaminant to occur, and for the breakdown reactions to be fully completed.
However, many treatment materials are themselves expensive. It can be desirable therefore to mix the treatment material with a less expensive material, and preferably a material which is chemically inert or passive with respect to the contaminant breakdown reaction. One material that might be used, for instance, is the sand/gravel that has been taken out of the trench.
It should be noted that it is difficult to make a trench of less than about a meter in width; on the other hand, however, even if it were possible practically to construct narrower trenches, a wide trench is preferred in order that the residence time be kept high.
In summary, one of the conventional ways of preparing a trench is this: pile-drive two parallel barriers; extract the material from between
REFERENCES:
patent: 4877357 (1989-10-01), Verstraeten
patent: 5054961 (1991-10-01), Sherman
patent: 5057227 (1991-10-01), Cohen
patent: 5106233 (1992-04-01), Breaux
patent: 5132021 (1992-07-01), Alexander
patent: 5240348 (1993-08-01), Breaux
patent: 5354149 (1994-10-01), Breaux
Cherry John A.
Gillham Robert W.
Vales Enoch S.
Novosad Stephen J.
University of Waterloo
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