LNAPL MANAGEMENT: SHORT-COURSE INTRODUCTION

Transcription

1 LNAPL MANAGEMENT: SHORT-COURSE INTRODUCTION CHARACTERIZING AND MANAGING LNAPL SITES: October 5, 2016 Tools & Strategies for Risk-based Decision Making

2 Workshop Agenda Fundamentals of multiphase fluids in porous media LNAPL conceptual site models (LCSM) and risk-based approach Tools for investigation, LCSM development and remedial evaluation Case study examples Risk-based LNAPL regulatory programs OM&M, operational efficiency and lifecycle considerations Liability Management Considerations

3 LNAPL Science & Policy Background 1980s 1990s Work on hysteresis, soil physics Early multiphase modeling Fluorescence tools (1990s) API LNAPL Toolkit EPA RTDF MA LSPA white papers ASTM LCSM guidance (2006) ITRC guidance (2009) Policy shifts (ongoing) ASTM guidance: Est. LNAPL Transmissivity (2013) ASTM LCSM guidance update (2014) ITRC guidance update (underway) API NSZD guidance (underway)

4 Definitions NAPL = non-aqueous phase liquid composed of one or more organic compounds that are immiscible or sparingly soluble in water ( separate phase ) LNAPL = light non-aqueous phase liquid with specific gravity <1 (less dense than water) > Common LNAPLs: gasoline, diesel and other fuels, lubricants, and crude oil > Product, oil, PSH (phase separated hydrocarbons), SPH (separate phase hydrocarbons), LPH (liquid phase hydrocarbons), are older equivalent terms > Multi-component hydrocarbon mixtures produce COCs in the environment

5 Misconceptions about LNAPL in the Subsurface There is no LNAPL on site because it is not measured in wells LNAPL enters the pores just as easily as groundwater All pores in an LNAPL plume are filled with LNAPL LNAPL floats on the water table or the capillary fringe like a pancake and doesn t penetrate below the water table Thickness in the well is exaggerated by a factor of 4 LNAPL thickness in a well is always equal to the formation thickness LNAPL plumes always spread directly down the water table gradient LNAPL plumes only stabilize after residual oil saturations exist everywhere You can recover all LNAPL in contact with groundwater Based in part on ITRC (2009) and Garg (2010)

6 Development of LNAPL Conceptual Models Pancake Conceptual Model > 1970 s 80 s > LNAPL occupies 100% of porosity > In-well LNAPL thickness is about 4 times thicker in formation > Influenced regulatory policies, esp. practicability endpoints Paradigm shift to a Saturation Profile Conceptual Model > Early 1990 s > Based upon multiphase processes & replaced pancake model > Now influencing regulatory policies

7 Basis of Saturation Profile Model LNAPL co-exists (competes) with air/water in the pore network Fluid wettability, capillary forces, saturations, and relative permeabilities come into play Fluid saturations and relative permeability can be quantified Properties vary spatially Assumes homogenous fluids, porous media, and vertical equilibrium.

8 Comparison of Conceptual Models

9 Implications of Paradigm Shift By itself, in-well LNAPL thickness is meaningless Oil saturation profile depends on > Matrix properties > Fluid properties > In-well thickness Oil saturation profile effects volume, mobility, recoverability, source stability, and longevity Oil saturation varies significantly in space and time, but can be measured or approximated using available tools (API LDRM)

10 Saturation

11 Capillary Pressure = Capillary Rise As pore size gets smaller, capillary rise gets larger. Water at less-than atmospheric pressure Water at atmospheric pressure

12 Capillary Pressure Curves Air/Water

13 van Genuchten Curves for USDA Soils Note: Calculated using USDA soil properties taken from the Carsel and Parrish (1988) Database

14 Implications for LNAPL Summary Sand and coarse-grained soils hold water less tightly and have lower θ r > Smaller displacement head > LNAPL more easily displaces water in pores > Greater LNAPL saturations Silt, clay and fine-grained soils hold water more tightly and have higher θ r > Larger displacement head > Difficult for LNAPL to displace water and enter pores > Smaller LNAPL saturations

15 Residual LNAPL Saturation LNAPL Saturation (S o ) is fraction of pore space occupied by LNAPL Residual LNAPL Saturation (S or ) is fraction of pore space occupied by LNAPL that cannot be mobilized under an applied gradient Vertical Elevation Soil Column S or Oil Saturation (%) So>Sor So<Sor Adapted from ITRC 2009

16 Volume Comparison Calculated LNAPL volume for a constant in-well product thickness of 1 meter in a 100 square meter area 10,000 9,000 Pancake Conceptualization Mobile Oil Volume (gallons) 8,000 7,000 6,000 5,000 4,000 3,000 Saturation Profile Conceptualization: Mobile Oil Volume Function of Soil Type 2,000 1,000 0 Coarse Sand Fine Sand Silt Sand

17 Volume Comparison Oil Saturation Profiles for Same Volume of Gasoline in Different Soil Types 6 5 Elevation above the Oil-Water Interface (ft) Clayey Silt Silt Fine Sand Coarse Sand 0 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% Oil Saturation

18 Relative Permeability 1 Relative Permeability Irreducible Water Saturation NAPL Water Residual Oil Saturation Core of LNAPL Plume > Highest LNAPL saturations > Highest K ro > Highest flow rate Edge of LNAPL Plume > Low LNAPL saturations > Minimal K ro > Low flow rate % Water Saturation NAPL Saturation 100% 0 As LNAPL saturation approaches residual saturation, relative permeability for LNAPL approaches zero

19 Oil Transmissivity Oil Transmissivity (T o or T n ) is the average oil conductivity multiplied by thickness of free oil zone (dimen. L 2 /T) T o = K o b, K o = hydraulic conductivity of oil b = thickness of free oil zone Describes the bulk movement and recovery, but not discrete velocity at the pore scale at plume fronts Dependent on the hydrostatic conditions (not a constant) > In general, the more disperse an LNAPL plume becomes vertically and laterally, the smaller the transmissivity

20 Summary of Attributes of the Saturation Profile Conceptual Model Key parameters > 5 soil properties (K w,s rw,vg-α, vg-n, porosity) > 4 fluid properties (ρ ο, µ ο, air/oil surface tension, and oil/water interfacial tension) > Observed in-well LNAPL thickness Oil saturation profile calculated with these parameters Oil saturation controls oil volume and relative permeabiltity K o, T o, and M o can then be estimated

21 Displacement Head and LNAPL Migration There is a minimum LNAPL displacement entry pressure or displacement head that must be overcome for LNAPL to migrate into water-wet pores; this minimum displacement head can be related to the thickness of LNAPL in the formation If LNAPL thickness is less than this minimum thickness, then no LNAPL movement into water-wet pores occurs Field scale observations of LNAPL are consistent with LNAPL plumes that stop spreading laterally due to displacement entry pressure Water acts as a capillary barrier against continued LNAPL spreading

22 Effect of Soil Type on LNAPL Migration Migration of Gasoline in Different Soils Simulated release of 1 million gallons of gasoline over 5 years in two different soil types

23 Effect of Fluid Type on LNAPL Migration Migration of Different Product Types Simulated release of 1 million gallons of gasoline and fuel oil over 5 years in a sandy aquifer

24 Effect of Water Table Changes Increased understanding of trapping of oil in pore network that creates residual oil as water table fluctuates Rise in Water Table Fall in Water Table Oil Trapped by Capillary Forces Depth Depth Oil Trapped by Water Displacement % Oil in Pore Space % Oil in Pore Space

25 Effect of Water Table Changes Changes in Observed Well Product Thickness Due to Water Table Changes

26 LNAPL Thickness in Well vs Water Table Elevation Diagnostic Plots Confined Aquifer Conditions Matched potentiometric surface and LNAPL thickness response (must factor in density ratio of the two fluids) LNAPL thickness increases as water table rises Adapted from ITRC Water Table Elevation (ft) LNAPL Thickness (ft) /24/ /6/1999 4/19/2001 LNAPL Thickness (ft) Unconfined Confined Potentiometric Surface (ft)

27 Confined LNAPL Conditions LNAPL thickness in well increases with increase in water level Bottom filling of well Monitoring well acts like giant pore Clay Clay Clay Gravel LNAPL Gravel LNAPL Water Water

28 Summary of Fundamentals LNAPL Transmissivity is a key parameter for LNAPL mobility & recoverability > Practical hydraulic recovery range of ft 2 /day (ITRC) > Can be readily determined via skimming or baildown testing Saturation model is basis for volume, mobility, transmissivity and recoverability estimates > Specific volume of LNAPL within soil will be greater in coarse than fine grained soil for a given LNAPL thickness Measurable LNAPL thickness in a well does not necessarily indicate mobility > LNAPL plumes generally come to stable configurations over relatively short periods of time > LNAPL plume may be stable at the LNAPL fringe, but there may be local re-distribution within the LNAPL core

29 QUESTIONS? Thank You.