WO2007125186A1

WO2007125186A1 - Method of decontaminating a water table and device for implementing it

Info

Publication number: WO2007125186A1
Application number: PCT/FR2007/000643
Authority: WO
Inventors: Jean-Claude Setier; Jean-Christophe Renat; Jean-Louis Pornain; Frédéric Perie; Denis Millette
Original assignee: Total S.A.; Traitement Valorisation Decontamination (Tvd)
Priority date: 2006-04-19
Filing date: 2007-04-17
Publication date: 2007-11-08
Also published as: FR2900146B1; FR2900146A1

Abstract

The invention relates mainly to a method of decontaminating a water table having an outflow and containing contaminants, said method comprising the injection of oxygen into a permeable underground filtering body placed in said water table in order to degrade the contaminants within the filtering body. The invention also relates to a device suitable for decontaminating a water table.

Description

METHOD OF DEPOLLUTING A PHERATIC TABLET AND DEVICE FOR IMPLEMENTING THE SAME

FIELD OF THE INVENTION

The present invention relates to a process for the depollution of a saturated zone or a water table having a flow and a device for the implementation thereof, in particular for degrading dissolved hydrocarbons.

TECHNICAL BACKGROUND

The decontamination or decontamination of groundwater can be carried out by the implementation of very diverse techniques, generally complex. Most of these techniques involve extracting groundwater, treating it on the surface and returning it to the soil. Other techniques offer a treatment of pollutants in situ. In all cases, the treatment may be physico-chemical or biological (biodegradation).

For example, US 5037240 discloses an in situ treatment based on the injection into the ground of chemical reagents or bacteria, nutrients and air through tubular elements, to allow local degradation of various contaminants.

US 5221159 discloses a combination of several depollution methods, namely, inducing an injection / extraction airflow into the soil to cause volatile contaminants; the treatment of these at the surface by microorganisms in culture; the removal of soil microorganisms followed by their cultivation and reinjection into the soil for biodegradation; direct extraction of contaminants.

EP 0502460 discloses a method of treating groundwater in situ, comprising placing a receptacle of active material in the soil for treating contaminants such as chromium oxide, uranium oxide or ferrous sulphate. The process is more particularly carried out under anaerobic conditions.

Document US 5362400 describes another in situ treatment method, in which a circulation of water is artificially created in the water table by pumping and in which oxygen is supplied to the medium in order to create an oxidation / precipitation zone or reduction. The document relates more particularly to the treatment of iron, manganese, aluminum, fluorine.

Document US Pat. No. 5,525,008 describes a process for the depollution of hydrocarbons, in which an oxidizing reactive solution is injected into the soil. This reactive solution may in particular be based on hydrogen peroxide.

US 5605634 discloses a system for the diffusion into the soil of treatment substances circulating in buried tubes. For example, ^"it is intended to inject toluene as nutrient for bacteria to promote the degradation of trichlorethylene by them.

US 5623992 discloses a method of biodegradation of petroleum by microorganisms in the soil. Biodegradation is stimulated by the injection of soluble nitrogen and oxygen-containing gases into aeration columns.

US 6102621 discloses a method for the degradation and immobilization of contaminants in soil and groundwater, based on the injection into the soil of a mixture comprising a chemical oxidant KMnO ₄ type, a transport fluid and a hydrophilic material of the cement or clay type, in order to promote an oxidative chemical reaction in situ.

In US 6268205 is proposed the injection into the soil of materials ensuring a controlled release of hydrogen peroxide (which ensures the oxidation of organic contaminants) as well as a controlled release of oxygen in the longer term (which stimulates biodegradation phenomena).

In US 6274048 is described the treatment of dense contaminants trichlorethylene type by the injection into an aquifer of a solution of potassium permanganate.

Document US 2003/0207440 describes a groundwater decontamination system in which a gas containing O ₂ is injected into the soil. This gas promotes the formation of volatile contaminants that are recovered via an extraction well and treated by surface installations.

US 2005/0077242 discloses the use of a controlled release coated oxidation agent, in situ or ex situ.

In addition, Solinst sells a system for injecting various substances into the soil by diffusion, called Waterloo Emitter ™. Thus, it is possible to inject oxygen directly into a water table to promote the biodegradation phenomena. Similarly, iSOC ™ technology, developed by InVentures Technologies Incorporated, provides hydrophobic microporous hollow fibers for infusing a water table with a gas.

The various known techniques mentioned above have a number of disadvantages. In many cases, these techniques involve the injection into the soil of chemicals or microorganisms that may present some environmental risk. Moreover, those of the preceding techniques which consist in stimulating microorganisms already present in the soil can give results sometimes unpredictable or random, insofar as it is difficult to control the growth of populations of microorganisms. In particular, it may be necessary to stimulate these populations on a vast geographical area in order to obtain a satisfactory depollution performance. More generally, there is a real need to design a new efficient and simple groundwater depollution process, particularly in the context of the decontamination of dissolved hydrocarbons.

SUMMARY OF THE INVENTION

The invention relates first of all to a method of decontaminating a water table having a flow and containing contaminants, said method comprising injecting oxygen into a permeable underground filter bed placed in said sheet to degrade the contaminants within the massive filtering.

Advantageously, the contaminants flow in the form of a plume and the filtering mass is placed so as to intercept the plume, substantially transversely to the direction of flow.

Preferably, the filtering mass comprises at least one reactive part and, optionally, a protective part upstream of the reactive part relative to the direction of the flow, the oxygen injection occurring at least partially in the part reactive.

Preferably, the oxygen injection is of purely diffusive type.

Preferably, the oxygen injection is carried out through a microporous polymeric membrane.

Preferably, the amount of oxygen injected into the filter bed per unit of time is adapted so that the reactive part is under aerobic conditions.

Advantageously, the contaminants include dissolved hydrocarbons, particularly benzene and / or ethylbenzene and / or toluene and / or xylene.

The subject of the invention is also a process for the depollution of a water table having a flow and containing contaminants composed of dissolved hydrocarbons flowing in the form of a plume, said method comprising passing the plume of contaminants through a permeable underground filter bed placed in the sheet, said filter bed comprising at least a reactive portion and a protective portion and said method comprising contaminant degradation in the reactive portion of the filter bed.

Preferably, said dissolved hydrocarbons comprise benzene and / or ethylbenzene and / or toluene and / or xylene.

According to a preferred embodiment of the process according to the invention, the reactive part of the filtering mass has a total porosity of at least 50% decomposing into an air porosity representing 70 to 90% of the total porosity and a porosity with water representing 10 to 30% of the total porosity.

Advantageously, the reactive part of the filtering mass serves as a support for the development of native bacteria, said native bacteria ensuring the degradation of contaminants.

Advantageously, the reactive part of the filtering mass comprises one or more materials chosen from peat, pozzolans, corn cobs, tree bark.

Advantageously, the protective part of the filtering mass comprises one or more materials chosen from gravel, chippings and any material consisting of quartz or silica whose size is between 20 and 40 millimeters.

The invention furthermore relates to a device for decontaminating a water table having a flow and containing contaminants, said device comprising: a permeable subterranean filter bed; an oxygen injection system 5 in the filter bed. According to an advantageous embodiment of the device according to the invention, the filtering mass comprises at least one reactive part 1 and optionally a protective part 2 and in which the oxygen injection system at least partially supplies the reactive part 1 with oxygen. .

According to an advantageous embodiment of the device according to the invention, the oxygen injection system 5 comprises a source of supply of oxygen under pressure as well as diffusion oxygen injection means arranged in the solid mass. filtering.

According to an advantageous embodiment of the device according to the invention, the diffusion oxygen injection means comprise one or more pipes containing oxygen under pressure and whose wall is a microporous polymeric membrane.

According to an advantageous embodiment of the device according to the invention, the pipes are connected in parallel with the oxygen supply source under pressure.

The invention also relates to a pollution control device for a water table having a flow and containing contaminants, said device comprising a permeable underground filtering mass, said filtering mass having at least one reactive part 1 and a protective part 2.

According to a preferred embodiment of the device according to the invention, the reactive part 1 of the filtering mass has a total porosity of at least 50% decomposing into an air porosity representing 70 to 90% of the total porosity and a porosity with water representing 10 to 30% of the total porosity.

Advantageously, the reactive part 1 of the filtering mass serves as a support for the development of native bacteria.

Advantageously, the reactive part 1 of the filtering mass comprises one or more selected materials among peat, pozzolana, corn cobs, tree bark.

Advantageously, the protective part 2 of the filtering mass comprises one or more materials chosen from gravel, chippings and any material made of quartz or silica whose size is between 20 and 40 millimeters.

Advantageously, the filtering mass is covered with a protective layer 10 such as a geotextile.

The invention also relates to a method for installing a permeable underground filter bed in a water table having a flow and containing contaminants flowing in the form of a plume, for the purpose of cleaning up the sheet, said method comprising the steps of: determining span and velocity parameters of the plume as well as concentration of contaminants by measurement and / or modeling; choice of a threshold level of concentration of contaminants; determination of the location, the orientation and the dimensions of the filter mass as well as a quantity of oxygen to be injected per unit of time in the filter mass, according to said determined parameters and as a function of said chosen threshold level, in order to the concentration of contaminants downstream of the filter bed is below the chosen threshold level; set up in the sheet of the filter mass whose location, orientation and dimensions were determined in the previous step.

According to a preferred embodiment of said method, the contaminants comprise dissolved hydrocarbons, in particular benzene and / or ethylbenzene and / or toluene and / or xylene. According to a preferred embodiment of said method, the filter mass is as defined above.

The present invention overcomes the disadvantages of the state of the art. In particular, it provides a simple and effective method of decontaminating dissolved hydrocarbons which, although implemented in situ, does not necessarily require active treatment of a large geographical area.

The invention solves in particular the problem of the depollution of a water table having a flow and contaminated by a source of pollution (the dilution of the contaminating elements in the sheet creating a plume of contamination in the direction of flow).

The problem is solved thanks to the development of a permeable underground filter bed intended to intercept and treat the polluting plume locally and actively.

According to certain particular embodiments, the invention also has the advantageous characteristics listed below: the decontamination is accomplished by in situ biodegradation using exclusively native bacteria (naturally present in the treated ground), without having to introduce additional microorganisms into the saturated zone; the filtering mass, thanks to a judicious choice of material, serves as a growth support for the populations of autochthonous bacteria, which develop much more efficiently than outside the filtering mass; the stimulation of the bacterial growth, preferably carried out by oxygen distribution in the saturated zone, is 00643

advantageously limited to the zone of the filtering mass, which makes it possible to use a distribution device of limited size; the oxygen distribution in question is advantageously carried out by diffusion, which makes it possible, on the one hand, to avoid the problems related to the formation of gas bubbles in the water table, that is to say an effect of "Venting", and secondly to minimize or even avoid clogging of the filtering mass; the different parameters of the invention are adapted case by case according to the type of soil, the type of sheet to be treated and the type of pollution, and this in order to reduce the concentration of contaminants to a desired level, as low as it is ; advantageously, the invention can be implemented in an "ecological" manner, using entirely natural and local materials, chosen according to the type of region where the invention is implemented; the invention can be implemented with very little material on the surface, which can be advantageous especially in a hostile environment; in addition the maintenance is simple and inexpensive.

BRIEF DESCRIPTION OF THE FIGURES

Figure la and Figure Ib are schematic representations of a filter bed according to the invention, respectively in top view and in lateral section.

Figure 2 is a schematic representation of an oxygen distribution element used in the context of the invention.

FIG. 3 is a schematic representation of a groundwater treatment pilot according to the invention (see example). DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The invention is now described in more detail and without limitation in the description which follows.

Massive filtering

Referring to Figures la and Ib ₇ there is described here a filter mass according to the invention set up in a saturated zone comprising a water table. The filtering mass comprises a reactive part 1 and, optionally, a protective part 2. When it is present, the protective part 2 is located upstream of the reactive part 1, the upstream term referring to the flow of the water table. whose direction and direction are indicated by the gray arrow in the figures.

The reactive part 1 consists of one or more materials mainly intended to support the development of indigenous microorganisms. These materials can be chosen according to the criteria that follow.

The permeability of the materials is preferably close to that of the surrounding saturated zone, so as not to disturb significantly the flow of the water table; preferably the coefficient of permeability of the materials is between ⁴ and 10 ^"1 m / s.

The materials have a large exchange surface, generally in the form of porosity, to provide effective support for bacterial growth. The total porosity (0otot) of the material is preferably at least 50%. The total porosity of the material (0tot) is preferably decomposed into an air porosity (0air) representing 70 to 90% of 0tot and a porosity with water (0eau) representing 10 to 30% of 0tot. The materials must have satisfactory resistance to flow or leaching.

Examples of suitable materials include peat, pozzolans, corn cobs, tree bark and especially pine bark. Specifically, the material must have a limited biodegradability whose kinetics of biodegradation is much lower than that of dissolved hydrocarbons.

Optionally, the filter bed may comprise two or more reactive portions succeeding one another in the direction of flow or transversely to the direction of flow.

When present, the protective portion 2 may comprise one or more materials that are selected to perform the following functions:

- precipitate and fix ions (iron ...);

filtering fine particles entrained in the flow;

- To avoid a variation in speed of water flow too much water when it passes through the filter, due to too high permeability differential between the materials of the reactive part 1 and the surrounding area;

- More generally avoid leaching of the reactive part 1 and avoid clogging.

Examples of suitable materials include gravel, chippings and any materials made of quartz or silica ranging in size from 20 to 40 millimeters. By size is meant the average dimension (or d ₅₀ ) of grains or particles of materials, as determined for example by sieving.

It may be useful to maintain the filter mass by a neutral material holding assembly surrounding the reactive part 1 of the solid mass. This holding assembly (constituted in the example of the figure by a grid 4 and tubes 3) makes it possible to stabilize the massive with respect to the ground without disturbing the flow of the sheet. By neutral material is meant any inert material having no influence on the environment.

An oxygen injection (or distribution) system 5 is provided to supply oxygen to the medium, particularly at the level of the filter bed. Preferably, the oxygen injection system 5 provides an injection of oxygen in situ by diffusion (according to Fick's law). This system may consist of essentially vertical elements that provide the oxygen supply of the medium. These elements can be regularly spaced, aligned and distributed in one or more successive rows. Each row is for example substantially perpendicular to the flow direction of the web and still intercepts the polluting plume.

According to a preferred embodiment, and with reference to FIG. 2, each element comprises a hollow protective cylinder comprising a solid portion 11 and a pierced portion of orifices 12, which contains a pressurized oxygen injection pipe. . The pierced portion of orifices 12 is immersed in the sheet. The water of the web can therefore freely enter this part of the hollow cylinder and come into contact with the oxygen injection pipe.

The portion of pipe 13 that is not immersed in the water table may be designed in any material impermeable to oxygen, for example polyethylene. On the other hand, the portion of pipe 14 located in the zone of contact with the water table may be formed of a microporous polymeric membrane, of silicone for example. This membrane must have an oxygen permeability. In the case of silicone for example, the membrane is permeable to oxygen at a pressure of less than about 2.5 bar. The pressurized oxygen injection pipe may be wound on a support 15 which may be simply a cylinder of circular section, for example of high density polyethylene, or a cross-section cylinder (or of another equivalent form), in order to maximize the surface of the injection pipe in contact with the environment.

It is possible to provide that the injection pipes belonging to different elements form the same oxygen supply circuit (in series) or it is possible to provide for a single oxygen supply line to feed the pipes in parallel. injection of the various elements, which are then preferably in a non-emergent form. This parallel configuration is advantageous from the point of view of the pressure drop. Means for supplying oxygen under pressure are provided on the surface. They supply the aforementioned pipes with oxygen at a pressure greater than 1 bar and less than 2.5 bars. The injection of dissolved oxygen by diffusion makes it possible to avoid bubbling and coalescence phenomena.

It is also possible in the filtering mass 6 control organs for measuring one or more physicochemical parameters (water height, flow characteristics, concentration of contaminants ...). These control members 6 can be integrated with piezometers for example.

One or more eyes 7 may allow to take samples if necessary.

The filter bed is disposed on a bedrock called substratum 8 and is advantageously separated from the surface lands 9 which cover it by means of a geotextile 10, which avoids the penetration of the surface lands in the filter bed.

Method of decontaminating a water table

A site comprising a water table with a flow and a contamination by dissolved hydrocarbons is considered here. The dissolved hydrocarbons are mainly benzene, toluene, ethylbenzene and xylene (denoted BTEX). When the contamination comes from a continuous source (relatively point or extended), the entrainment of contaminants by the flow of the water table creates a plume of pollution.

A first phase of the decontamination process consists of conducting a precise examination of the terrain to model the pollution plume. This modeling results in a three-dimensional mapping of the BTEX concentration and provides the flow velocity of the water table. The model is considered representative when the calculated quantities are substantially equal to the values measured on site inside the piezometers.

A second phase consists in choosing the characteristics of the filtering mass to be put in place: location, orientation, dimensions, type of material (x) (in particular for the reactive part of the massif). This choice is made according to the scale of the depollution that is desired. Typically, it is chosen to dispose the filtering mass so that it is essentially perpendicular to the flow direction of the plume and so that it essentially intercepts the entire plume. The choice of materials and the choice of the size of the reactive part of the mass in the direction of flow (thickness of the reactive part of the mass) determine the decontamination rate, ie the reduction factor of the concentration of BTEX at the crossing of the filter bed.

This decontamination rate can be predicted using previous laboratory experiments, where the effectiveness of different media to promote bacterial growth and BTEX decontamination can be compared on a reduced scale (the level of contamination and the velocity of water being chosen identical to the site). It is particularly interesting to test as supports materials naturally present near the site to clean up.

It is then possible to choose the type of support and the thickness of the filter bed (that is to say indirectly the residence time of the contaminants in the massif) as a function of the final residual level of contaminants considered acceptable for the site to pollute.

The actual installation of the filtering mass is carried out by digging a cavity in the soil of appropriate dimensions, possibly placing a holding device, filling the cavity with one or two compartments (upstream and downstream) of selected materials, and by covering the massif with a geotextile and then surface earth. The oxygen injection system is then put in place. The density of the oxygen injection elements must be sufficient to obtain a suitable concentration of dissolved oxygen in the filter mass and thus the development of populations of indigenous aerobic bacteria necessary for the desired level of decontamination.

For example, the invention can make it possible to pass from a natural population of bacteria of 10 ³ -10 ⁴ CFU (Colony Forming Unit) to a population of 10 ⁷ -10 ⁹ CFU in the reactive part of the filtering mass.

It is useful to carry out various measurements upstream of the filter bed, downstream of it and inside it, in order to determine not only the actual decontamination rate of the BTEX during the passage of the plume through the massive, but also in order to control the evolution of the different hydrodynamic and physicochemical parameters of the site.

For example, preferably the amount of oxygen to be dispensed should be such that the medium is aerobic.

EXAMPLE The following example illustrates the invention without limiting it.

Site to clean up

The site to be cleaned up is located in a sedimentary basin. The permeable alluvium consists of large pebbles and gravel with a sandy matrix and is 4 to 5 m thick. They rest on a marly layer (called substratum above) essentially impermeable on which flows the water table. The free water table is between 1.5 and 2.5 m deep. The general hydrodynamic profile of the water table is determined using measurements via. about sixty piezometers implanted on the site and nearby and supplemented by simulations under Visual Modflow. In particular: the speed of flow of the sheet is 1 m / day on average; the average density of the aquifer is of the order of 1700 kg / m ³ ; the water recharge is of the order of 400 mm / year; the longitudinal dispersivity is

2.3 m (the transverse and vertical dispersivities being respectively 10 and 100 times lower); the pH of the study area is 6.5. The study area is contaminated by a point source of volatile aromatic hydrocarbons. In what follows, the concentration of BTEX is measured according to standard NF-ISO 11423-1 at various points on the site. The index of total hydrocarbons is also measured according to standard NF 9377-2.

Setting up a driver

The choice of materials to be used in the filter bed has been made based on the results of biodegradation tests (mineralization) of hexadecane and benzene conducted in laboratories on three types of peat, on maize cobs, on pozzolans and on pine bark. Pine bark has been chosen as the active material because it shows the best biodegradation index.

The positioning of the pilot is subject to field constraints. It is determined according to the results of the modeling under Visual Modflow in order to guarantee the interception of the BTEX plume, perpendicular to the flow direction of the aquifer. The dimensions of the pilot are: 8 m long (transverse to the flow), 3 m high (in the alluvium from the marl), 3 m wide parallel to the flow of which 2 m for the reactive part downstream and 1 m for the protective part upstream. The area to be excavated is subjected to continuous pumping at 70 m ³ / h in order to dry it during the works.

Casings are arranged on the periphery of the pilot to ensure the maintenance of the natural terrain without disturbing the hydrodynamic regime of the sheet, with a regular spacing of 1 m between the casings. The upstream portion of the pilot is filled with alluvial gravel 20/40 rolled. The pine bark is arranged in the downstream part.

Since the natural concentration of dissolved oxygen in the aquifer is almost zero, an oxygen injection system is put in place: three rows of high-density polyethylene strainer casings are placed upstream of the gravels, downstream of the gravels , and in the center of the reactive part of the filtering mass (see figure la). Oxygen diffusers formed of microporous silicone tubes are arranged in parallel in the strainer casings. The supply of the oxygen diffusers is carried out on the surface by a bottle of O ₂ under pressure. Between the bottle and the diffusers is disposed a pressure chamber formed of a stainless steel tube of greater diameter than that of silicone tubes. This minimizes the pressure drops. In each strainer tubing, a spiral support is provided to hold the silicone tubes and increase the tube / web exchange surface. For example, the oxygen flow rate is between 0,2xl0 ^"3 L. ^{s" 1} and 0 ₇ 4xl0 ^"3 L. ^{s -1} under a pressure of 2 barg.

Results

Figure 3 provides a representation of the site. The arrow gives the direction of flow of the tablecloth; the dashed rectangle corresponds to the location of the filter bed. The marks (PZl, PZlW ...) correspond to piezometers used in particular for measuring the concentration of pollutants in the sheet.

The comparison between the measurement in PZl and the measurement in PZ4 makes it possible to estimate the decontamination rate at the crossing of the filtering mass. The comparison between the measurement in PZlW and the measurement in PZ6 serves as a control, because these piezometers are located laterally away from the filtering mass.

Table 1 below gives the results of the contamination measurements a few months after the installation of the filter bed, that is to say after having waited the necessary time to recover a normal hydrodynamic regime, but before having started the oxygen distribution process stimulates biodegradation. The results in question (which differ little from the results of measurements carried out on the virgin ground before the setting up of the filtering mass) thus serve as reference compared to an active filtering mass. 43

19

Table Contamination Measures Before Oxygen Supply

Table 2 below provides the results of the contamination measurements after the injection of oxygen into the filter bed, that is to say after having activated the biodegradation which transforms the filter bed into an active filter bed.

The reduction of BTEX in the filter mass is of the order of 1000 μg / L taking into account the fluctuations of the source of contamination. In the present case, to obtain a complete depollution, all other things being equal, it would have been necessary to double the width of the filtering mass.

Table Contamination measures after oxygen supply

Claims

A method for the remediation of a water table having a flow and containing contaminants, said method comprising injecting oxygen into a permeable underground filter bed placed in said web to degrade contaminants within the filter bed.

The method of claim 1, wherein the contaminants flow in the form of a plume and the filter bed is positioned to intercept the plume, substantially transverse to the direction of flow.

3. Method according to claim 2, wherein the filtering mass comprises at least one reactive part and, optionally, a protective part upstream of the reactive part relative to the direction of the flow, the injection of oxygen s'. performing at least partially in the reactive part.

4. Method according to one of claims 1 to 3, wherein the oxygen injection is purely diffusive type.

5. Method according to one of claims 1 to 4, wherein the oxygen injection is carried out through a microporous polymeric membrane.

6. Method according to one of claims 1 to 5, wherein the amount of oxygen injected into the filter bed per unit of time is adapted so that the reactive part is under aerobic conditions.

7. Process according to one of claims 1 to 6, in which the contaminants comprise dissolved hydrocarbons, in particular benzene and / or ethylbenzene and / or toluene and / or xylene.

8. Method according to one of claims 3 to 7, wherein the reactive portion of the filter mass has a total porosity of at least 50% decomposing to an air porosity representing 70 to 90% of the total porosity and a porosity to water of 10 to 30% of the total porosity.

9. Method according to one of claims 3 to 8, wherein the reactive portion of the filtering mass serves as a support for the development of native bacteria, said native bacteria ensuring the degradation of contaminants.

10. A method according to one of claims 3 to 9, wherein the reactive portion of the filter bed comprises one or more materials selected from peat, pozzolans, maize cobs, tree bark.

11. Method according to one of claims 3 to 10, wherein the protective part of the filtering mass comprises one or more materials selected from gravel, chippings and any material consisting of quartz or silica whose size is between 20 and 40 millimeters.

12. Device for decontaminating a water table having a flow and containing contaminants, said device comprising: a permeable subterranean filter bed; an oxygen injection system (5) in the filter bed.

13. Device according to claim 12, wherein the filtering mass comprises at least one reactive part (1) and optionally a protective part (2) and wherein the oxygen injection system at least partially feeds the reactive part (1). ) in oxygen.

14. Device according to claim 13, wherein the oxygen injection system (5) comprises a source of supply of oxygen under pressure as well as diffusion oxygen injection means arranged in the filter bed. .

15. Device according to claim 14, wherein the diffusion oxygen injection means comprise one or more pipes containing oxygen under pressure and whose wall is a microporous polymeric membrane.

16. Device according to claim 15, wherein the pipes are connected in parallel with the source of supply of oxygen under pressure.

17. Device according to one of claims 13 to 16, wherein the reactive portion (1) of the filter mass has a total porosity of at least 50% decomposing into an air porosity representing 70 to 90% of the total porosity and a porosity to water representing 10 to 30% of the total porosity.

18. Device according to one of claims 13 to 17, wherein the reactive part (1) of the massif Filtering serves as a support for the development of indigenous bacteria.

19. Device according to one of claims 13 to 18, wherein the reactive portion (1) of the filter bed comprises one or more materials selected from peat, pozzolans, maize cobs, tree bark.

20. Device according to one of claims 13 to 19, wherein the protective portion (2) of the filter bed comprises one or more materials selected from gravel, chippings and any material consisting of quartz or silica whose size is included between 20 and 40 millimeters.

21. Device according to one of claims 12 to 20, wherein the filter bed is covered with a protective layer (10) such as a geotextile.

22. A method of installing a permeable underground filter bed in a water table having a flow and containing contaminants flowing in the form of a plume, for the purpose of depolluting the sheet, said method comprising the following steps: plume range and velocity parameters as well as contaminant concentration, by measurement and / or modeling; choice of a threshold level of concentration of contaminants; determination of the location, the orientation and the dimensions of the filter mass as well as a quantity of oxygen to be injected per unit of time in the filter mass, according to said determined parameters and as a function of said chosen threshold level, in order to that the contaminant concentration downstream of the filter bed is below the chosen threshold level; set up in the sheet of the filter mass whose location, orientation and dimensions were determined in the previous step.

23. The process according to claim 22, wherein the contaminants comprise dissolved hydrocarbons, in particular benzene and / or ethylbenzene and / or toluene and / or xylene.