Abstract:
The application relates to a sustainable and green remedial approach for in situ remediation. The system and method use directionally drilled horizontal wells filled with granular reactive media generally installed in the direction of groundwater flow for groundwater remediation. “Flow-focusing” behavior is leveraged to capture and passively treat proportionally large volumes of groundwater in situ. The system and method perform well in low hydraulic conductivity environments. Reactive media are selected according to the contaminants to be treated and site characteristics The application further relates to a source area bypass system comprising one or more horizontal wells, constructed in a manner to allow unimpacted or cleaner groundwater to bypass a nonaqueous phase liquid zone or high-concentration source area of contamination and discharge downgradient. Reactive media may or may not be used in a source area bypass system. In some configurations, groundwater pumps may be installed to enhance performance.

Description:
FIELD OF THE INVENTION 
     The application relates to a sustainable and green remedial approach for in situ remediation. The system and method use directionally drilled horizontal wells filled with granular reactive media generally installed in the direction of groundwater flow for groundwater remediation. “Flow-focusing” behavior is leveraged to capture and passively treat proportionally large volumes of groundwater in situ. The system and method perform well in low hydraulic conductivity environments where the success of other in situ remediation methods is controlled by aquifer injectability. Reactive media are selected according to the contaminants to be treated and site characteristics. Energy conservation and other considerations result in considerable cost savings compared to current in situ remediation systems. According to another embodiment of the invention, a source area bypass comprises one or more horizontal wells, constructed in a manner to allow unimpacted or cleaner groundwater to bypass a nonaqueous phase liquid zone or high-concentration source area of contamination and discharge downgradient. Reactive media may or may not be used in a source area bypass. In some configurations, groundwater pumps may be installed to enhance performance. Core elements of green remediation according to the United States Environmental Protection Agency are achieved. 
     BACKGROUND OF THE INVENTION 
     During the past two decades the science and implementation of in situ groundwater remediation through the subsurface injection of liquid remedial agents has advanced substantially, and these techniques are now successfully employed in many settings. Most commonly, these strategies involve the injection of a specialized reagent solution into an aquifer in order to create a zone within the aquifer where enhanced biotic and/or abiotic reactions occur in situ to detoxify or destroy target contaminants. This general strategy is often referred to as an in situ reactive zone (IRZ) approach. 
     Enhanced reductive dechlorination (ERD) is one of the most widely-applied methods for treating chlorinated volatile organic compound (CVOCs) contaminants. In practice, ERD typically involves injecting an electron donor (usually a biodegradable form of organic carbon such as molasses, corn syrup, or vegetable oil) into the subsurface. Other similar commonly-employed strategies include the use of electron donors to support the biologically-mediated in situ conversion and precipitation of hexavalent chromium to trivalent chromium, and the injection of various chemical oxidants for the direct oxidation of applicable contaminants. 
     Although several important factors affect the implementability and potential success of an injected remedial fluid-based approach, relatively high hydraulic conductivity site conditions is one of the most important and desired site features, because this property controls the ease at which remedial fluids can be injected. For example, literature describing in situ remediation system design considerations identifies high aquifer hydraulic conductivity as a critical site property and screening criterion. See, e.g., Morse, J. J., Alleman, B. C., Gossett, J. M., Zinder, S. H., Fennelll, D. E., Sewell, G W., and Vogel, C. M., 1998. Draft technical protocol: A Treatability test for evaluating the potential applicability of the reductive anaerobic biological in situ treatment technology (RABITT) to remediate chloroethenes. Prepared for Environmental Security Technology Certification Program (ESTCP); Suthersan, S. S., 2002. Natural and enhanced remediation systems. CRC Press, Lewis Publishers, Boca Raton, Fla., 419 pp.; Gossett, J. M., Zinder, S. H., Fennel, D. E., Morey, C., and Adamson, D. T., 2003. Cost and performance report: Reductive anaerobic biological in situ treatment technology (RABITT) treatability testing. Prepared for Environmental Security Technology Certification Program (ESTCP); Faris, B., Vlassopoulos, D., and ITRC—In Situ Bioremediation Team, 2003. A Systematic approach to in situ bioremediation in groundwater. Remediation Journal 13: 27-52; Payne, F. C., and S. S. Suthersan., 2005. In Situ Remediation Engineering. CRC Press, Boca Raton, Fla. 511 pp; Payne, F. C., J. Quinnan, S. T. Potter. 2008. Remediation Hydraulics. CRC Press, Boca Raton, Fla. 
     In most cases, sites with low natural bulk hydraulic conductivity values (typically less than approximately 5 ft/day) are not considered ideal candidate sites for injected liquid-reagent-based in situ remedial strategies. This is primarily because at these sites, injection times necessary to deliver adequate reagent volume under non-fracturing pressures are prohibitively long. Consequently, many sites which might otherwise be good candidates for these types of strategies (that is, the contaminants are appropriate, geochemical conditions are favorable, and site use/infrastructure is compatible) are eliminated from conventional IRZ approaches or deemed less suitable solely because of the hydraulic challenges associated with remedial amendment delivery and distribution. 
     Injected reagent-based strategies are also challenged by the difficulty in delivering and distributing the reagent to the target zones within the plume. To some degree, this challenge has been overcome by optimizing the delivery and distribution of soluble substrates, through a renewed focus on the fundamental properties that govern groundwater movement and solute distribution at the remedial system scale. For example, it is now understood that large injection volumes are necessary to achieve lateral coverage due to the recognition that transverse dispersion of injected reagent is generally insignificant. See, e.g., Payne et al., 2008, supra. However, an important implication of this concept is that conventional batch-volume injection methods will not be technically-appropriate or cost-effective for large plumes. Large plume treatment with injected fluid reagents can only be practically accomplished when the flow field is artificially controlled by extraction-injection systems that facilitate larger well spacing. See, e.g., Suthersan, S. S., C. E. Divine, and S. T. Potter. 2009. Remediating large plumes: Overcoming the scale challenge. Ground Water Monitoring &amp; Remediation 29, no 1: 34-43. 
     Furthermore, it is increasingly recognized that contaminant mass flux and discharge information provides the most useful measure of plume strength and potential risk to off-site receptors, and remedial approaches that focus on flux will increasingly be preferred to most effectively reduced risk. See, e.g., Nichols, E. M., 2004. In a state of (mass) flux. Ground Water Monitoring &amp; Remediation 24, no. 3: 4-5; Suthersan, S. S., C. Divine, J. Quinnan, and E. Nichols. 2010. Flux-informed remediation decision making Ground Water Monitoring &amp; Remediation 30, no 1: 45-50. 
     Finally, despite the widespread success of injected remedial reagent approaches for select contaminants, there still remain a large number of contaminants (e.g., select metals and radionuclides, polycyclic aromatic hydrocarbons, explosives and energetics, chlorides) that are not treatable, or their treatment is relatively undemonstrated by currently-available liquid-based remedial agents. 
     Additionally, while many soluble substrates (i.e., lactate, molasses and other organic carbon sources) are ideally suited to an injection approach, they are not ideal with respect to reaction rates since their effectiveness in contaminant destruction relies on promoting microbiological activity. 
     The concept of placing reactive media in vertically-oriented passive wells has been previously theoretically evaluated. See, e.g., Wilson, R. D., D. M., Mackay, and J. A. Cherry. 1997. Arrays of unpumped wells for plume migration control by semi-passive in situ remediation. Ground Water Monitoring &amp; Remediation 21, no. 3: 185-193; Hudak, P. F. 2008. Evaluation of reactive well networks for remediating heterogeneous aquifers. J Environ Sci Health 43, no 7: 731-737; Hudak, P. F. 2009. Interior versus exterior configurations of passive wells with filter cartridges for cleaning contaminated groundwater. See Remediation 20, no. 1: 133-141. While theoretically plausible, the concept is not practical or cost-effective for most sites because the treatment widths for individual reactive wells are very small (roughly twice the well diameter) due to negligible flow-focusing, and therefore, many wells would be necessary to achieve a system width appropriate for typical plumes. Furthermore, the in-well residence times for these systems are very short (a few hours to a few days) and this may not be long enough to achieve treatment of some contaminants. 
     Passive reactive barrier (PRB) technology is well-developed and numerous field applications have been installed over the past decade. See USEPA 1999. Field applications of in situ remediation technologies: Permeable reactive barriers. EPA542-R-99-002, April 1999. Generally, PRB design consists of a reactive media-filled trench oriented orthogonal to the groundwater flow direction and is intended primarily to “cut off” the plume and control contaminant mass discharge at a discrete transect location, and many implementations utilize a “funnel-and-gate” system configuration to focus the treatment zone and reduce the volume of reactive media needed. Solids that have been used for in situ remediation include iron particles, limestone rock, oxide minerals, and particulate organics (e.g., mulch). These are generally emplaced in a trench-type PRB across the path of groundwater with the reactive solid media mixed with sand or gravel. However, this approach at applying solids in the treatment of plumes is not ideal: trenching is an expensive and difficult operation, reactive solid media have limited life-span and rehabilitation of the barrier is often not feasible, and often large portions of a plume cannot be treated because of limitations on trenching at a site. Other reactive media are also not available for use in trenches, such as ion-exchange resins and granular activated carbon (solids used successfully in ex-situ applications) due to the media cost and the need for large quantities to fill a trench. 
     Some limited work has been performed to evaluate the hydraulic performance of trench-based PRB systems oriented at non-orthogonal angles. See Edwards, D. A., and V. V. Dick. 1998. Method for directing groundwater flow and treating groundwater in situ. U.S. Pat. No. 5,833,388. Issued November 10; Edwards, D. A., V. V. Dick, J. W. Little, and S. L. Boyle. 2001. Refractive flow and treatment systems: Conceptual, analytical, and numerical modeling. Ground Water Monitoring &amp; Remediation 21, no. 3: 64-70. Edwards et al. present a limited hydraulic capture analysis of a technique termed “refractive flow and treatment” (RFT), which consisted of fully-penetrating trenches installed in bedrock at oblique angles to ambient groundwater flow direction, and filled with highly-permeable inert material. The authors simulated the flowfield and hydraulic capture associated with “X” and chevron-shaped trench orientations and demonstrated that the trenches induce flow-focusing behavior. In concept, this captured water could be routed to a discrete zone where groundwater would be treated in situ by several possible technologies. Very recently, Hudak conducted some limited modeling of hypothetical reactive media-filled fully-penetrating PRBs used to treat the leading edge of synthetic plumes. See Hudak, P. F., 2010. Viability of longitudinal trenches for capturing contaminated groundwater. Bull Environ Contam Toxicol. Therein, Hudak evaluated several alternative orientations and his results suggest that PRB trenches oriented parallel to groundwater flow could result in faster treatment for the entire synthetic plume due to shorter average travel distances to the trench and associated reactive material. 
     While further work is needed to evaluate the potential benefits and the limitations of alternative orientations, there are important cost and implementability limitations associated with conventional PRBs. Trenching is relatively expensive as costs are particularly sensitive to trench depth. Additionally, the practical maximum trench depth attainable by available trenching technology is about 100 ft. Further, there are challenges associated with achieving adequate contaminant residence times for conventional orientations, and the hydraulic performance of many existing systems has been impacted by clogging and reduction in permeability, which can result in undesirable alteration of the flowfield and even plume spreading and bypass around the PRB. Reduction of hydraulic conductivity and reactivity in zero-valent iron columns by oxygen and TNT. Ground Water Mon. &amp; Remd. 25(1): 129-136. Lastly, access issues such as buildings and utilities may limit the use of trenches. See, e.g., Johnson, R. L., P. G. Tratnyek, R. Miehr, R. B. Thoms, and J. Z. Bandstra, 2005. 
     SUMMARY OF THE INVENTION 
     The invention relates to a horizontal treatment (HRX) well system (also known as a horizontal in-well treatment (HIT) remediation system) comprising horizontal wells that can be installed within plumes of contaminated groundwater and oriented in the general direction of groundwater flow. Each well is filled with reactive treatment media (comprising, e.g., iron, granular activated carbon, zeolite). A flow-focusing phenomenon can be created by the high in-well hydraulic conductivity of the well and engineered reactive media relative to the surrounding aquifer hydraulic conductivity that passively draws impacted groundwater into the horizontal wells through a screen disposed substantially at the up-gradient portion of each well such that impacted groundwater is treated as it flows through the horizontal well. Treated groundwater then exits each horizontal well through a screen disposed substantially along the down-gradient section of the well. 
     According to one embodiment, the invention comprises a system for in situ remediation, comprising a plurality of well pipes disposed into an aquifer or plume to be treated for contamination, wherein further at least one part of one well pipe is disposed substantially horizontal to contaminated groundwater flow with respect to vertical and parallel to contaminated groundwater flow when viewed from the ground surface, wherein further at least one well pipe comprises a first section and a second section, wherein the first section comprises a first filter portion and a solid wall portion which is substantially permeable to groundwater flow, wherein the first filter portion is situated up-gradient of the flow of the contaminated groundwater within the aquifer or plume and the second section comprises a second filter portion and a solid wall portion which is substantially permeable to groundwater flow, wherein the second filter portion is situated down-gradient of the flow of the contaminated groundwater; wherein reactive media are placed into the interior of the well pipes, wherein the hydraulic conductivity of the reactive media situated in the interior of the well pipes is greater than the average hydraulic conductivity of the aquifer, wherein a portion of the flowing contaminated groundwater funnels into the first filter of the first section and exits through the second filter of the second section, wherein further the contaminated groundwater is treated by the reactive media as it passes through the interior of the well pipes. In one embodiment, the first section of the well pipe and the second section of the well pipe are disposed to form a “V” shaped or elongated “U” shaped well within the aquifer. In one embodiment, the well pipe further comprises a third section between the first section and the second section disposed substantially horizontal to the flow of groundwater in the contaminated aquifer or plume. In one embodiment, the third section comprises a solid section substantially impermeable to the contaminated groundwater or other environmental effects. 
     According to the method of the invention, a plume to be remediated is identified. A HRX well system comprising a plurality of wells having up-gradient and down-gradient screens or filters are installed such that they intersect the groundwater flow of the plume. The HRX well system details are designed depending on site conditions and specific design requirements, including but not limited to the HRX well system treatment width; the total contaminant mass to be treated; and contaminant concentration and mass reduction goals down-gradient of the HRX well system. Known horizontal directional drilling techniques can be used to drill and install the HRX wells. Reactive media can be placed in the interior of the HRX wells either before or after they are installed. 
     According to one embodiment, the invention comprises a method for in situ remediation, comprising: identifying an aquifer to be remediated; determining site qualities of the aquifer, including average hydraulic conductivity and the rate and direction of flow of contaminated groundwater through the aquifer; drilling a plurality of boreholes from the ground surface into the aquifer; inserting a plurality of well pipes into the boreholes, wherein further at least one part of one well pipe is disposed substantially horizontal to contaminated groundwater flow with respect to vertical and parallel to contaminated groundwater flow when viewed from the ground surface, wherein further at least one well pipe comprises a first section and a second section, wherein the first section comprises a first filter portion and a solid wall portion which is substantially permeable to groundwater flow, wherein the first filter portion is situated up-gradient of the flow of the contaminated groundwater; and the second section comprises a second filter portion and a solid wall portion which is substantially permeable to groundwater flow, wherein the second filter portion is situated down-gradient of the flow of the contaminated groundwater; and thereafter emplacing reactive media into the interior of the well pipes, wherein the hydraulic conductivity of the reactive media situated in the interior of the well pipes is greater than the average hydraulic conductivity of the aquifer, wherein a portion of the flowing contaminated groundwater funnels into the well pipes through the first filter of the first section, passes through the interior of the well pipes and exits through the second filter of the second section, wherein further the contaminated groundwater is treated by the reactive media as it passes through the interior of the well pipes. In one embodiment, the first section of the well pipe and the second section of the well pipe are disposed to form a “V” shaped or elongated “U” shaped well within the aquifer. In one embodiment, the well pipe further comprises a third section between the first section and the second section disposed substantially horizontal to the flow of groundwater in the contaminated aquifer or plume. In one embodiment, the third section comprises a solid section substantially impermeable to the contaminated groundwater or other environmental effects. 
     In another embodiment of the invention, a source area bypass (SAB) system comprises one or more horizontal wells, constructed in a manner to allow un-impacted or cleaner groundwater to bypass a non-aqueous phase liquid (NAPL) zone or high-concentration source area of contamination and discharge down-gradient of the contamination area. Reactive media may or may not be used in a SAB system. In some configurations, groundwater pumps may be installed to enhance performance. 
     The HRX well system compares favorably in sustainability analyses relative to conventional remedial approaches. Because the HRX well system operates passively in situ, recurring and cumulative energy requirements and carbon footprint are believed to be much lower compared to many other remedial alternatives. Additionally, recurring material use and waste generation are believed to be much lower, and the life-cycle water consumption would be essentially zero. Because the system is in situ and therefore requires minimal above-ground infrastructure and surface access, retention and potential enhancement and incorporation of natural resources would be high. Relative safety would be high due to the reduced number of wells required (i.e., fewer drilling locations and less drilling time), minimal above-ground construction (and associated electrical requirements and heavy equipment activity), absence of electrical or other power requirements, and the reduced site activities and operation and maintenance visits. Finally, the life-cycle costs for a HRX well system would be substantially lower than most conventional alternative remedial strategies, particularly if remedial performance goals are focused on mitigating risk through reducing contaminant mass flux and discharge. Although approximate, HRX well technology is a sustainable and green remedial approach. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The invention will be described with reference to the accompanying drawings, in which like elements are referenced with like numerals. 
         FIG. 1  provides an illustration of a HRX well system according to one embodiment of the invention. 
         FIG. 2  depicts a comparison of calculated treatment widths for individual HRX wells with varying pipe diameters oriented parallel to groundwater flow direction for aquifers of varying hydraulic conductivity. 
         FIG. 3A  depicts a cross-sectional view of an example model output including a HRX well system configuration, simulated groundwater elevation contours, and simulated concentrations of a representative contaminant plume. 
         FIGS. 3B and 3C  depict 3-dimensional model output (groundwater elevations and contaminant concentration distributions) in plan-view showing treated water exiting a HRX well system at the end of a 900-day simulation for parallel flow conditions according to one embodiment of the invention. 
         FIG. 4A  is a side-view (top portion) and a plan-view (bottom portion) depiction from a 3-dimensional numerical model output of the predicted potentiometric groundwater elevation caused by a HRX well system having a total length of 70 feet and hydraulic conductivity at a ratio of 100:1 for the reactive media within the well compared to the average hydraulic conductivity of the aquifer. 
         FIG. 4B  is a side-view (top portion) and a plan-view (bottom portion) depiction of the predicted treatment area of the HRX well system of  FIG. 4A  after 1 year. 
         FIG. 5A  is a side-view (top portion) and a plan-view (bottom portion) depiction from a 3-dimensional numerical model output of the predicted potentiometric groundwater elevation caused by a HRX well system having a total length of 70 feet and hydraulic conductivity at a ratio of 1000:1 for the reactive media within the well compared to the average hydraulic conductivity of the aquifer. 
         FIG. 5B  is a plan-view depiction of the predicted treatment area of the HRX well system of  FIG. 5A  after 1 year. 
         FIG. 6A  is a side-view (top portion) and a plan-view (bottom portion) depiction from a 3-dimensional numerical model output of the predicted potentiometric groundwater elevation caused by a HRX well system having a horizontal (flat bottom) length of 200 feet and hydraulic conductivity at a ratio of 1000:1 for the reactive media within the well compared to the average hydraulic conductivity of the aquifer. 
         FIG. 6B  is a side-view (top portion) and a plan-view (bottom portion) depiction of the predicted treatment area of the HRX well system of  FIG. 6A  after 1 year. 
         FIG. 7  depicts predicted relative contaminant concentration versus time downstream of an HRX well system for well orientation parallel to the general groundwater flow direction. 
         FIG. 8A  conceptually depicts the cumulative cost of a HRX well system compared to a pump-and-treat system over time;  FIG. 8B  conceptually depicts the cumulative water use of a HRX well system compared to a pump-and-treat system over time;  FIG. 8C  conceptually depicts the cumulative carbon footprint of a HRX well system compared to a pump-and-treat system over time; and  FIG. 8D  conceptually depicts the cumulative energy consumption of a HRX well system compared to a pump-and-treat system over time. 
         FIG. 9  provides an illustration of a SAB system according to one embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention relates to a horizontal treatment (HRX) well system (also known as a horizontal in-well treatment (HIT) remediation system) comprising horizontal wells that are installed within plumes of contaminated groundwater and oriented in the general direction of groundwater flow. Each well is filled with reactive treatment media (comprising, e.g., iron, granular activated carbon, zeolite). A flow-focusing phenomenon is created by the high in-well hydraulic conductivity of the well and engineered reactive media relative to the surrounding aquifer hydraulic conductivity that passively draws impacted groundwater into the horizontal wells through a screen disposed substantially at the up-gradient portion of each well such that impacted groundwater is treated as it flows through the horizontal well. Treated groundwater then exits each horizontal well through a screen disposed substantially along the down-gradient section of the well. 
     According to one embodiment, the invention comprises a system for in situ remediation, comprising a plurality of well pipes disposed into an aquifer or plume to be treated for contamination, wherein further at least one part of one well pipe is disposed substantially horizontal to contaminated groundwater flow with respect to vertical and parallel to contaminated groundwater flow when viewed from the ground surface, wherein further the well pipes comprise a first section and a second section, wherein the first section comprises a first filter portion and a solid wall portion which is substantially permeable to groundwater flow, wherein the first filter portion is situated up-gradient of the flow of the contaminated groundwater within the aquifer or plume and the second section comprises a second filter portion and a solid wall portion which is substantially permeable to groundwater flow, wherein the second filter portion is situated down-gradient of the flow of the contaminated groundwater; wherein reactive media are placed into the interior of the well pipes, wherein the hydraulic conductivity of the reactive media situated in the interior of the well pipes is greater than the average hydraulic conductivity of the aquifer, wherein a portion of the flowing contaminated groundwater funnels into the well pipes through the first filter of the first section and exits through the second filter of the second section, wherein further the contaminated groundwater is treated by the reactive media as it passes through the interior of the well pipes. In one embodiment, the first section of the well pipe and the second section of the well pipe are disposed to form a “V” shaped or “U” shaped well within the aquifer. In one embodiment, the well pipe further comprises a third section between the first section and the second section disposed substantially horizontal to the flow of groundwater in the contaminated aquifer or plume. In one embodiment, the third section comprises a solid section substantially impermeable to the contaminated groundwater or other environmental effects. 
     According to one embodiment of the invention, a HRX well comprises a pipe of a diameter selected according to site conditions and specific design requirements. In one embodiment and without limiting the invention in any way, pipe diameters range from 8 inches to 18 inches. The pipes constituting the HRX wells may comprise any suitable material. In one embodiment and without limiting the invention in any way, pipes of the HRX wells are constructed of polyvinylchloride (PVC); other well materials may include high-density polyethylene (HDPE) or stainless steel. 
     A typical HRX well design will either require a pre-pack screen or, under the appropriate conditions, be installed with no sand pack and allow a natural filter pack develop. The pipes of the HRX wells further comprise an up-gradient filter and a down-gradient filter. The up-gradient filter comprises slotted or perforated pipe and is designed to allow groundwater to freely flow into the interior of the well while holding back the sandpack and native aquifer materials. The specific design and construction details of the filter are based on aquifer characteristics and design modeling. The down-gradient filter comprises slotted or perforated pipe and is designed to allow groundwater to freely flow into the well while holding back the sandpack and native aquifer materials. The specific design and construction details of the filter are based on aquifer characteristics and design modeling. 
     Reactive media are placed in the interior of the HRX wells. The reactive media are selected such that a differential is created between the hydraulic conductivity within the horizontal well and the aquifer. This hydraulic conductivity differential creates a “flow focusing” phenomenon which passively draws impacted groundwater into the HRX wells where it is treated with the reactive media as it passes through the HRX well. The residence time of the impacted groundwater within the HRX well can be determined according to site conditions and design requirements. 
     As described by Darcy&#39;s law, the passive flow rate through a horizontal well (Q HRX ) is:
 
 Q   HRX   =K   HRX   πr   HRX   2   i   HRX   (1)
 
where K HRX  is the hydraulic conductivity of the reactive media within the well, r HRX  is the radius of the horizontal well, and i HRX  is the hydraulic gradient along the well (approximately equal to the ambient aquifer hydraulic gradient, i A  for wells that are mostly horizontal and oriented parallel to groundwater flow). The vertically-averaged treatment width (w AVE ) for an individual well within a HRX well system can then be approximated by:
 
                     W   AVE     =       Q   HRX         K   A     ⁢     b   A     ⁢     i   A                 (   2   )               
where K A  is the average hydraulic conductivity of the aquifer and b A  is the aquifer thickness intercepted by the horizontal well. The maximum potential average velocity v AVE  and minimum average residence time T AVE  can be estimated by:
 
                     v   AVE     =       Q   HRX       φ   HRX               (   3   )                 T   AVE     =       L   HRX       v   HRX               (   4   )               
where φ HRX  is the porosity of the reactive media within the horizontal well and L HRX  is the average particle travel length within the he horizontal well. These equations provide only approximate estimates; actual velocity and particle travel distances profiles vary with depth and are influenced by the specific well design and aquifer conditions.
 
     The flow field orientation with respect to the HRX well orientation can be varied according to one embodiment of the invention. The performance metrics for the various angled flow fields presented in Table 1 include well treatment width at the water table and ten feet below the water table, the total contaminant mass treated at the end of 900 days, and the total mass treated at 900 days normalized to the treatment well length. 
                                     TABLE 1                       Treatment       Total Mass           Treatment   Width Ten   Total Mass   Treated per           Width at the   Feet Below   Treated at   Length at       Flow Field   Water Table   Water Table   900 days   900 days       Orientation   (feet)   (feet)   (lbs)   (lbs/ft-day)                   Parallel Flow   49   41   2522   0.0075       22.5 Degrees   44   44   2532   0.0075         45 Degrees   33   42   2346   0.0069       67.5 Degrees   33   33   2144   0.0063         90 Degrees   —   —   1389   0.0044                    
The results indicate greater treatment width and total mass treated is obtained when the treatment well is oriented parallel to the flow field.
 
     Reactive media that can be used in the HRX well system include any granular form of media. Solids promote reactions at the reactive media surface-water interface, and are more attractive than liquid agents as reactive substrates due to their compatibility with a wider range of groundwater chemistry conditions and rapid reactivity. Generally, solid reactive media do not rely on microbial processes to be effective. Solids, emplaced within the HRX wells to promote groundwater contact, are isolated from aquifer soil and generally do not interact with soil, thereby minimizing generation of treatment byproducts (as is often the case for reactive substrates injected directly into the aquifer). Because of its demonstrated ability to treat a broad range of contaminants, iron is one of the most promising reactive media types for this application. Table 2 provides a summary of reactive media available and associated target contaminants that could potentially be used in a HRX well system. 
     
       
         
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Reactive Media Options 
                 Contaminant 
               
               
                   
               
             
             
               
                 Zero valent iron (ZVI) 
                 Chorinated solvents (CVOCs) 
               
               
                   
                 Nitrate 
               
               
                   
                 Perchlorate 
               
               
                   
                 Metals 
               
               
                   
                 Radionuclides 
               
               
                 Bimetallics (ZVI + palladium (Pd), 
                 CVOCs 
               
               
                 platinum (Pt), or nickel (Ni)) 
               
               
                 Granulated activated carbon (GAC) 
                 Halomethanes 
               
               
                 Ion Exchange Resins 
                 Brines 
               
               
                 Zeolite 
                 Radionuclides (including cesium and 
               
               
                   
                 strontium) 
               
               
                 Phosphates 
                 Metals and radionuclides 
               
               
                   
               
             
          
         
       
     
     Other potential reactive media include magnetite, molecular sieve, mulch and apatite. Limestone or magnesium oxide may be used to generate alkalinity to treat acidic groundwater; conversely reactive sulfide minerals (such as iron sulfide) may be used to generate acidity to treat alkaline groundwater. Reactive media can be initially placed and subsequently replaced by the use of a Jet/Vac truck and/or tremie pipe. 
     Treatment widths of tens of feet of a plume per horizontal well are achievable under passive flow conditions in typical site settings, including lower-permeability settings. In contrast to injected fluid-based in situ strategies, the treatment performance of the HRX well system increases with decreasing aquifer hydraulic conductivity (due to the increased permeability contrast and flow-focusing). The results also demonstrate that the average treatment width is sensitive to well diameter and impacted aquifer thickness. Finally, these results are based on passive operation which relies on flow focusing due to the engineered permeability contrast; however, hydraulic capture and treatment width could be significantly increased by installing a pump with the intake at the up-gradient end of the horizontal well and the pumped water pushed through reactive media within the horizontal well (i.e., no effluent is brought to land surface). 
     According to the method of the invention, a plume to be remediated is identified. A HRX well system comprising a plurality of wells having up-gradient and down-gradient screens or filters are installed such that they intersect the groundwater flow of the plume. The HRX well system details are designed depending on site conditions and specific design requirements, including but not limited to the HRX well system treatment width; the total contaminant mass to be treated; and contaminant concentration and mass reduction goals down-gradient of the HRX well system. Known horizontal directional drilling techniques can be used to drill and install the HRX wells. Reactive media can be prepacked or can be placed in the interior of the HRX wells once they are installed. 
     According to one embodiment, the invention comprises a method for in situ remediation, comprising: identifying an aquifer to be remediated; determining site qualities of the aquifer, including average hydraulic conductivity and the rate and direction of flow of contaminated groundwater through the aquifer; drilling a plurality of boreholes from the ground surface into the aquifer; inserting a plurality of well pipes into the boreholes, wherein further at least one part of one well pipe is disposed substantially horizontal to contaminated groundwater flow with respect to vertical and parallel to contaminated groundwater flow when viewed from the ground surface, wherein further at least one well pipe comprises a first section and a second section, wherein the first section comprises a first filter portion and a solid wall portion which is substantially permeable to groundwater flow, wherein the first filter portion is situated up-gradient of the flow of the contaminated groundwater; and the second section comprises a second filter portion and a solid wall portion which is substantially permeable to groundwater flow, wherein the second filter portion is situated down-gradient of the flow of the contaminated groundwater; and thereafter emplacing reactive media into the interior of the well pipes, wherein the hydraulic conductivity of the reactive media situated in the interior of the well pipes is greater than the average hydraulic conductivity of the aquifer, wherein a portion of the flowing contaminated groundwater funnels into the first filter of the first section, passes through the interior of the well pipes and exits through the second filter of the second section, wherein further the contaminated groundwater is treated by the reactive media as it passes through the interior of the well pipes. In one embodiment, the first section of the well pipe and the second section of the well pipe are disposed to form a “V” shaped or elongated “U” shaped well within the aquifer. In one embodiment, the well pipe further comprises a third section between the first section and the second section disposed substantially horizontal to the flow of groundwater in the contaminated aquifer or plume. In one embodiment, the third section comprises a solid section substantially impermeable to the contaminated groundwater or other environmental effects. 
     HRX wells can be installed using currently known horizontal directional drilling (HDD), which is commonly used to install pipelines for water, sewer, gas, and conduits for fiber optics without using the traditional trenching method. Over the past two decades horizontal drilling methods have been used in a wide range of environmental applications. Drilling technology has been adequately developed and is readily available throughout the United States. Emplacement of reactive media in the HRX wells can be accomplished by the use of a tremie pipe or the use of a Jet/Vac truck. 
     Costs to install a HRX well system are expected to be similar to vertical well pump and treat systems. However, like any remediation well, costs to drill and install a HRX well will vary based on the hydrogeologic conditions, the length and diameter of the well, and the materials used to complete the well. 
     A pilot hole can be drilled by mud rotary drilling. In one embodiment, entry is approximately 15° from the ground surface. Once the depth of placement of the horizontal wells within the plume or aquifer is reached, the pilot hole is then reamed to the desired borehole diameter, ranging from 2 inches to 36 inches. Depth is generally not a limitation. The horizontal well pipes may comprise HDPE, SCH40 and SCH80 PVC or stainless steel. The specific HRX well system placement is dependent on the plume characteristics, site geology, and the results of pre-design modeling. 
     A performance monitoring program can comprise the installation of monitoring wells immediately up- and down-gradient of the HRX well system at various locations relative to predicted treatment widths as well as perpendicular to the flow of the treated clean water to monitor hydraulic head and water quality. Additionally, surface and/or downhole Electrical Resistivity (ER) can be used to map the treatment zone because in many applications water quality within the treatment zone may have a significantly reduced electrical conductivity signature due to reduction and removal of sulfate and precipitation of other minerals. To assess performance within the horizontal wells, inexpensive point velocity probes could be placed within the reactive media to quantify flow, velocities, and residence times. The geochemical performance of the reactive media could be confirmed through porewater chemistry collected through permanent sampling ports installed within the horizontal well. Additionally, a series of electrodes could be placed at regular intervals within the horizontal well to easily monitor changes in the reactive media electrical resistivity through time. This information can be used to verify where and how fast geochemical reactions are occurring, and provide information regarding reactive media depletion rates. 
     In another embodiment of the invention, a source area bypass (SAB) system comprises one or more horizontal wells, constructed in a manner to allow unimpacted or cleaner groundwater to bypass a non-aqueous phase liquid (NAPL) zone or high-concentration source area of contamination and discharge down-gradient of the contamination area. Horizontal wells of an SAB system comprise an up-gradient filter and a down-gradient filter. SAB system horizontal wells are either void in the interior of any media or else contain media which have a higher hydraulic conductivity than the groundwater which creates flow funneling of the groundwater through the horizontal wells. The exterior surfaces of the horizontal wells are clad such that they are impermeable to the source area of contamination, thus shielding the groundwater flowing through the horizontal wells from the contaminated source area. 
       FIG. 1  provides an illustration of a HRX well system according to one embodiment of the invention. HRX well system  100  comprises a series of horizontal wells  110   i  disposed within the flow of contaminated groundwater  120  of a plume  130  to be treated. Reactive media  140  are placed inside horizontal wells  110   i . Horizontal well  110   i  comprises an up-gradient filter  150   i  and a down-gradient filter  160   i  which allows contaminated groundwater  120  to flow through the horizontal well  110   i  where it is treated by reactive media  140 . Horizontal well  110   i  further comprises a sandpack  105   i  that may be prepacked or may be allowed to form after installation. Horizontal well  110   i  yet further comprises an up-gradient section  115   i  disposed at an angle in relation to ground surface  145 ; a middle section  125   i  substantially parallel to the flow of the contaminated groundwater  120 ; and a down-gradient section  135   i  disposed at an angle in relation to ground surface  145 . Pump  170  may optionally be disposed within horizontal well  110   i  to increase the flow of groundwater  120  into horizontal well  110   i . 
       FIG. 2  depicts a comparison of calculated treatment widths for individual HRX wells oriented parallel to groundwater flow direction for aquifers of varying hydraulic conductivity. The results demonstrate that the average treatment width is sensitive to well diameter and impacted aquifer thickness. These results are based on passive operation which relies on flow focusing due to the engineered permeability contrast; however, hydraulic capture and treatment width could be significantly increased by installing a pump with the intake at the up-gradient end of the horizontal well and the pumped water pushed through reactive media within the horizontal well (i.e., no effluent is brought to land surface). 
     Equation 2 and the calculated treatment widths shown in  FIG. 2  provide initial insight to HRX well performance; however, they ignore the vertical flow component and do not provide a three-dimensional representation of performance. Therefore, a numerical groundwater flow and contaminant transport model was used to assess the effectiveness of horizontal treatment wells as flow-focusing features for several possible well configurations under a variety of groundwater flow conditions. The main objective of the modeling was to assess the well performance metrics (contaminant mass treated and reduction of down-gradient mass discharge) as a function of the permeability (hydraulic conductivity) contrast between the reactive media in the horizontal well and the surrounding formation; of the flow field orientation with respect to the well orientation; and of the required reactive media residence times. 
     Simulations were performed using MODFLOW, a three-dimensional numerical groundwater flow model. In addition, the modeling codes MODPATH and MT3DMS were used to delineate groundwater flow paths and to simulate the migration and treatment of dissolved-phase contaminants. MODFLOW and associated companion codes simulate all relevant physical processes that control the migration of contaminants in saturated groundwater flow systems. Environmental Visualization System (EVS) was used to generate the enhanced three-dimensional visualizations of select numerical modeling simulations to further support interpretation and effective communication of quantitative model output. 
       FIG. 3A  depicts a cross-sectional view of an example model output including a HRX well system configuration, simulated groundwater elevation contours, and simulated concentrations of a representative simulation. Untreated groundwater is shown at  300 , while treated groundwater after flowing through the HRX well is shown at  310 . 
       FIGS. 3B and 3C  depict 3-dimensional model results of a plan view of treated water exiting a HRX well at the end of a 900-day simulation for parallel flow conditions according to one embodiment of the invention. In  FIG. 3B , background  320  represents untreated water and treated water is shown at  330 . In  FIG. 3C , treated water volume is seen at  340 . These figures show the variation in the distribution of treated water discharged from a HRX well in three dimensions. The model results confirm the treatment widths indicated by Equation 2 and demonstrate that significant contaminant concentration and mass discharge reductions occur almost immediately upon initial contact with the HRX well. In just over one week, concentration of contaminants is calculated to be reduced by an order of magnitude down-gradient of the HRX well. 
       FIG. 4A  is a side-view (top portion) and a plan-view (bottom portion) depiction from a 3-dimensional numerical model output of the predicted potentiometric groundwater elevation and flowfield  400  caused by the HRX well. The HRX well comprises two sections forming a “V” shape and together totaling 70 feet in length, where the hydraulic conductivity of the reactive media within the well to the average hydraulic conductivity of the aquifer (plume) is 100:1. In this simulation, aquifer thickness=25 ft.; groundwater velocity in the aquifer (plume)=0.13 ft/day; and groundwater velocity in the HRX well=8 ft/day. 
       FIG. 4B  depicts 3-dimensional numerical model results of the HRX well shown in  FIG. 4A  after 1 year. This graph is a side-view (top portion) and a plan-view (bottom portion) depiction of the predicted treatment area  410  of the HRX well. 
       FIG. 5A  is a side-view (top portion) and a plan-view (bottom portion) depiction from a 3-dimensional numerical model output of the predicted potentiometric groundwater elevation and flowfield  500  caused by the HRX well. The HRX well comprises two sections forming a “V” shape and together totaling 70 feet in length, where the hydraulic conductivity of the reactive media within the well to the average hydraulic conductivity of the aquifer (plume) is 1000:1. In this simulation, aquifer thickness=25 ft.; groundwater velocity in the aquifer (plume)=0.13 ft/day; and groundwater velocity in the HRX well=42 ft/day. As compared to the simulation conditions used to generate  FIG. 4A , there is a greater effect on the groundwater flowfield and the treatment width is greater. 
       FIG. 5B  depicts 3-dimensional numerical model results of the HRX well shown in  FIG. 5A  after 1 year. This graph is a side-view (top portion) and a plan-view (bottom portion) depiction of the predicted treatment area  510  of the HRX well. As compared to the simulation conditions used to generate  FIG. 4B , there is a greater effect on the groundwater flowfield  500  and the treatment width is greater. 
       FIG. 6A  is a side-view (top portion) and a plan-view (bottom portion) depiction from a 3-dimensional numerical model output of the predicted potentiometric groundwater elevation and flowfield  600  caused by the HRX well. The HRX well comprises three sections comprising two sections disposed at an angle to the ground surface and one 200 foot long section disposed substantially horizontal to the ground surface and the flow of contaminated groundwater within the aquifer or plume, where the hydraulic conductivity of the reactive media within the well to the average hydraulic conductivity of the aquifer (plume) is 1000:1. In this simulation, aquifer thickness=25 ft.; groundwater velocity in the aquifer (plume)=0.13 ft/day; and groundwater velocity in the HRX well=100 ft/day. As compared to the simulation conditions used to generate  FIG. 5A , the well geometry is different, resulting in changes to the groundwater flowfield  600 . 
       FIG. 6B  depicts 3-dimensional numerical model results of the HRX well shown in  FIG. 6A  after 1 year. This graph is a plan-view depiction of the predicted treatment area  610  of the HRX well. As compared to the simulation conditions used to generate  FIG. 5B , the effects on the groundwater flowfield  600  and HRX well treatment zone are somewhat different. 
       FIG. 7  depicts predicted relative contaminant concentration versus time downstream of an example HRX well for a well orientation parallel to the general groundwater flow direction. 
       FIG. 8A  conceptually depicts the cumulative cost of an example HRX well system compared to a pump-and-treat system over time;  FIG. 8B  conceptually depicts the cumulative water use of a HRX well system compared to a pump-and-treat system over time;  FIG. 8C  conceptually depicts the cumulative carbon footprint of a HRX well system compared to a pump-and-treat system over time; and  FIG. 8D  conceptually depicts the cumulative energy consumption of a HRX well system compared to a pump-and-treat system over time. 
       FIG. 9  provides an illustration of a SAB system according to one embodiment of the invention. SAB system  900  comprises a series of horizontal wells  910   i  disposed in a manner to allow unimpacted or cleaner groundwater  920  to bypass a non-aqueous phase liquid (NAPL) zone or high-concentration source area of contamination  930  and discharge down-gradient of the contamination area  930 . Reactive media (not shown) may be placed inside horizontal well  910   i . Horizontal well  910   i  comprises an up-gradient filter  950   i  and a down-gradient filter  960   i  and is clad such that it is impermeable to the non-aqueous phase liquid (NAPL) zone or high-concentration source area of contamination  930 . Horizontal well  910   i  further comprises a sandpack  905   i  that may be prepacked or may be allowed to form after installation. Horizontal well  910   i  yet further comprises an up-gradient section  915   i  disposed at an angle in relation to ground surface  945 ; a middle section  925   i  substantially parallel to the flow of the unimpacted or cleaner groundwater  920 ; and a down-gradient section  935   i  disposed at an angle in relation to ground surface  945 . Pump  970  may optionally be disposed within horizontal well  910   i  to increase the flow of groundwater  920  into horizontal well  910   i . 
     The foregoing embodiments have been presented for the purpose of illustration and description only and are not to be construed as limiting the scope of the invention in any way.