Advanced Methods for Mass Flux Characterization in NAPL Zones - PDF document

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 Presented by Michael J. Gefell Acknowledgements • Anchor QEA Innovation Program • Kevin Russell, Mark Mahoney, Dimitri Vlassopoulos, and Masa Kanematsu (Anchor QEA, LLC) • David S. Lipson (HRS Water Consultants) Advanced Methods for Mass Flux Characterization in NAPL Zones 2 Presented by Michael J. Gefell Nonaqueous Phase Liquids • 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 Advanced Methods for Mass Flux Characterization in NAPL Zones 3 Presented by Michael J. Gefell

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. Advanced Methods for Mass Flux Characterization in NAPL Zones 4 Presented by Michael J. Gefell Why NAPL Mobility and Dissolved Phase Mass Flux Matter • 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 Advanced Methods for Mass Flux Characterization in NAPL Zones 5 Presented by Michael J. Gefell Near-Shore, Bank, and Subaqueous NAPL – Little Room for Error Shoreline Bank Subaqueous Advanced Methods for Mass Flux Characterization in NAPL Zones 6 Presented by Michael J. Gefell

NAPL Mobility Advanced Methods for Mass Flux Characterization in NAPL Zones 7 Presented by Michael J. Gefell 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 Experimental photographs – Can be remobilized by water pumping ( after Schwille, F., 1988. Dense Chlorinated Solvents , or open boreholes Lewis Publishers, Chelsea Michigan, 146 p.) Advanced Methods for Mass Flux Characterization in NAPL Zones 8 Presented by Michael J. Gefell Laboratory NAPL Mobility Testing • 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 Advanced Methods for Mass Flux Characterization in NAPL Zones 9 Presented by Michael J. Gefell

Laboratory NAPL Mobility Test Samples • Typically 2 inches long by 1.5-inch diameter • Often selected based on core photography • Usually highest apparent NAPL saturation Photograph courtesy of PTS Laboratories (Houston, Texas) UV light – NAPL fluoresces white light – NAPL has natural color Advanced Methods for Mass Flux Characterization in NAPL Zones 10 Presented by Michael J. Gefell Laboratory Test Gradients Extremely High • 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 If no NAPL is produced from sample, NAPL is residual (immobile), but what if some NAPL is produced under these test conditions? Advanced Methods for Mass Flux Characterization in NAPL Zones 11 Presented by Michael J. Gefell Interpreting Laboratory NAPL Mobility Test Results Advanced Methods for Mass Flux Characterization in NAPL Zones 12 Presented by Michael J. Gefell

Interpreting NAPL Mobility Test Results • Calculate NAPL effective hydraulic conductivity • Distinguish residual from pooled NAPL using multiple lines of evidence Advanced Methods for Mass Flux Characterization in NAPL Zones 13 Presented by Michael J. Gefell NAPL Effective Hydraulic Conductivity (K n ) • Darcy’s Law • K n = Q n / (Ai) Q n = avg. NAPL flow rate = ∆ V n / t [L 3 /T] A = cross sectional area for flow [L 2 ] i = lab test hydraulic gradient [L/L] • K n accounts for: – Soil/sediment pore sizes – NAPL viscosity – NAPL saturation – NAPL relative permeability 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. Advanced Methods for Mass Flux Characterization in NAPL Zones 14 Presented by Michael J. Gefell Multiple Lines of Evidence Line of Evidence Potentially Mobile Residual (Immobile) 1. NAPL produced Yes No during first test Conservative – test step (multi-step gradient much test) higher than ambient 2. NAPL produced No during 1,000xG centrifuge (single- step test only) >10 -7 cm/sec <10 -7 cm/sec 3. Calculated NAPL- effective hydraulic Effectively immobile conductivity Advanced Methods for Mass Flux Characterization in NAPL Zones 15 Presented by Michael J. Gefell

Multiple Lines of Evidence (cont.) Line of Evidence Potentially Mobile Residual (Immobile) 4. Decrease in >10% of initial <10% of initial saturation value NAPL saturation saturation value during test NAPL at or very near depletion to residual 5. Initial NAPL >30% <10% saturation Near upper end of Near low end of literature range for literature range for residual saturation residual saturation Advanced Methods for Mass Flux Characterization in NAPL Zones 16 Presented by Michael J. Gefell Binary Decision For Each Test Sample • 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 Advanced Methods for Mass Flux Characterization in NAPL Zones 17 Presented by Michael J. Gefell NAPL Mass Flux and Velocity • 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 ( dM n /dt ) and pore velocity ( v n ) in the field dM n /dt = Q n ρ n = K n i n A ρ n v n = K n i n / (nS) Q n = volumetric NAPL flow rate [L 3 /T] A = area of potential NAPL flow ρ n = NAPL density [M/L 3 ] perpendicular to flow direction [L 2 ] i n = net gradient in the field (includes n = porosity hydraulic gradient and for vertical S = NAPL saturation movement the “gradient due to gravity”) Advanced Methods for Mass Flux Characterization in NAPL Zones 18 Presented by Michael J. Gefell

Dissolved Concentration Measurements in NAPL Zones – Avoiding False Positives Advanced Methods for Mass Flux Characterization in NAPL Zones 19 Presented by Michael J. Gefell 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 Bottom figure source: Wilson, J.L., S.H. Conrad, W.R. Mason, W. Peplinski, and E. Hagan, 1990. dissolved concentrations Laboratory Investigation of Residual Liquid Organics from Spills, Leaks, and the Disposal of Hazardous Wastes in Groundwater. EPA/600/6- 90/004. April 1990. Advanced Methods for Mass Flux Characterization in NAPL Zones 20 Presented by Michael J. Gefell Porous Ceramics Are NAPL Barriers D C B A Pore Outer Size K Length Diameter Approximate ID Shape (µm) (cm/s) Porosity (cm) (cm) 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 Notes: *: Physical parameters estimated based on laboratory testing by Anchor QEA. All others provided by manufacturer. K: hydraulic conductivity Advanced Methods for Mass Flux Characterization in NAPL Zones 21 Presented by Michael J. Gefell