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HYDRO GEO CHEM INC. by www.hgcinc.com Systematic Remedial Methodology for Chlorinated VOC Contamination of Soils and Groundwater Underlying Desert Landfills.

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Presentation on theme: "HYDRO GEO CHEM INC. by www.hgcinc.com Systematic Remedial Methodology for Chlorinated VOC Contamination of Soils and Groundwater Underlying Desert Landfills."— Presentation transcript:

1 HYDRO GEO CHEM INC. by Systematic Remedial Methodology for Chlorinated VOC Contamination of Soils and Groundwater Underlying Desert Landfills Harold Bentley, Stewart Smith Hydro Geo Chem, Inc. Tucson and Scottsdale, Arizona Presented at the Desert Remedial Action Technologies Workshop Phoenix, Arizona October 2-4, 2007

2 HYDRO GEO CHEM INC. First, you’ve got to understand the plumbing! ---King Hubbard, Father of American Hydrogeology This presentation discusses a quantitative in- situ remediation methodology that relies on site-specific evaluation and numerical simulation to 1. develop critical insights regarding the conceptual model of the contamination problem, and 2. to develop and optimize a remedial engineering design that meets corrective action goals at maximum efficiency and minimum expense.

3 HYDRO GEO CHEM INC. Landfill groundwater contamination by volatile chlorinated organic compounds (VCOCs) and Freons is pervasive throughout the desert southwest. As an example of how serious this problem might be, the City of Tucson has concluded, based on a proactive series of extensive remedial investigations, that all of their 40- some unlined or partially-lined landfills have likely contaminated groundwater with VCOCs*. *R. Murray, City of Tucson, personal communication Statement of Problem

4 HYDRO GEO CHEM INC. Conceptual Model of Arid-Landfill Groundwater Contamination by Volatile Chlorinated Organic Compounds (VCOCs)

5 HYDRO GEO CHEM INC. PCE Beneath Silverbell Landfill, Tucson

6 HYDRO GEO CHEM INC. The generally observed distribution of the VCOCs has some interesting characteristics: 1.VCOCs are found up as well as down the groundwater gradient, an observation often interpreted as evidence for an upgradient, non-landfill VCOC source 2.The groundwater plume tends to be depleted in the total organic carbon and semi-volatiles usually associated with landfill leachate. (deprived of electron donors and the resulting anaerobic biodegradation, the VCOC plume will be persistent).

7 HYDRO GEO CHEM INC. The generally observed distribution of the VCOCs has some interesting characteristics: 3.The vadose zone (soil gas) concentrations of VCOCS typically increase with depth, suggesting that the source is at depth rather than in the landfill itself 4. The groundwater concentrations of VCOCS are highest at the water table and decrease with depth, implying that the VCOC source is above the water table

8 HYDRO GEO CHEM INC. QUESTIONS: 1. Can The Observed Soil and Water VCOC (PCE) Distribution Beneath a Landfill Result From Vapor Phase Movement of PCE Introduced into the Landfill When Active? 2. If So, What is the Most Cost Effective Way to Remove this VCOC Source?

9 HYDRO GEO CHEM INC. Using Field Data and Numerical Simulation to Evaluate the Conceptual Model of VCOC Groundwater Contamination

10 HYDRO GEO CHEM INC. The Numerical Model and its Assumptions Model Code: TRAMP (Bentley and Travis, 1989) 3-D variably saturated 2-Phase flow and transport uniform steady groundwater flow aerobic/anaerobic biodegradation VCOC (PCE) initially within Landfill only Model parameters derived from site-specific permeabilities, porosities, water content; literature values of PCE anaerobic biodegradation rates Transport Conditions: Advection resulting from landfill gas generation Diffusion Anaerobic biodegradation of PCE in landfill Run model for 20 years after Silverbell landfill closure

11 HYDRO GEO CHEM INC. Roger Rd Grant Rd Prince Rd Flowing Wells Rd Silverbell Rd N Plan View of 3-dimensional Model Grid 5000 feet GROUNDWATER FLOW DIRECTION

12 HYDRO GEO CHEM INC. Simulated Silverbell Landfill PCE Movement: After 2 Years Water table Direction of Groundwater flow

13 HYDRO GEO CHEM INC. Simulated Silverbell Landfill PCE Movement: After 20 Yrs Water table Direction of Groundwater flow

14 HYDRO GEO CHEM INC. Measured and Simulated Silverbell Landfill PCE Concentrations 20 Years after Closure

15 HYDRO GEO CHEM INC. Results of Silverbell Landfill Gas-Phase PCE Transport Simulations 20 years after closure the original source area (the landfill) is relatively free of VCOC while deep vadose soils continue to have relatively high levels of PCE. Gas and liquid advection and diffusion and anaerobic biodegradation are all found to be important in reducing landfill PCE concentrations and increasing concentrations at depth. We conclude that vadose-zone VCOCs beneath the landfill are the source of past and continuing groundwater contamination.

16 HYDRO GEO CHEM INC. SVE is clearly the most cost-effective VCOC source removal option Contamination is deep and covers a large area Vadose soils have a relatively high gas permeability Contaminants are volatile and therefore amenable to removal by SVE How best to implement SVE removal of the VCOC source is the rest of this presentation.

17 HYDRO GEO CHEM INC. Systematic Remediation of Deep Vadose-Zone PCE Contamination (Harrison Road Landfill, Tucson, Arizona)

18 HYDRO GEO CHEM INC. Three-Dimensional Model Structure Harrison Road Landfill, looking southwest LFG Wells SVI-1: Multi-level nested probes SVE-1: Multi-level nested probes VMW Multi- level nested probes Base of Landfill

19 HYDRO GEO CHEM INC. Technical Issues Regarding Sub-Landfill SVE for Removing Deep VCOCs Must allow for LFG generation in overlying landfill Must operate in conjunction with LFG collection system Must minimize air intrusion into landfill to prevent fires and maintain methanogenic efficiency An important goal is to minimize number of SVE wells. Cost of an SVE well, at this site, is greater than $50,000

20 HYDRO GEO CHEM INC. SVE Design Performance Criteria Provide early removal of deep vadose VCOC source to groundwater Minimize drawing VCOCs from shallower soils to deeper soils Minimize air intrusion into the overlying landfill to maintain the landfill’s anaerobic character BONUS: Removal of low-volatility organics from any leachate present beneath landfill by means of aerobic biodegradation (bioventing)

21 HYDRO GEO CHEM INC. Summary of Data Needed for Effective Design of Sub-Landfill SVE Horizontal and Vertical Soil Air Permeability Provide Achievable Subsurface Air Circulation Rate per SVE Well Determine Minimum Number of SVE Wells Needed to Achieve Desired Total Air Circulation Rate. Ratio of Horizontal to Vertical Air Permeability Affects Surface Leakage and Lateral Effectiveness of Individual SVE Wells

22 HYDRO GEO CHEM INC. Summary of Data Needed for Effective Design of Sub-Landfill SVE Subsurface VCOC Distribution Affects Location and Vertical Placement of SVE wells Soil Porosity, Moisture Content, Organic Carbon Content and VCOC Properties Affect Calculation of Total VCOC Mass and Cleanup Times Distributed Landfill Gas Generation Rates Affect induced SVE flow field

23 HYDRO GEO CHEM INC. Hydro Geo Chem’s Pneumatic Assessment Toolbox The numerical model TRAMP, a powerful 3-D integrated finite difference, distributed parameter model for assessing gas and liquid fate and transport. Includes aerobic and anaerobic biodegradation, thermodynamics, and liquid/gas phase changes. Capable of automatic parameter estimation and design optimization. ASAP, a proprietary pneumatic well test interpretation model. Includes automatic parameter estimation. The Baro-Pneumatic Method, an HGC-patented methodology for assessing landfill gas generation rates and permeabilities. Provides calibration data for numerical (TRAMP) model of landfill gas flow, useful for designing efficient gas collection and control systems.

24 HYDRO GEO CHEM INC. Obtaining Horizontal and Vertical Gas Permeabilities Conduct Pneumatic Well Tests Install wells as part of a trial SVE system Employ step-tests to determine well efficiencies Utilize monitoring wells, if possible, as observation wells Conduct Baro-pneumatic Tests Monitor barometric pressure and subsurface pressure responses to changes in barometric pressure. Obtain vertical permeabilities and LFG generation rates

25 HYDRO GEO CHEM INC. SVE Wellhead Construct Monitoring Well that can Later Double as SVE or SVI Well

26 Gas Porosity and Horizontal Permeability Estimates by Step-drawdown Test. (ASAP analysis) Gas extraction well SVE-A4; Observation well MP-12 WELL TEST RESULTS Pumping well screened 15’-30’ Monitoring well screened 15’-17’ Wells 24 feet apart Gas pumping rates 30, 80 scfm Horizontal permeability 25.1 darcies Vertical permeability 0.40 darcies Cover permeability = darcies Gas Porosity = 0.25 Started Pumping At 30 SCFM Increased Pumping to 80 SCFM Stopped Pumping

27 HYDRO GEO CHEM INC. 2700’ AMSL 2530’ AMSL 2630’ AMSL 2550’ AMSL K h = 15.0 darcies K v = 1.5 darcies Porosity = 35% K h = 15.0 darcies K v = 1.5 darcies Porosity = 35% K h = 15.0 darcies K v = 1.5 darcies Porosity = 35% K h = 20.0 darcies K v = 2.0 darcies Porosity = 35% K h = darcies K v = 15.0 darcies Porosity = 45% K h = 4.0 darcies K v = 4.0 darcies Porosity = 35% K h = 15.0 darcies K v = 1.5 darcies Porosity = 35% Landfill Landfill Cap SVI-1 SVE-1 VMW SVI-1 Screen Interval (203’-283’) SVI-1 Screen Interval (175’-180’) SVI-1 Screen Interval (95’-100’) SVI-1 Screen Interval (135’-140’) SVI-1 Screen Interval (45’-50’) SVE-1 Screen Interval 75’ -80’ SVE-1 Screen Interval 130’ - 135’ SVE-1 Screen Interval 150’ - 200’ SVE-1 Screen Interval 240’ - 245’ VMW - 1A Screen Interval (85’ - 90’) VMW - 1B Screen Interval (135’ - 140’) 1/14/99 Permeability Distribution for the Harrison Landfill Model Note: K H = horizontal permeability K V = vertical permeability H:\69200\Figures\Permeability Distribution.ppt

28 The Baro-Pneumatic Method: Measured Vertical Permeability and LFG Generation Rate at SVI-1, Harrison Landfill, Tucson LFG = 740 cfm K v (vertical permeability = 15 darcies (.015 cm/sec) φ g (gas porosity) ~ 0.24

29 HYDRO GEO CHEM INC. SVE Design Parameters Derived from SVE Performance Optimization by Numerical Modeling Need a total of 3 perimeter extraction wells and just one, central air injection well Injection well screened at deeper intervals than extraction wells Injection wells and extraction wells operate at same rate of flow (250 scfm each.

30 HYDRO GEO CHEM INC. Simulation of VCOC Remediation Progress (Month.Year) SVI SVE

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33 HYDRO GEO CHEM INC. Comparing Optimized Harrison Landfill SVE Design Layout to Conventional “Radius of Influence” (ROI) Layout Based on SVE well tests, and interpretation by our well pneumatics software, ASAP, the achievable SVE pumping rate for each well is 250 standard ft3/min (scfm) and the ROI of each well is 200 feet (at a steady-state vacuum of 0.03 inches H 2 O). Setting a well grid at 300 feet between wells provides 25% overlap of the circles defined by a 200-foot ROI. The resulting Harrison Landfill ROI well array is illustrated in the following slide.

34 ● 200-FOOT ROI WELLS (TOTAL OF 30) MODEL-OPTIMIZED SVI/SVE WELL LOCATIONS (TOTAL OF 4) ● ● ● ● VCOC-CONTAMINATED VADOSE ZONE ●  Site-specific and ROI SVE-Well Arrays

35 HYDRO GEO CHEM INC. Cost Comparison of Optimized Harrison SVE Design to Conventional “Radius of Influence” (ROI) Design The SVE well array resulting from the use of a 200- foot ROI is comprised of 30 SVE wells, each costing (in 1999) more than $50,000. Thus the total cost for this conventionally designed system’s well construction exceeds $1.5 million dollars, which compares poorly to the $200,000 well cost for the optimized 4-well system. This estimate does not account for the increased costs associated with valving, sumps, gas collection manifolds, and O&M of the more complex ROI- designed system.

36 HYDRO GEO CHEM INC. Piping (PVC,ABS,Steel, HDPE) SVE Blower (Lampson spark-proof) Injection Blower (Roots positive displacement) Electricity (NEMA4 Enclosures) Sumps and Off-gas Treatment Trenching and Cover SVE Engineering Design

37 HYDRO GEO CHEM INC. SVE System Construction

38 HYDRO GEO CHEM INC. HDPE Welding

39 HYDRO GEO CHEM INC. Drainage Crossing Design Details

40 HYDRO GEO CHEM INC. 1,000 scfm SVE Installation

41 HYDRO GEO CHEM INC. PCE Reduction in Deep Soils (VMW-2)

42 HYDRO GEO CHEM INC. PCE Reduction in Vadose-Zone Soils (VMW-1)

43 HYDRO GEO CHEM INC. PCE Reduction in Harrison Rd. Groundwater (WR-348A)

44 HYDRO GEO CHEM INC. Mass Removed: Source Removal Completed as of 8/2002; SVE System Shut Down Deep SVE System Performance at Harrison Landfill

45 HYDRO GEO CHEM INC. Conclusions Most unlined arid-zone landfills have and are contaminating groundwater with VCOCs. Landfill-origin chlorinated VCOCs reach groundwater via gas phase transport. Removal of the landfill origin vadose zone VCOCs sources can be accomplished by SVE. Using pneumatic data collection and SVE simulation to develop the engineering design results in Significant capital and operational cost savings High collection system efficiency and more rapid remediation

46 HYDRO GEO CHEM INC. Suggested Further Reading Can also be found in: Walter, Gary R Fatal Flaws in Measuring Landfill Gas Generation Rates by Empirical Well Testing [PDF] J. Air & Waste Management, , p 461Fatal Flaws in Measuring Landfill Gas Generation Rates by Empirical Well Testing [PDF] Bentley, H.W., S. Smith, J. Tang, and G.R. Walter A Method for Estimating the Rate of Landfill Gas Generation by Measurement and Analysis of Barometric Pressure Waves. Proceedings of the 18th International Conference on Solid Waste Technology and Management, Philadelphia, Pennsylvania, March , 2003 Walter, G.R., Geddis, A.M., Murray, R., Bentley, H.W Vapor Phase Transport as a Groundwater Contamination Process at Arid Landfill Sites [PDF]. Proceedings of the 18th International Conference on Solid Waste Technology and Management, Philadelphia, Pennsylvania, March 23-26, 2003Vapor Phase Transport as a Groundwater Contamination Process at Arid Landfill Sites [PDF] Bentley, H.W., S.J. Smith, and T. Schrauf Baro-pneumatic Estimation of Landfill Gas Generation Rates at Four Operating Landfills. Proceedings, SWANA’s 28th Annual Landfill Gas Symposium, March 7-10, 2005 Smith, S.J., H.W. Bentley, and K. Reaves Systematic Design of Methane Migration Control Systems. Proceedings, 29th Annual SWANA Landfill Gas Symposium, St. Petersburg FL, March pp.

47 HYDRO GEO CHEM INC. Contact Information Harold W. Bentley, Ph.D. Principal Scientist Hydro Geo Chem, Inc. 51 W. Wetmore Road Ste 101 Tucson, AZ Phone: x 111 Cell: FAX: Website: Stewart Smith, MS. Associate Hydrogeologist Hydro Geo Chem, Inc. 51 W. Wetmore Road Ste 101 Tucson, AZ Phone: x 111 FAX: Website:


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