Abstract:
A groundwater treatment system for collecting a non-aqueous-phase liquid (NAPL) in a flow path of contaminated water in a body of water, the contaminated water moving toward a permeable reactive barrier (PRB) including a treatment agent for treating the contaminated groundwater. The groundwater treatment system including a collection layer positioned up-gradient of the PRB and permeable to the NAPL and the contaminated water. The collection layer including a NAPL collecting element to inhibit the NAPL from flowing to the PRB.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims benefit from U.S. Provisional Application 61/600,246 filed Feb. 17, 2012, the entire contents of which are incorporated herein by reference for all purposes. 
    
    
     BACKGROUND 
     The invention relates generally to devices and methods for trapping and removing non-aqueous-phase liquids (NAPLs) from groundwater or sub-surface soil prior to the treatment of the dissolved-phase contaminants by a permeable reactive barrier (PRB). 
     PRBs are often placed in the path of contaminated groundwater in order to remove dissolved contaminants. PRBs provide a cost-effective means of treating groundwater without mechanical systems. 
     PRBs are typically configured to remove specific contaminants (i.e., target compounds) as called for on a case-by-case basis. The specific target compound may or may not be a dissolved NAPL. That is to say, the target compound that the PRB intends to remove may be a NAPL at a very low concentration in the groundwater. The following application will discuss the target compound as the contaminant that the PRB is designed to remove, which may or may not include a NAPL. The following discussion will refer to NAPLs as contaminants that the PRB is not configured to process. 
     In the cases described above, NAPL may be migrating through the subsurface in conjunction with associated dissolved-phase contaminants (i.e., target compounds) in the groundwater. In those cases, the NAPL can cause the PRB to fail in part or in whole, either by 1) NAPL consuming the treatment chemical in the PRB matrix which was intended for low (dissolved) concentrations of target compound or 2) NAPL forming a barrier that is impermeable to groundwater on the up-gradient side of the PRB, or 3) physically coating or in other ways fouling the reactive surfaces of the PRB. 
       FIG. 1  shows a typical PRB groundwater treatment system  10 . The PRB groundwater treatment system is positioned adjacent a water table  14  and includes a low permeability cap  18  and a PRB  22  positioned in the groundwater flow path. 
     In some instances, hydraulically permeable PRBs  22  are also used to treat groundwater that has been contaminated by hazardous materials such as pesticides, volatile organic compounds (VOCs), polycyclic aromatic hydrocarbons (PAHs), polychlorinated dioxins (PCDDs), polychlorinated furans (PCDFs), polychlorinated biphenyls (PCBs), heavy metals, synthetic organic compounds, and the like. 
     Generally, NAPLs may be broken into two categories; light non-aqueous-phase-liquids (LNAPLs) and dense non-aqueous-phase-liquids (DNAPLs). LNAPLs will tend to the top portion of the ground water flow path while DNAPLs will tend to the bottom portion of the groundwater flow path. 
     PRBs  22  typically include a permeable reactive layer that has been designed to treat the target contaminants. The reactive material may include, but is not limited to, zero valent iron (ZVI), activated carbon, apatite, organoclay, and/or various type of degradable organic material. 
       FIG. 1  shows LNAPLs  26  and DNAPLs  30  in a contaminated groundwater flow  34  but not yet in contact with the PRB  22 . In the situation depicted in  FIG. 1 , the PRB  22  is functioning normally and is not yet fouled with NAPLs  26 ,  30 . The contaminated groundwater  34  flows into the PRB  22 , the PRB  22  treats a target compound, as desired, and treated groundwater  38  exits the PRB  22 . 
     Referring to  FIG. 2 , when unexpectedly high concentrations of the target compound and/or NAPLs (e.g., LNAPL  26  or DNAPL  30 ), prematurely consume the treatment capacity of the PRB  22 , the design life of the PRB  22  is decreased. That is, functional failure occurs sooner than planned. The PRB  22  is designed to address dissolved (lower) concentrations of the target compound, rather than higher concentrations of the target compound in addition to or including NAPLs (e.g., LNAPL  26  or DNAPL  30 ).  FIG. 2  shows that concentrations of NAPLs  26 ,  30  may permeate the PRB  22 . Once the PRB  22  is fouled, some groundwater passing through the PRB  22  will be properly treated (i.e., treated groundwater  38 ), but some groundwater exiting the PRB  22  will be partially treated groundwater  42 . That is to say, the partially treated groundwater  42  still contains appreciable levels of undesirable compounds. 
     Referring to  FIG. 3 , the DNAPL  30  coats or physically fouls the treatment chemical of the PRB  22 , the design life of the PRB  22  is decreased as the treatment chemical is unavailable to react with the target compound(s) in a dissolved state. The design life of the PRB  22  is calculated assuming uniform flow of contaminated groundwater  34  through all parts of the PRB  22 . If the flow of contaminated groundwater  34  is focused on only portions of the treatment PRB  22 , the full calculated treatment capacity of that PRB  22  is not realized because the focused flow overwhelms those portions of the PRB  22  receiving all the flow and contamination. This increased localized water flow rate can cause premature breakthrough of the treatment media. In addition, a partial blocked PRB  22  will lower the effective transmissivity of the PRB  22  and potentially force a portion of the contaminated groundwater  34  to flow around the PRB  22 , resulting in a partial failure (e.g., as shown at the bottom of  FIG. 3 ). 
     Referring to  FIG. 4 , the PRB  22  is completely blocked by accumulation of DNAPL  30  and LNAPL  26 . The contaminated groundwater  34  bypasses the PRB  22  altogether without any treatment. The contaminants in the water are not treated or removed, essentially rendering the PRB  22  ineffective. 
     Referring again to  FIG. 4 , in the case of NAPL  26 , 30  migrating through the water-permeable PRB  22 , the NAPL  26 , 30  migrates through the PRB  22  without any treatment. The contaminants in the NAPL  26 , 30  are free to dissolve into the groundwater down-gradient from the PRB  22 , essentially rendering the PRB  22  ineffective. 
     BRIEF SUMMARY OF THE INVENTION 
     From the above, it should be apparent that the art needs devices and methods that trap and collect the NAPLs  26 , 30  before they reach the treatment media of the PRB  22 . Such trapping would permit the PRB  22  to function properly. 
     The problem that has been previously unrecognized and unresolved with PRBs that are intended to be hydraulically permeable is the continuing migration of NAPLs that may consume the treatment chemical, physically foul the matrix, or completely or partially obstruct the passage of water. It is instructive to understand that when a NAPL impinges on a PRB intended for treating dissolved concentrations the NAPL may migrate through the water-permeable PRB after only being partially treated. 
     In one aspect, the present invention provides an apparatus and a method for trapping NAPL migrating toward a PRB in a collection layer before it encounters the treatment media of the PRB. The method comprises the step of interposing in the path of the NAPL, a NAPL collection device that may include a permeable collection sump and/or a hanging baffle. The permeable collection sump is suited to collect dense non-aqueous-phase liquid (DNAPL) and the hanging baffle is suited to collect light non-aqueous-phase liquid (LNAPL). 
     In a related aspect, in some embodiments the invention may incorporate devices and methods to remove the NAPL, including physical removal, or removal through oxidation, volatilization or other means. 
     The foregoing and other aspects and advantages of the invention will appear from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown by way of illustration a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention, however, and reference is made therefore to the claims and herein for interpreting the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The invention will be better understood and features, aspects and advantages other than those set forth above will become apparent when consideration is given to the following detailed description thereof. Such detailed description makes reference to the following drawings. 
         FIG. 1  is a side sectional view of a prior art PRB immediately after installation; 
         FIG. 2  is a side sectional view of a prior art PRB after NAPL has consumed a large portion of the treatment capacity; 
         FIG. 3  is a side sectional view of a partially NAPL-blocked prior art PRB; 
         FIG. 4  is a side sectional view of a completely NAPL-blocked prior art PRB; 
         FIG. 5  is a side sectional view of a water-permeable PRB according to the present invention; 
         FIG. 6  is a side sectional view of another water-permeable PRB according to the present invention, treating contaminated groundwater and collecting LNAPL; 
         FIG. 7  is a side sectional view of another water-permeable PRB according to the present invention, treating contaminated groundwater and collecting DNAPL; and 
         FIG. 8  is a side sectional view of another water-permeable PRB according to the present invention. 
     
    
    
     While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention relates to the inventors&#39; observation that NAPL can prematurely consume the treatment capacity of a PRB, physically foul it, and or block the flow of water through it. Also NAPL, impinging on a PRB, can pass directly through it, only partially treated thereby defeating its intended purpose. If the PRB is blocked partially or completely, groundwater will be diverted around the PRB either partially or completely. A blockage could also cause failure of the PRB by focusing groundwater flow through a section of the PRB causing premature depletion of the treatment chemical. NAPL could also migrate through a water-permeable PRB defeating the treatment component and allowing a source of groundwater contamination to migrate down-gradient (that is, downstream) from the PRB. 
       FIG. 5  shows a groundwater treatment system  50  situated adjacent a water table  54  and in a flow of contaminated ground water  58  that includes LNAPLs  62  and DNAPLs  66 . The groundwater treatment system  50  includes a low permeability upper cap  70 , a PRB  74  having an top end  75  engaging the upper cap  70  and a bottom end  77 , the PRB  74  depending vertically from the upper cap  70 , a NAPL collection layer  78  depending vertically from the upper cap  70  and having an upper portion  79  engaging the upper cap  70  and a lower portion  81 , the upper portion  79  defining an upper LNAPL collecting portion  63 , the lower portion  81  defining a lower DNAPL collecting portion  67 , and an impermeable barrier or containment baffle  82  positioned between the PRB  74  and the collection layer  78 . As shown in  FIG. 5 , ground water  58  flows through but not beneath the PRB  74  and the collection layer  78 , which are keyed into an impermeable layer. 
     The collection layer  78  has a coarser grain size than either the PRB  74  or the aquifer through which the contaminated groundwater  58  is flowing. The collection layer  78  provides a condition where LNAPLs  62  can easily rise to the top portion of the collection layer  78  and DNAPLs  66  will travel with gravity to a bottom portion of the collection layer  78 . The thickness of the collection layer  78  can be arranged such that the expected flow of DNAPL  66  and LNAPL  62  will not reach the PRB  74  but will rather accumulate in the collection layer  78 . 
     An LNAPL sump  86  is positioned in the top portion of the collection layer  78  and is arranged such that LNAPL  62  collects in the LNAPL sump  86 . When a predetermined volume of LNAPL  62  has collected, or upon a predetermined time interval, or upon another triggering event, a removal device  90  (e.g., pump, vacuum, et cetera) is fluidly connected to the LNAPL sump  86  and the LNAPL  62  stored therein is removed. 
     A DNAPL sump  94  is positioned in the bottom portion of the collection layer  78  and is arranged such that DNAPL  66  collects in the DNAPL sump  94 . The DNAPL sump  94  may be positioned below the lower extremity of the PRB  74  to provide a larger sump volume while avoiding contamination of the lower portion of the PRB  74 . However, the DNAPL sump  94  does not have to be positioned below the PRB  74 . When a predetermined volume of DNAPL  66  has collected, or upon a predetermined time interval, or upon another triggering event, a removal device  98  (e.g., pump, vacuum, et cetera) is fluidly connected to the DNAPL sump  94  and the DNAPL  66  stored therein is removed. 
     The containment baffle  82  is positioned between the PRB  74  and the collection layer  78  to inhibit LNAPL  62  from travelling through the collection layer  78  and fouling the PRB  74 . As the LNAPL  62  flows into the collection layer  78  and collects in the LNAPL sump  86 , the containment baffle  82  maintains the LNAPL  62  in the collection layer  78  and inhibits crossover to the PRB  74 . 
     In operation, the contaminated groundwater  58  flows through the collection layer  78  where LNAPLs  62  and DNAPLs  66  are collected. The contaminated groundwater  58  is then treated in the PRB  74  and treated ground water  96  exits. The groundwater treatment system  50  is shown bound on a bottom end, but need not be. 
       FIG. 6  illustrates another construction of the invention. A groundwater treatment system  100  includes a low permeability cap  70 ′, a PRB  74 ′, a NAPL collection layer  78 ′, and an impermeable barrier or containment baffle  82 ′ positioned between the PRB  74 ′ and the collection layer  78 ′. The groundwater treatment system  100  may be used where only LNAPLs  62  are a concern. 
     An LNAPL sump  86 ′ is positioned in the top portion of the collection layer  78 ′ and is arranged such that LNAPL  62  collects in the LNAPL sump  86 ′. When a predetermined volume of LNAPL  62  has collected, or upon a predetermined time interval, or upon another triggering event, the removal device  90  (e.g., pump, vacuum, et cetera) may be fluidly connected to the LNAPL sump  86 ′ and the LNAPL  62  stored therein removed. 
     The containment baffle  82 ′ is positioned between the PRB  74 ′ and the collection layer  78 ′ to inhibit LNAPL  62  from travelling through the collection layer  78 ′ and fouling the PRB  74 ′. As the LNAPL  62  flows into the collection layer  78 ′ and collects in the LNAPL sump  86 ′, the containment baffle  82 ′ maintains the LNAPL  62  in the collection layer  78 ′ and inhibits crossover to the PRB  74 ′. 
       FIG. 7  illustrates another construction of the invention. A groundwater treatment system  200  includes a low permeability cap  70 ″, a PRB  74 ″, and a NAPL collection layer  78 ″. The groundwater treatment system  200  may be used where only DNAPLs  66  are a concern. 
     A DNAPL sump  94 ″ is positioned in the bottom portion of the collection layer  78 ″ and is arranged such that DNAPL  66  collects in the DNAPL sump  94 ″. The DNAPL sump  94 ″ may be positioned below the lower extremity of the PRB  74 ″ to provide a larger sump volume while avoiding contamination of the lower portion of the PRB  74 ″. However, the DNAPL sump  94 ″ does not have to be positioned below the PRB  74 ″. When a predetermined volume of DNAPL  66  has collected, or upon a predetermined time interval, or upon another triggering event, the removal device  98  (e.g., pump, vacuum, et cetera) may be fluidly connected to the DNAPL sump  94 ″ and the DNAPL  66  stored therein removed. 
       FIG. 8  shows a groundwater treatment system  300  similar to the groundwater treatment system  50  but wherein the PRB  74 ″′ is spaced from the collection layer  78 ″′. All components are marked the same as discussed above with respect to the groundwater treatment system  50  and referenced with triple prime (′″) reference numbers. 
     The present invention provides devices and methods to inhibit NAPLs from fouling, blocking, or otherwise rendering PRBs ineffective. To this end, a collection layer permeable to the NAPL and the contaminated water is disposed up-gradient (that is, upstream) of the PRB. The collection layer collects the NAPL and includes a collecting element (for example, a sump and/or a baffle) to draw the NAPL away from the collection layer or otherwise inhibit the NAPL from flowing to the PRB. 
     The present device could also be constructed with two separate trenches separated by a nominal distance of native material. The up-gradient trench would be constructed to trap LNAPL and DNAPL, while the down-gradient trench would be constructed with treatment media either installed in the trench or blended into the native soil, as shown in  FIG. 5 . 
     By “water table,” we mean a surface where water pressure equals atmospheric pressure (that is, the “surface” of the body of water). 
     From the above description, it should be apparent that the present invention provides improved devices and methods for preventing NAPL from causing functional failure of a PRB intended for treating target chemicals dissolved in migrating groundwater. 
     The invention has been described in connection with what are presently considered to be the most practical and preferred embodiments. However, the present invention has been presented by way of illustration and is not intended to be limited to the disclosed embodiments. Accordingly, those skilled in the art will realize that the invention is intended to encompass all modifications and alternative arrangements within the spirit and scope of the invention as set forth in the appended claims.