WO2000048755A1

WO2000048755A1 - Remediation of contaminated groundwater

Info

Publication number: WO2000048755A1
Application number: PCT/US2000/003840
Authority: WO
Inventors: Walter Farman Farmayan; Mark Dwight Akeny; Marjorie Sungwon Hong
Original assignee: Shell Oil Company
Priority date: 1999-02-18
Filing date: 2000-02-15
Publication date: 2000-08-24
Also published as: AU2881600A

Abstract

A system is described for achieving hydraulic containment and remediation of water table contaminant plumes, based upon pumping from groundwater and recycling extracted water through the root zone, rhizosphere and/or vadose zone by surface and/or subterranean infiltration, which system comprises: (1) one or more extraction wells to allow pumping of contaminated groundwater to the surface and to create a cone of depression in the saturated zone, (2) a surface and/or subterranean infiltration gallery to reinject the pumped groundwater, and to irrigate the vegetation, (3) a zone or stand of vegetation (dryland or wetland) to provide for ultimate water removal from the soil column by evapotranspiration thereby allowing a cone of depression to be maintained in the saturated zone, (4) a vertical reflux where excess water, not taken up by plants infiltrates back to groundwater and (5) a potential reaction zone in (a) the root zone/rhizosphere where enhancement to indigenous microbial degradation of groundwater contaminants may occur, or below the root zone where infiltration may enhance physical/chemical/microbial disappearance of groundwater.

Description

DESCRIPTION REMEDIATION OF CONTAMINATED GROUNDWATER Technical Field

The invention relates to a system for groundwater remediation comprising hydraulic containment and phytoremediation of groundwater contaminant plumes, by pumping of the contaminated water from the saturated zone(s) of an aquifer and recycling of the extracted water by surface and/or subterranean infiltration through the root zone and/or rhizosphere established by surface vegetation, and/or the vadose zone . Background Art

Phytoremediation, which involves using plants or trees to help cleanup contaminated sites, has been used to remediate contaminated sites located in unsaturated zone and/or shallow contaminated groundwater, which can be reached readily by roots of plants or trees with the agricultural techniques, and ranging from the surface downwards to 10-15 ft. below the surface. Roots can be encouraged to grow into the capillary zone and preferentially draw water from the contaminated aquifer. In this situation contaminants are drawn to the plants or trees directly by root water uptake in close physical vicinity of the aquifer and contaminated groundwater.

However, there are settings which comprise deep groundwater sites which can be 30 feet (sometime 20 feet, sometimes 10 feet, depending on the location or much further below the surface, where the groundwater may not be readily accessible by roots of surface vegetation with agricultural techniques such as "deep rooting techniques." Moreover, there are contaminated groundwater sites with building constructions on the surface such as a chemical plant, a refinery, or a house, where an area for planting is not readily available above the contaminant plume. Still further, the remediation project may last for extended period of time, while the contaminants may continue migrating offsite thereby expanding the area of contamination.

The "Pump and Treat" method has also been used to treat contaminated groundwater where contaminated groundwater is extracted from the saturated zone by pumping through extraction wells. The extracted contaminated groundwater is subsequently treated by a variety of conventional wastewater treatment processes such as steam distillation, air stripping, and bioremediation. The operational costs for this method can be very high, since it may be costly to pump extracted water over long distances or piping runs to a remote surface water treatment facility, especially when the project extends over ten, twenty or more years. Moreover, the costs for building new treatment facilities, if needed, can also be very high.

Therefore, there remains a need for a cost effective system suitable for remediating deep groundwater as well as shallow groundwater which would eradicate the above-mentioned concerns and shortcomings of the prior art processes. Disclosure of the Invention

A system for remediating groundwater contaminant plumes involving pumping from the saturated zone(s) and recycling the extracted water through the root zone, rhizosphere, and/or vadose zone by surface and/or subterranean infiltration. The system comprises: (1) one or more extraction wells to allow pumping of contaminated groundwater to the surface and to create a saturated zone cone of depression, (2) a surface and/or subterranean infiltration gallery or water distribution system to reinject the pumped groundwater and to irrigate the root zone/rhizosphere of the surface vegetation, (3) a zone or stand of vegetation (dryland or wetland) to provide for ultimate water removal from the soil column by evapotranspiration thereby allowing a cone of depression to be maintained in the saturated zone, (4) a vertical reflux zone where excess water, not taken up by vegetation infiltrates back to groundwater, and (5) a potential reaction zone in (a) the root zone/rhizosphere where enhancement to indigenous microbial degradation of groundwater contaminants may occur, or below the root zone where infiltration may enhance physical/chemical/microbial disappearance of groundwater contaminants and/or (b) the vegetation itself via the action of plant enzymes which may break down or accumulate groundwater contaminants. The vegetation at the contaminated sites, which may have bioaccumulated contaminants from soil and groundwater, can be cut or harvested, subsequently composted, dried, incinerated, thereby reducing the quantity of mass to be taken offsite. Brief Description of the Drawings Fig. 1 is a cross-sectional side view of a prior art process where the contaminant plumes located in the unsaturated zone and shallow groundwater are in the close vicinity of the root zone, and contaminants are drawn toward the plants or trees directly by root water uptake; Fig. 2 is a side cross-sectional view of a vertical hydraulic containment and contaminant capture for deep groundwater utilizing a system according to an embodiment of the present invention, wherein the vegetation is directly above the zone of contamination; Fig. 3 is a side cross-sectional view of a system utilizing pumping and plants where an area for planting is not readily available above the contaminant plume according to another embodiment of the present invention; and

Fig. 4a, 4b, and 4c are side cross-sectional views of vertical phyto-hydraulic capture system of the present invention using various water pumping/irrigation/reflux schemes .

Detailed Description of the Invention

The present invention is a method for remediating contaminated subsurface soil and/or groundwater which provides for contaminated groundwater capture combined with phyto- and bioremediation using a system comprising:

(1) one or more extraction wells in the saturated zone of concern to allow pumping of contaminated groundwater to the surface and to create a saturated zone cone of depression; (2) a surface and/or subterranean infiltration gallery to re- inject the pumped groundwater and to irrigate the root zone/rhizosphere created by a zone or stand of vegetation as described in (3) below;

(3) a zone or stand of vegetation such as plants and trees which can be vertically in line with the contaminated plumes or can be located at an off site to provide for ultimate water removal from the soil column by evapotranspiration thereby allowing a cone of depression to be maintained in the saturated zone; (4) a vertical reflux where excess water, not taken up by plants infiltrates back to groundwater, and

(5) a potential bioremediation zone comprised of either or both (a) degradation/removal of organic/inorganic contaminants by indigenous microbial activity or other physical/chemical processes enhanced by the presence of roots in the root zone or by infiltration in the vadose zone within or underlying the root zone, and (b) degradation of contaminants by plant enzymes or accumulation/binding of contaminants in plants as groundwater contaminants move through the vegetation. The number of wells and areal extent of the system can be tailored to site-specific patterns of deep groundwater and vadose zone (source area contamination) . There are no restrictions on size areal extent. Multiple systems could be installed within a broad area as well to cover localized plumes or hot spots.

The pumping rates of the wells will be determined by evapotranspiration demand and moisture reflux requirements. Evapotranspiration demand can be estimated by conventional evapotranspiration theory. Moisture reflux requirements must be tailored to site specific vadose zone moisture saturation requirements, which will vary with height or extent of the unsaturated zone, optimal conditions for microbial activity, and contaminant type. For inorganic contaminants reflux levels are likely to be minimal to minimize flushing/ infiltration below the root zone. For hydrocarbon contaminants, amenable to biodegradation, extension of near saturated conditions deep into the unsaturated zone, possibly all the way to groundwater may or may not be advantageous depending upon microbial response as well as potential for deep flushing back to groundwater.

Referring to Fig. 1, the apparatus used reflects the prior art for hydraulic containment and contaminant remediation of shallow groundwater by plants. In Fig. 1, three major zones are found: an unsaturated zone 1, a capillary zone 2 and a saturated zone 3. In addition, a capture zone 4 is pictured to draw contaminated groundwater 5. Contaminant plumes 6, which can be in the saturated zone or unsaturated zone, provide the source of the contaminants. A root zone 7 starts under a tree grove 8 and can extend through the unsaturated zone 1 and into the capillary zone 2. These roots are in close proximity to the source of the contamination, which can be in the saturated zone or unsaturated zone. Infiltration 9 of the water resulting from precipitation 10 occurs as part of this process. Evapotranspiration 11 results in a loss of water from the system. Referring to Figs. 4a, 4b and 4c, the infiltration zone or gallery can comprise a wide variety of designs, including but not limited to surface infiltration ponds in diked vegetated areas 16, surface sprinkler systems 16, subterranean irrigation/drainage tile 17, and subterranean system of water emitters including vertical injection wands 18. The particular design chosen will depend upon site specific concerns and needs including costs/economics, size/extent of the remediation, soil hydraulic properties, etc.

In Figs. 4a, 4b and 4c, three preferred embodiments of the invention are shown. As shown, a cone of depression is formed in the capture zone for each embodiment. A water reflux 24 process below a zero flux plane 20 (a plane which corresponds to reversal of the soil water gradient, and below which soil water travels only downwards is shown. An in-situ remediation zone 21 containing roots 22 provides an active zone for contaminant removal is also shown. The diked vegetated zone 16 in 4a, which receives and distributes water from the extraction well also can contain water holding tanks 23. In Fig. 4b a horizontal subterranean irrigation/drainage tile 17 is used to distribute the recycled water into the infiltration gallery. In Fig. 4c, a manifold of vertical injection wands (perforated vertical pipe) 18 is used to distribute the extracted water to the infiltration gallery.

The infiltration gallery can have added features to enhance the bioremediation activity of the root zone and rhizosphere. Such features include aeration of water by spray jets, oxygen-releasing compounds or other means, and addition of fertilizers or other amendments to enhance biological activity or uptake.

The zone or stand of vegetation can comprise a broad range of plant types or species including but not limited to phreatophyte trees such as hybrid poplars, grasses, a complex wetlands ecosystem, evergreen plant species such as pine trees and spruce trees, or other vegetations, and will again depend upon site specific requirements such as desired root zone depth, dryland vegetated area versus wetland, sensitivity and/or uptake efficiencies for specific contaminant types, etc. Performance of the vegetation in terms of evapotranspiration may be estimated from evapotranspiration theory. The positioning of the vegetated zone/evapotrans-piration sink/infiltration gallery will be determined by site specific remediation requirements. In some cases it will be advantageous to locate the vegetated zone directly above the extraction well, to minimize piping runs and costs. In other instances, advantage may be gained by locating the vegetated area away from the pumping well area based upon land availability, or possibly the location of upgradient vadose zone source zones, amenable to in-situ bioreme- diation/treatment . The positioning of the vegetated plot/infiltration gallery is not restricted to a location directly above the extraction wells. The vegetated plot/infiltration/reflux zone can be, for example, located upgradient of the extraction well directly over a perceived vadose zone contaminant source area. Referring to Fig. 3, as a specific embodiment of the present invention, where vegetation or an area for planting vegetation is not available directly above or in the vicinity of the plume, a pump 12 can be used to extract water and create a cone of depression 19, and a line can be routed to distribute this extracted water for uptake by vegetation in an area offsite which is located more than 10 miles, preferably from about one mile to about 10 miles, more preferably from about 100 feet to about one mile, still more preferably from about 10 to about 100 feet from the area or the surface boundary of contamination. The surface boundary of contamination is defined as the outer periphery/perimeter of ground surface area directly under which a contaminated saturated zone of concern lies. In other words, i.e., beyond the surface boundary of contamination, no arbitrary action level or regulatory action level is set. This specific embodiment is applicable to both shallow groundwater or deep groundwater. Note that this specific embodiment in Fig. 3 contains common elements to other embodiments of the invention including a reaction zone 15, sub-irrigation 14 and infiltration 9. The vertical extent and width of the vertical ground- water reflux/bioremediation zone (in the vadose zone) can be tailored to site-specific requirements, of size, saturation and infiltration rate. Site-specific stratigraphy and spatial variation of hydraulic properties will also impact the extent of the reflux zone. The size and physical extent can be estimated using well-established unsaturated/saturated flow theory.

Referring to Fig. 2, as a specific embodiment of the present invention, the present system provides for hydraulic containment of deep groundwater plumes by installing a pump 12 near or in an extraction well 13 to create a capture zone 4 with hydraulic containment of the contaminated groundwater; the contaminated groundwater is extracted to above the ground surface or to a subterranean zone below ground surface and reinjected back via above-ground irrigation or sub- irrigation 14; and simultaneously utilizes evapotranspiration 11 of surface vegetation 8 as the final or ultimate water sink (outlet) in the process.

The present system in Fig. 2 may also create a vertical in-situ reaction zone 15 where refluxed, recycled or offsite re-injected groundwater contaminants can be degraded by indigenous, applied (exogeneous) or enhanced microbial degradation or within the vegetation itself by plant enzymes. Alternatively, in the case of inorganic contaminants, an in- situ bio-accumulation zone, wherein plants absorb and retain the contaminant, can be established corresponding to the vegetative root zone 7.

The vertical reflux zone effectively acts as an in-situ bioreaction zone and can begin with the vegetation and extend the entire height of the unsaturated zone from the infiltration gallery to the water table. The width of the in- situ reaction zone is effectively controlled or determined by the areal extent of the infiltration gallery and/or vegetated area . Level of saturation below the plant root or water uptake zone can be controlled by a reflux ratio or recycled water in excess of evapotranspiration demand. The net increase in the vadose zone saturation below the plant root uptake zone will depend upon the difference in pumping and evapotranspiration rate, the size or height of the vadose zone, some radial moisture spreading beyond the vegetation/infiltration gallery footprint, and the specific hydraulic/moisture properties of the unsaturated zone soil column, which can vary spatially due to changes in stratigraphy and lithology. This level of saturation is considered important because there can be a direct impact to microbial activity in the unsaturated zone. The optimal level of saturation will also impact pumping capacity requirements.

In one aspect of the present invention, the present system provides containment and remediation of organic and hydrocarbon plumes. In another aspect of the present invention, the present technology is applied to inorganic contaminants as well, including but not limited to nitrates, salts and heavy metals. In the case of the latter, the amount of vertical reflux or infiltration could be deliberately limited to minimize flushing/infiltration of inorganic contaminants back to groundwater. Control of saturation levels below the root zone to near-residual levels would minimize deep infiltration. In this case the root zone would act as a bio-accumulation zone rather than as a biodegradation zone. Plants from the vegetated area can subsequently be harvested to remove accumulated inorganics. The harvesting procedure can range from simple surface mowing/cutting to complete uprooting depending upon the nature of the contaminant (s) , where in the plant tissue they bio-accumulate, and toxicity response of the plants to contaminants over time. The plants or trees from the vegetated area can subsequently, but not necessarily, be cut and then composed, dried or incinerated, thereby reducing the quantity of mass to be taken offsite. As a specific embodiment of the present invention, the system is utilized to remediate oxygenates contaminated plumes, MTBE (methyl t-butyl ether) plumes, TBA (t-butyl alcohol) contaminated plumes or plumes contaminated with mixtures of oxygenates such as mixtures of TBA and MTBE. The present process can be designed and tailored to site- specific conditions of meteorology, geology and stratigraphy using basic principles of unsaturated/saturated groundwater flow and evapotranspiration theory.

The invention provides a number of attractions from the point of view of environmental remediation:

The invention minimizes the need for costly pumping of extracted water over long distances or piping runs to a surface water treatment facility.

The invention provides for in-situ bioremediation in a relatively tight or confined space of the soil column by creating a vertically oriented water treatment/reflux zone .

The invention combines phytoremediation with hydraulic capture for deep groundwater (groundwater not directly accessible by roots) .

The system can be designed to maintain contaminated groundwater below ground surface by the use of a subsurface infiltration gallery/infiltration system to minimize concerns related to potential direct surface receptor exposure pathways.

Claims

1. A system for remediating a groundwater contaminant plume comprising:

(a) at least one extraction well for pumping contaminated groundwater to surface;

(b) a water distribution system to reinject the pumped groundwater and to irrigate vegetation of (c) below; and

(c) a stand of vegetation for uptake of (i) water, (ii) contaminants, or (iii) both water and contaminants from a soil column by evapotranspiration.

2. The system of claim 1, wherein said system further comprises a reaction zone adapted for remediating by a process selected from the group consisting of (i) enhancement of indigenous microbial degradation of the contaminants occurs, (ii) plant enzymes further break down or accumulate contaminants, or (iii) the combination of (i) and (ii) .

3. The system of claim 1, wherein the water distribution system comprises surface infiltration ponds in diked vegetated areas.

4. The system of claim 1, wherein the water distribution system comprises surface sprinkler systems.

5. The system of claim 1, wherein the water distribution system comprises subterranean irrigation tile.

6. The of claim 1, wherein the water distribution system comprises spray jets for the aeration of the water.

7. The system of claim 1, wherein the stand of vegetation is selected from the group consisting of phreatophyte trees, poplars, grasses, evergreen plant species, complex wetlands and the mixtures thereof.

8. The system of claim 1, wherein the stand of vegetation is about 10 feet to 10 miles away from the surface boundary of the contaminant plume.

9. The system of claim 8, wherein the stand of vegetation is about 10 to 100 feet away from the surface boundary of the contaminant plume.

10. The system of claim 8, wherein the stand of vegetation is located about directly above the extraction well (s) .

11. The apparatus system of claim 8, wherein the stand of vegetation is about 1 to 10 miles away from the surface boundary of the contaminant plume .

12. The system of claim 8, wherein the stand of vegetation is greater than 10 miles away from the surface boundary of the contaminant plume.

13. A method for remediating a groundwater contaminant plume involving pumping contaminated water from a saturated zone and recycling the extracted water through a vegetative stand comprising the steps of: - providing a stand of vegeta ion; providing an extraction well in the saturated zone of concern; providing a water distribution system; pumping of the contaminated groundwater from the extraction well to the unsaturated zone to create a cone of depression; reinjecting the pumped groundwater to the water distribution system and irrigating said stand of vegetation; - removing water from the soil column via evapotranspiration in said stand of vegetation thereby allowing the cone of depression to be maintained in the saturated zone; infiltrating excess water back to the groundwater through a reflux zone.

14. The method of claim 13, wherein the water distribution system is a surface infiltration gallery.

15. The method of claim 13, wherein the water distribution system is a subterranean infiltration gallery.

16. The method of claim 13, wherein said method further comprises a step of degrading the groundwater contaminants in the root zone by enhanced indigenous microbial action.

17. The method of claim 13, wherein said method further comprises a step of degrading the groundwater contaminants in the root zone by action of plant enzymes.

18. The method of claim 13, wherein said method further comprises a step of accumulating groundwater contaminants in plants .

19. The method of claim 13, wherein the vegetation in the stand is selected from the group consisting of phreatophyte trees, poplars, evergreen plant species, grasses, complex wetlands, and mixtures thereof.

20. The method of claim 13, wherein the contaminant is organic .

21. The method of claim 13, wherein the contaminant is inorganic.

22. The method of claim 21, wherein the contaminant is selected from the group consisting of (i) heavy metal (s) , (ii) salt(s), (iii) nitrate (s) and (iv) mixtures thereof.

23. The method of claim 20, wherein the contaminant is an oxygenate.

24. The method of claim 20, wherein the contaminant is methyl tertiary butyl ether (MTBE) or t-butyl alcohol (TBA) .

25. The method of claim 13, wherein said method further comprising a step of accumulating of contaminants in plant (s) and disposing of contaminated vegetation.