Advanced Methods for Mass Flux Characterization in NAPL Zones - - PowerPoint PPT Presentation

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Advanced Methods for Mass Flux Characterization in NAPL Zones - - PowerPoint PPT Presentation

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Advanced Methods for Mass Flux Characterization in NAPL Zones Presented by Michael J. Gefell (Anchor QEA, LLC) Colorado Environmental Management Society January 8, 2019 Advanced Methods for Mass Flux Characterization in NAPL Zones 1

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Advanced Methods for Mass Flux Characterization in NAPL Zones Presented by Michael J. Gefell

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Advanced Methods for Mass Flux Characterization in NAPL Zones

Presented by Michael J. Gefell (Anchor QEA, LLC)

Colorado Environmental Management Society January 8, 2019

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Advanced Methods for Mass Flux Characterization in NAPL Zones Presented by Michael J. Gefell

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  • Anchor QEA Innovation Program
  • Kevin Russell, Mark Mahoney, Dimitri Vlassopoulos,

and Masa Kanematsu (Anchor QEA, LLC)

  • David S. Lipson (HRS Water Consultants)

Acknowledgements

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Advanced Methods for Mass Flux Characterization in NAPL Zones Presented by Michael J. Gefell

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  • Dense nonaqueous phase liquids (DNAPLs)

– Chlorinated solvents (TCE, PCE, TCA, and methylene chloride) – Coal tar and oil tar – Wood-treating materials (creosote and PCP) – PCBs – Some pesticides

  • Light nonaqueous phase liquids (LNAPLs)

– Most petroleum hydrocarbons

  • Very common to have one or more of these at

hazardous waste sites

Nonaqueous Phase Liquids

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Advanced Methods for Mass Flux Characterization in NAPL Zones Presented by Michael J. Gefell

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Generic NAPL Zone and Dissolved Phase Plume

Source: U.S. Environmental Protection Agency, 2009. Assessment and Delineation of DNAPL Source Zones at Hazardous Waste Sites. EPA/600/R-09/119. September 2009.

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Advanced Methods for Mass Flux Characterization in NAPL Zones Presented by Michael J. Gefell

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  • If not properly characterized and accounted for,

migrating NAPL or dissolved chemicals can:

– Pose an ecological or human health risk that does not yet exist – Cause a remedy to fail

  • Our focus today

– Can NAPL move? If so, what is the NAPL mass flux? – What are the dissolved-phase concentrations of NAPL components?

  • We want quantitative answers

Why NAPL Mobility and Dissolved Phase Mass Flux Matter

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Near-Shore, Bank, and Subaqueous NAPL – Little Room for Error

Shoreline Bank Subaqueous

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Advanced Methods for Mass Flux Characterization in NAPL Zones Presented by Michael J. Gefell

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NAPL Mobility

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Advanced Methods for Mass Flux Characterization in NAPL Zones Presented by Michael J. Gefell

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Residual and Pooled NAPL – Crucial Distinction for NAPL Mobility

  • Residual

– Immobile – Small, disconnected droplets and ganglia – Does not flow into well – <15% to 25% of porosity NAPL-filled

  • Pooled

– Potentially mobile – Flows into well or borehole – >15% to 25% of porosity NAPL-filled – Stability of NAPL pools can be tenuous – Can be remobilized by water pumping

  • r open boreholes

Experimental photographs

(after Schwille, F., 1988. Dense Chlorinated Solvents, Lewis Publishers, Chelsea Michigan, 146 p.)

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Advanced Methods for Mass Flux Characterization in NAPL Zones Presented by Michael J. Gefell

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  • Centrifuge

– Relatively low cost – 1 gravity ≈ hydraulic gradient of 1 – 1,000 gravities ≈ hydraulic gradient of 1,000

  • Water-drive

– Rigid wall (intermediate cost) – Flexible wall (higher cost)

  • Tests can have multiple steps with increasing

centrifuge spin rate or water injection rate

Laboratory NAPL Mobility Testing

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Advanced Methods for Mass Flux Characterization in NAPL Zones Presented by Michael J. Gefell

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UV light – NAPL fluoresces white light – NAPL has natural color

  • Typically 2 inches long by 1.5-inch

diameter

  • Often selected based on core

photography

  • Usually highest apparent NAPL

saturation

Laboratory NAPL Mobility Test Samples

Photograph courtesy of PTS Laboratories (Houston, Texas)

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Advanced Methods for Mass Flux Characterization in NAPL Zones Presented by Michael J. Gefell

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  • To complete tests in a reasonable time frame,

laboratory test gradients are often much stronger than field conditions

  • Centrifuge typically 10G to 1,000G
  • Water-drive hydraulic gradients up to 100s

Laboratory Test Gradients Extremely High

If no NAPL is produced from sample, NAPL is residual (immobile), but what if some NAPL is produced under these test conditions?

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Interpreting Laboratory NAPL Mobility Test Results

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  • Calculate NAPL effective hydraulic conductivity
  • Distinguish residual from pooled NAPL using

multiple lines of evidence

Interpreting NAPL Mobility Test Results

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  • Darcy’s Law
  • Kn = Qn / (Ai)

Qn = avg. NAPL flow rate = ΔVn / t [L3/T] A = cross sectional area for flow [L2] i = lab test hydraulic gradient [L/L]

  • Kn accounts for:

– Soil/sediment pore sizes – NAPL viscosity – NAPL saturation – NAPL relative permeability

NAPL Effective Hydraulic Conductivity (Kn)

Source: Gefell, M.J., K. Russell, and M. Mahoney, 2018. “NAPL Hydraulic Conductivity and Velocity Estimates Based on Laboratory Test Results.” Groundwater 56(5): 690–694.

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Multiple Lines of Evidence

Line of Evidence Potentially Mobile Residual (Immobile)

  • 1. NAPL produced

during first test step (multi-step test) Yes Conservative – test gradient much higher than ambient No

  • 2. NAPL produced

during 1,000xG centrifuge (single- step test only) No

  • 3. Calculated NAPL-

effective hydraulic conductivity >10-7 cm/sec <10-7 cm/sec Effectively immobile

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Multiple Lines of Evidence (cont.)

Line of Evidence Potentially Mobile Residual (Immobile)

  • 4. Decrease in

NAPL saturation during test >10% of initial saturation value <10% of initial saturation value NAPL at or very near depletion to residual

  • 5. Initial NAPL

saturation >30% Near upper end of literature range for residual saturation <10% Near low end of literature range for residual saturation

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  • Assign points for potentially mobile and residual
  • Most points “wins”
  • Ties go to potentially mobile NAPL interpretation (to

be conservative)

  • Remember to include a reasonable number of tests

and test the most notable NAPL-containing soil or sediment

Binary Decision For Each Test Sample

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  • If all NAPL mobility tests indicate NAPL is residual (i.e.,

immobile), NAPL mass flux is interpreted as zero

  • If some tests indicate potentially mobile NAPL, calculate

potential NAPL mass flux (dMn /dt) and pore velocity (vn) in the field dMn /dt = Qn ρn = Kn in A ρn vn = Kn in / (nS)

NAPL Mass Flux and Velocity

Qn = volumetric NAPL flow rate [L3/T] ρn = NAPL density [M/L3] in = net gradient in the field (includes hydraulic gradient and for vertical movement the “gradient due to gravity”) A = area of potential NAPL flow perpendicular to flow direction [L2] n = porosity S = NAPL saturation

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Dissolved Concentration Measurements in NAPL Zones – Avoiding False Positives

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NAPL Can Exaggerate “Aqueous” Concentrations

  • NAPL enters push-point samplers

and wells

  • NAPL coats hydrophobic passive

samplers

  • Aqueous concentrations calculated

from soil or sediment samples can exceed effective solubility

  • NAPL can cause reported or

inferred dissolved concentrations to be biased high—above true dissolved concentrations

Bottom figure source: Wilson, J.L., S.H. Conrad, W.R. Mason, W. Peplinski, and E. Hagan, 1990. Laboratory Investigation of Residual Liquid Organics from Spills, Leaks, and the Disposal of Hazardous Wastes in Groundwater. EPA/600/6- 90/004. April 1990.

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Porous Ceramics Are NAPL Barriers

ID Shape Pore Size (µm) K (cm/s) Porosity Length (cm) Outer Diameter (cm) Approximate Cost

A* Tube 11.2 8 × 10-5 0.22 24 4.9 $20 B Tube 2.5 9 × 10-6 0.45 17 4.0 $100 C Tube 2.5 9 × 10-6 0.45 8.9 2.2 $40 D Disk 2.5 9 × 10-6 0.45 NA 2.2 $40

A B C D

Notes: *: Physical parameters estimated based on laboratory testing by Anchor QEA. All others provided by manufacturer. K: hydraulic conductivity

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PNAPL Pwater Pwater Pwater Wettability and Displacement Pressure

Fundamentals of NAPL Exclusion

Source: Wilson, J.L., S.H. Conrad, W.R. Mason, W. Peplinski, and E. Hagan, 1990. Laboratory Investigation of Residual Liquid Organics from Spills, Leaks, and the Disposal of Hazardous Wastes in Groundwater. EPA/600/6-90/004. April 1990.

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Laboratory Test of Sampling Water in Contact with NAPL

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  • Aquarium with well-

graded sand, 0.5M NaCl water, and 10% creosote NAPL saturation

  • Duplicate samples

– Diffusion-based water samples at 10, 20, and 31 days – Pumped water samples also collected from ceramic tubes at 31 and 60 days

Porewater Sampling Tests With Diffusive Equilibration and Pumping (With NAPL)

Magnetic stirrer

Source: Gefell, M.J., M. Kanematsu, D. Vlassopoulos, and D.S. Lipson, 2018. “Aqueous-Phase Sampling with NAPL Exclusion Using Porous Ceramic Cups.” Groundwater 56(6): 847–851.

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Porewater Sampling Tests With Diffusive Equilibration and Pumping (With NAPL) (cont.)

VOCs PAHs

Source: Gefell, M.J., M. Kanematsu, D. Vlassopoulos, and D.S. Lipson, 2018. “Aqueous-Phase Sampling with NAPL Exclusion Using Porous Ceramic Cups.” Groundwater 56(6): 847–851.

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Field Testing in Monitoring Wells at NAPL Sites

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  • USEPA Region 1 Superfund

Site

  • Chlorinated solvents and

petroleum-based aromatics

  • Tested in three wells with

historical DNAPL

  • 30-day ceramic sampler

equilibration

  • Comparative HydraSleeve

samples

Field Test 1 – Diffusion Groundwater Sampling September 2018

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Field Test 1 – Results

R2 = 0.99

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  • Petroleum LNAPL site in Colorado
  • BTEX compounds
  • Tested below LNAPL layer in two

wells and in two other wells without LNAPL

  • Purged five ceramic sampler

volumes before sampling

  • Comparative low-flow samples at

wells without LNAPL

Field Test 2 – Pumped Groundwater Sampling September 2018

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Field Test 2 – Results

One sampler-and-tubing volume = 450 mL Purge Before Sampling

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Field Test 2 – Results (cont.) Groundwater Concentrations in Wells with LNAPL (from ceramic samplers)

Compound (mg/L) Well 1 Well 2 Benzene 7.54 16.4 Toluene 22.1 23.0 Ethylbenzene 2.39 1.60 Xylenes 28.5 15.3 Total BTEX 60.5 56.3

  • 17% of total LNAPL mass detected in VOC analysis (Method 8015)
  • BTEX was 11% of total mass
  • Calculation of effective solubility challenging and unreliable based on

available data

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Field Test 2 – Results (cont.) Groundwater Concentrations in Wells Without LNAPL

  • 13% to 25% relative percent difference, within typical acceptability

range for laboratory MS/MSDs Compound (μg/L) Well 3 Ceramic Well 3 Low-Flow Benzene 63.0 71.8 Toluene 3.14 3.89 Ethylbenzene 26.0 29.6 Xylenes 15.7 20.2 Total BTEX 108 125

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Field Test 2 – Results (cont.) Groundwater Concentrations in Wells Without LNAPL

  • Ceramic sampler results lower - sampler was apparently at

waterline in well; air observed in discharge tube may have caused some VOC loss Compound (mg/L) Well 4 Ceramic Well 4 Low-Flow Benzene 2.77 6.35 Toluene 0.173 0.359 Ethylbenzene 0.307 0.811 Xylenes 1.41 2.88 Total BTEX 4.66 10.4

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  • Mass flux drives risk and remediation
  • Laboratory-based NAPL mobility tests provide data

to quantify the NAPL effective hydraulic conductivity, NAPL flux, and velocity

  • Use multiple lines of evidence to interpret residual

versus potentially mobile NAPL

  • Any NAPL included in water samples can bias high

the reported aqueous concentrations

  • Capillary barrier materials such as inert, porous

ceramics, can help avoid impacts due to NAPL in collecting aqueous-phase samples

Summary and Conclusions

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Michael J. Gefell Anchor QEA, LLC Lakewood, Colorado 303-984-6250 mgefell@anchorqea.com

Questions?