WO2000035608A1

WO2000035608A1 - Ground water decontamination using lower-side thermal barrier

Info

Publication number: WO2000035608A1
Application number: PCT/US1999/029895
Authority: WO
Inventors: Craig L. Eaker; George M. Becker; Randall S. Weidner; Terry C. Sciarrotta
Original assignee: Edison International
Priority date: 1998-12-17
Filing date: 1999-12-15
Publication date: 2000-06-22
Also published as: AU2844700A

Abstract

The invention is a method for removing heavy organic contaminants trapped within a confining layer (24). A typical underground water system (10) which includes top soil (12) above a water table (14). The top soil includes non-saturated, non-confining layers (16) and non-saturated, confining layers (18). An uppermost saturated non-confining layer (20) and an uppermost saturated confining layer (22) are disposed below the water table. A lower-most confining layer (24) is bound from above by an upper non-confining layer (26) and from below by a lower non-confining layer (28). Heavy organic contaminants which are introduced to the soil at a point (30), fan outward to form a plume (32) which gravitates downwardly towards the water table (14). The lower portion of the contaminated layer (33) is heated from below. Steam injection wells (34) provide heat to the non-confining layer below the contaminated layer. Extraction wells (36) evacuate the heated contaminant. By heating the lower portion (33), of the contaminated layer (24) from below, a temperature barrier (40) prevents cold ground water from upswelling and quenching the heated contaminant. The lower portion of the injection and extraction wells have perforations (42) to allow the flow of steam and/or steam/contaminant mixture.

Description

Form PCT/ISA/210 second sheet Jul 1998 * GROUND WATER DECONTAMINATION

USING LOWER-SIDE THERMAL BARRIER

FIELD OF THE INVENTION

This invention relates generally to methods for removing organic contaminants from underground water formations and, specifically, to methods for removing organic contaminants trapped within an underground confining layer.

BACKGROUND OF THE INVENTION

The urgency of removing organic contaminants from underground water formations has become increasingly more apparent during the last three decades. This urgency has spawned a tremendous effort to improve ground water decontamination techniques.

Despite this effort, one particular family of organic contaminants remains extremely difficult to remove from underground water formations. This family includes heavy, organic chemicals with specific gravities greater than 1.0. These large molecule contaminants are commonly termed "dense non-aqueous phase liquids," and are most frequently referred to by the acronym "DNAPLs." Such heavy organic contaminants are frequently trapped within low permeability confining layers within the underground water formations, and stubbornly resist all known methods to efficiently remove them from these confining layers.

The mechanism by which these heavy organic contaminants are trapped within the confining layers is not fully understood, but it is believed to rely heavily upon "sorption" mechanisms whereby the heavy organic materials are sorbed into or onto particles and voids within the confining layer. Whatever the mechanism, the heavy organic contaminants are almost wholly immobilized within the confining layer and cannot be flushed out by any known physical means. The only serious attempts to remove heavy organic contaminants in situ involve the application of heat to the confining layer. As explained below, prior art attempts regarding large scale in situ decontamination using heat have been largely unsuccessful.

In addition to the difficulties caused by the immobilization of the heavy organic contaminants within the confining layer, decontamination of the confining layer is complicated by the fact that the heavy organic contaminants are heavier than water. Accordingly, when and if the heavy organic contaminants are somehow freed from the confining layer, they tend to sink lower into the underground water formation, rather than rising to the top of the formation (as do all other organic contaminants). Because of the difficulty of removing heavy organic contaminants from a confining layer, and because of the danger of the heavy organic contaminants gravitating deeper into the underground water formation, confining layer concentrations of as little as a few parts per million are often considered significant.

Most known previous large scale attempts at in situ removal of heavy organic contaminants from confining layer have attempted to mobilize the heavy organic contaminants by conductive heating using a lateral sweeping of steam from above the confining layer or from within the confining layer by resistive heating. It has been the conventional wisdom within the industry that confining layers containing heavy organic contaminants must never be heated at the lower boundary of the confining layer. The reason for this has been the belief that such heating, if it succeeds in mobilizing the heavy organic contaminants, will permit the heavy organic contaminants to gravitate out of the confining layer and sink deeper and deeper into non-confining layers and non-contaminated confining layers located below the confining layer.

Unfortunately, all efforts to remove heavy organic contaminants by such "conductive" heating of the confining layer have proved unsuccessful. No matter how much heat is provided to the confining layer from the top or from within the confining layer, the heavy organic contaminants and other heavy organic chemicals tend to remain trapped within the confining layer.

Thus, the only "successful" remediation of such confining layers has occurred through non-in situ remediation methods involving the actual physical removal of the watery solids within the confining layer to the surface (where solids and liquids can be decontaminated by above- ground treatment methods.) Thereafter, the solids and liquids must generally be physically restored to the underground formation. These methods are understandably very time-consuming and exceedingly expensive.

Accordingly, there is a need for a much more efficient method of removing heavy organic contaminants from confining layers within underground water formations.

SUMMARY

The invention satisfies this need. The invention is a method for removing organic contaminants trapped within a confining layer comprising the steps of (a) continuously maintaining the temperature at the lower boundary of the confining layer at a temperature sufficient to mobilize the organic contaminants within the confining layer while preventing their downward travel, and (b) removing the mobile organic contaminants from the confining layer and from above the confining layer.

The temperature at the lower boundary of the confining layer is controlled to "slow cook" the confining layer from below. The heat provided at the lower boundary of the confining layer is preferably just enough to upwardly mobilize contaminates within the confining layer. In this way, the heating at the lower boundary of the confining layer effectively forms a "thermal barrier" which prevents the downward migration of heavy organic contaminants.

The "slow cooking" of the confining layer has been found to chemically and physically alter the heavy organic compounds, thereby accelerating their removal. The altered compounds have a much less propensity for being "trapped" within the confining layer and so they are easily mobilized by upward flows of vapors and liquids. Moreover, such compounds tend to have a specific gravity less than that of liquid water, so that they are buoyed upwardly through the confining layer.

Once mobilized, the organic contaminants can be easily withdrawn from the confining layer, such as via a perforated conduit disposed in the confining layer or in the non-confining layer immediately above the confining layer. In the first commercial test of the method, it is anticipated that the method will substantially reduce the costs of heavy organic material decontamination (as compared to cost projections based upon conventional techniques) of greater than 50% . Moreover, the estimated time to complete the decontamination is estimated to be reduced by at least 90%.

DRAWINGS

These features, aspects and advantages of the present invention will become better understood with regard to the following description, appended claims and accompanying figures where:

Figure 1 is a cross-sectional side rendition of an underground water formation showing features of the invention.

DETAILED DESCRIPTION

The following discussion describes in detail one embodiment of the invention and several variations of that embodiment. This discussion should not be construed, however, as limiting the invention to those particular embodiments. Practitioners skilled in the art will recognize numerous other embodiments as well.

The invention is a method for removing organic contaminants trapped within a lowermost contaminated confining layer comprising the steps of (a) continuously maintaining the temperature at the lower boundary of the contaminated confining layer at a temperature sufficient to mobilize the organic contaminants within the confining layer, and (b) removing the mobilized organic contaminants from the confining layer and from above the confining layer. The term "trapped" as used herein is meant to connote the fact that the organic contaminants are strongly retained within the contaminated confining layer and will stubbornly resist attempts to mobilize them by physical flushing or even by heat applied within the contaminated confining layer. By the term "confining layer, " it is meant a layer having relatively low hydraulic conductivity so that the layer significantly restricts the movement of ground water either into or out of adjacent layers. By the term "lowermost contaminated confining layer," it is meant a confining layer having trapped therein organic contaminants in a concentration of at least 100 ppm, and typically greater than 500 ppm, in an underground water formation wherein there is no additional contaminated confining layers within about 500 feet below the contaminated confining layer wherein organic contaminants having a concentration greater than about 100 ppm is trapped. Where the units "ppm" or "ppb" is used herein to describe the concentration of organic contaminants within an underground water formation layer, it is meant to connote "weight parts per million (or per billion) per weight parts of dry soil."

The method is applicable to an underground water system such as illustrated in Figure 1. Figure 1 is a cross-sectional diagrammatic representation of a typical underground water system 10 which includes top soil 12 above a water table 14. The top soil 12 includes non-saturated non-confining layers 16 and non-saturated confining layers 18. Below the water table 14 is an uppermost saturated non-confining layer 20 and an uppermost saturated confining layer 22. Below the uppermost saturated confining layer 22 is a lower-most contaminated confining layer 24 bound from above by an upper non-confining layer 26 and from below by a lower non-confining layer 28.

Figure 1 illustrates the consequences of contaminating the top soil 12 by the addition to the top soil of heavy organic contaminants at an input point 30. After initial contamination of the top soil 12, the heavy organic contaminants fan out to form a plume 32 which gravitates downwardly towards the water table 14. After reaching the water table 14, the heaviest of the organic contaminants — the organic contaminants having a specific gravity greater than that of water — continue to gravitate downwardly through the formation 10. At each confining layer, the heavy organic contaminants become temporarily trapped. Eventually, however, the organic contaminants 8

gravitate down to the lower-most contaminated confining layer 24. In Figure 1, these contaminants are shown concentrated in a contaminated portion 33 of the lower-most contaminated confining layer 24.

All known prior art methods of large scale in situ remediation of the contaminated confining layer 24, such as illustrated in Figure 1, have relied on the introduction of steam to the contaminated confining layer 24 via peripheral steam injection wells 34 or on the resistive heating of the interior of the contaminated confining layer 24. The intent of such prior art methods is to heat the organic contaminants within the contaminated confining layer 24 in an attempt to mobilize them and/or to flush them towards an extraction well 36. Unfortunately, such prior art methods have been unsuccessful. One theory for this (postulated by the inventors) is that the extracting of water from and above the confining layer 24 via the extraction well 36 induces the upward flow of cold waters 38 from the lower non-confining layer 28 into the contaminated confining layer 24. Such upward flow of cold water 38 effectively quenches any attempts to conductively heat the contaminated confining layer 20 to temperatures sufficiently high to mobilize the contaminants.

In the invention, sufficient heat is applied along the entirety of the contaminated confining layer 24 below the contaminated portion 33 to compensate for any upward flow of cold waters 38 from the lower non-confining layer 28. The heating of the contaminated confining layer 24 from below runs contrary to all of the previous teachings regarding the decontamination of heavy organic materials from lower-most contaminated confining layers. Those teachings hold that one should never heat the contaminated portion of a lower-most contaminated confining layer from below because, once the contaminants are mobilized, they will tend to gravitate downwardly out of the contaminated confining layer and into the layers below. In the invention, however, the downward gravitation of mobilized contaminants is eliminated by maintaining substantially the entirety of the underside of the contaminated portion 33 of the contaminated confining layer 24 at a temperature sufficient to form a "thermal barrier" 40 below the contaminated confining layer 24 which effectively prevents any downward flow of materials out of the contaminated confining layer 24.

To form the "thermal barrier" 40, the temperature at the lower boundary of the contaminated confining layer 24 is typically maintained between about 60°C and the boiling point of the water within the contaminated confining layer 24. Temperatures in this range are generally sufficient to mobilize a portion of the liquids within the contaminated confining layer 24. In the most typical operation utilizing the method of the invention, the lower boundary of the contaminated confining layer 24 is continuously maintained at a temperature between about 90°C and about 100°C.

The temperature need only be high enough to assure the upward mobilization of all organic contaminants at the lower boundary of the contaminated confining layer 24. Thus, the temperature need be no higher than the boiling point of water within the contaminated confining layer 24. The success of the invention appears to be as a result of the "slow cooking" of the contaminated confining layer 24 so as to physically and chemically change the contaminants within the contaminated confining layer 24 and induce their upward mobilization. Introducing excess heat at the lower boundary of the contaminated confining layer 24 is, therefore, generally wasteful.

Preferably, substantially the entirety of the lower boundary of the contaminated portion 33 of the contaminated confining layer 24 is continuously maintained at the temperature sufficient to mobilize the organic contaminants within the contaminated confining layer 24.

Maintaining the temperature at the lower boundary of the contaminated confining layer 24 requires the input of heat to the lower boundary of the contaminated confining layer 24. 10

This can be accomplished by electrical (resistive) heating or other exotic heating techniques. In almost all cases, however, it will be found to be most convenient and efficient to provide heat to the lower boundary of the contaminated confining layer 24 by injecting steam proximate to the lower boundary of the contaminated confining layer 24. Steam can be provided by one or more injection wells 42 which terminate proximate to the lower boundary of the contaminated confining layer 24. Generally, only enough steam is used to maintain the temperature at the lower boundary of the contaminated portion 33 of the contaminated confining layer 24. Excessive flows of steam, for example for the purpose of attempting to physically move contaminants with pressure within the contaminated confining layer 24, are ineffective and wasteful.

As noted above, in order to assure that heavy organic contaminants do not gravitate out of the contaminated confining layer 24 into non-contaminated layers below the contaminated confining layer 24, it is important that substantially the entirety of the lower boundary of the contaminated confining layer 24 be maintained at a temperature sufficient to assure upward mobilization of such contaminants. Thus, it is important in the invention to maintain effective temperature monitoring across the lower boundary of the contaminated portion of the contaminated confining layer 24. This can effectively be accomplished by disposing a plurality of spaced-apart thermal monitors across the lower boundary of the contaminated portion 33 of the contaminated confining layer 24. This may also be accomplished by utilizing electrical resistance tomography, such as taught in U.S. Patent No. 5,346,307, the entirety of which is incorporated herein by this reference.

As noted above, the complex mechanisms which work to trap the organic contaminants within the contaminated confining layer 24 are not fully understood. Accordingly, precisely how the invention works to "mobilize" these organic contaminants is not fully understood. 11

It is believed, however, that such mobilization is the result of physical and chemical changes to the organic contaminants caused by the upward rise of heated fluids from the lower boundary of the contaminated confining layer 24. After the heavy organic contaminants are "changed" by the process of the invention, the lighter organic materials are more easily mobilized within the contaminated confining layer 24 because they have a lesser propensity to be trapped by the low permeability materials within the contaminated confining layer 24 and because they generally have a specific gravity less than that of water (so that they are buoyed upward within the water within the contaminated confining layer 24). Thus, the concept of "mobilizing" the organic contaminants within the contaminated confining layer 24 is meant to be broadly construed to include the mobilization, not only of the original (heavy) organic contaminants, but also the lighter byproducts of those original contaminants.

The mobilized organic contaminants can be removed from the contaminated confining layer 24 via one or more extraction conduits 36 disposed within or slightly above the contaminated confining layer 24. Typically, the lower end of such extraction conduits 36 are perforated to allow the flow of fluids into the conduit 36. Extraction of materials from the contaminated confining layer 24 is generally facilitated by drawing a vacuum on the extraction conduit 36 such that the conduit 36 is maintained at a pressure less than the ambient pressure within the contaminated confining layer 24. Typically, the extraction conduit 36 is maintained at a pressure between 3 psia and about 14.7 psia.

The materials withdrawn from the contaminated confining layer 24 via such extraction conduits 36 generally include large quantities of water, both in the liquid and vapor phases. Typically, all liquids and vapor condensate are treated in above-ground treatment facilities after extraction via the extraction conduits 36. As noted above, it is considered wasteful to inject unnecessary quantities of steam at the lower boundary of the contaminated confining layer 24.

Claims

12Accordingly, the amount of water withdrawn from the contaminated confining layer 24 in the vaporous phase is carefully monitored to assure that excess quantities of steam are not being injected to the contaminated confining layer 24. Typically, the amount of vapor phase water extracted via the extraction conduits 36 is less than about 30 vol. % of the total water withdrawn via the extraction conduits 36.The method has been found to effectively remove heavy organic materials trapped within contaminated confining layers. The invention is effective in contaminated confining layers having concentrations of heavy organic contaminants of as little as a few parts per million, to greater man several percent.Having thus described the invention, it should be apparent that numerous structural modifications and adaptations may be resorted to without departing from the scope and fair meaning of the instant invention as set forth hereinabove and as described hereinbelow by the claims. 13What is Claimed is:

1. A method for removing organic contaminants trapped within a contaminated confining layer within an underground water formation, the contaminated confining layer being water saturated and being bounded from above by an upper non-confining layer and from below by a lower non-confining layer, the contaminated confining layer having trapped therewithin organic contaminants in a concentration of at least 100 ppm, the underground water formation having no additional confining layers within about 500 feet below the contaminated confining layer which has trapped therewithin organic contaminants in a concentration greater than about 100 ppm, the method comprising the steps of:

(a) continuously maintaining the temperature of the lower boundary of the contaminated confining layer at a temperature sufficient to mobilize the organic contaminants therewithin and to prevent the downward migration of mobilized organic contaminants across the lower boundary of the contaminated confining layer; and (b) removing the mobilized organic contaminants from the underground water formation.

2. The method of claim 1 wherein the temperature of the lower boundary of the contaminated confining layer is continuously maintained at a temperature between about 60 °C and the boiling point of water within the contaminated confining layer.

3. The method of claim 1 wherein the temperature of the lower boundary of the contaminated confining layer is continuously maintained at a temperature between about 90 °C and about 100 °C. 14

4. The method of claim 1 wherein the organic contaminants are disposed within a contaminated portion of the contaminated confining layer and wherein substantially the entirety of the lower boundary of the contaminated portion of the contaminated confining layer is continuously maintained at a temperature sufficient to mobilize the organic contaminants within the contaminated confining layer and to prevent the downward migration of mobile organic contaminants across the lower boundary of the contaminated confining layer.

5. The method of claim 1 wherein the lower boundary of the contaminated confining layer is continuously maintained at a temperature sufficient to vaporize a portion of the liquids within the contaminated confining layer.

6. The method of claim 1 wherein the lower boundary of the contaminated confining layer is continuously maintained at a temperature sufficient to mobilize the organic contaminants within the contaminated confining layer by injection of steam to the lower boundary of the contaminated confining layer.

7. The method of claim 1 wherein materials including the mobile organic contaminants are withdrawn from the contaminated confining layer via a conduit disposed above the lower boundary of die contaminated confining layer.

8. The method of claim 7 wherein the conduit is maintained at a pressure less than the ambient pressure within the contaminated confining layer.

9. The method of claim 7 wherein the conduit is maintained at a pressure between about 14.7 psia and about 3 psia. 15

10. The method of claim 7 wherein the materials withdrawn from the contaminated confining layer include liquid and vaporous water, and wherein less than about 30 vol. % of the water within the materials removed from the contaminated confining layer is in the vapor phase.

11. The method of claim 1 wherein the organic contaminants trapped within the contaminated confining layer comprise at least about 5 ppb heavy organic contaminants.

12. The method of claim 1 wherein the organic contaminants trapped within the contaminated confining layer comprise at least about 100 ppm heavy organic contaminants.

13. The method of claim 1 wherein the organic contaminants trapped within the contaminated confining layer comprise at least about 1000 ppm heavy organic contaminants.

14. A method for removing heavy organic contaminants trapped within a contaminated confining layer within an underground water formation, the contaminated confining layer being water samrated and being bounded from above by an upper non-confining layer and from below by a lower non-confining layer, the contaminated confining layer having trapped therewithin heavy organic contaminants in a concentration of at least 500 ppm, the underground water formation having no additional confining layers within about 500 feet below the contaminated confining layer which has trapped therewithin heavy organic contaminants in a concentration greater than about 100 ppm, the method comprising the steps of:

(a) injecting steam to the lower boundary of the contaminated confining layer so as to maintain the temperature at the lower boundary of the contaminated confining layer sufficient to vaporize a portion of the liquids within the contaminated confining layer and thereby 16

convert a significant portion of the heavy organic contaminants within the contaminated confining layer to contaminant organic materials having a specific gravity less than that of water; and

(b) removing the contaminant materials from the contaminated formation via a perforated conduit which is maintained at a pressure less than the ambient pressure within the contaminated confining layer.

15. The method of claim 14 wherein the temperature at the lower boundary of the contaminated confining layer is maintained sufficient to prevent the downward migration of mobile heavy organic contaminants across the lower boundary of the contaminated confining layer.

16. The method of claim 14 wherein the heavy organic contaminants are disposed within a contaminated portion of the contaminated confining layer and wherein substantially the entirety of the lower boundary of the contaminated portion of the contaminated confining layer is continuously maintained at a temperatore sufficient to mobilize the heavy organic contaminants within the contaminated confining layer and to prevent the downward migration of mobile heavy organic contaminants across the lower boundary of the contaminated confining layer.

17. The method of claim 14 wherein the temperature of the lower boundary of the contaminated confining layer is continuously maintained at a temperature between about 90 °C and about 100°C.

18. The method of claim 14 wherein the lower boundary of the contaminated confining layer is continuously maintained at a temperature sufficient to vaporize a portion of the liquids within the contaminated confining layer.