Estimating NAPL Hydraulic Conductivity and Migration Rate Based on Laboratory Test Results
Volume 9, Issue 3 | May 2021

Applied NAPL Science Review

Estimating NAPL Hydraulic Conductivity and Migration Rate Based on Laboratory Test Results

Editor: Lisa Reyenga, PE

ANSR Scientific Advisory Board
J. Michael Hawthorne, PG, Board Chairman, GEI Consultants, Inc.
Andrew J. Kirkman, PE, BP Corporation North America
Robert Frank, RG, Jacobs
Paul Cho, PG, CA Regional Water Quality Control Board-LA
Randy St. Germain, Dakota Technologies, Inc.
Dr. Terrence Johnson, USEPA
Brent Stafford, Shell Oil Co.
Douglas Blue, Ph.D., Imperial Oil Environmental & Property Solutions
Natasha Sihota, Ph.D., Chevron
Kyle Waldron, Marathon Petroleum
Danny D. Reible, Professor at Texas Tech University

Applied NAPL Science Review (ANSR) is a scientific ejournal that provides insight into the science behind the characterization and remediation of Non-Aqueous Phase Liquids (NAPLs) using plain English. We welcome feedback, suggestions for future topics, questions, and recommended links to NAPL resources. All submittals should be sent to the editor.

DISCLAIMER: This article was prepared by the author(s) in their personal capacity. The opinions expressed in this article are the author’s own and do not necessarily reflect the views of Applied NAPL Science Review (ANSR) or of the ANSR Review Board members.

Estimating NAPL Hydraulic Conductivity and Migration Rate Based on Laboratory Test Results

Michael J. Gefell, Anchor QEA, LLC

If aggressive laboratory NAPL mobility tests force NAPL to move out of test samples, that creates an opportunity to quantify key parameters for NAPL mobility assessment.

Fundamental parameters governing fluid flow through porous media are hydraulic conductivity, Darcy flux, and average lineal seepage velocity.  They are commonly estimated to evaluate groundwater flow. However, they also apply to non-aqueous phase liquid (NAPL) and can be estimated based on field NAPL recovery data or laboratory NAPL mobility test results using soil or sediment cores containing NAPL. This article focuses on estimating these parameters based on laboratory test results. This article considers NAPL movement (mobility) at the pore scale and NAPL movement (migration) at the NAPL body scale (ASTM Guide E3248 – 20).

Laboratory NAPL Mobility Tests

Two classes of NAPL mobility tests are centrifuge tests and water-drive tests. Centrifuge tests exert a centrifugal force to cause fluids to drain from the sample. Water-drive tests, also known as water‑flood tests (Niemet et al. 2015), involve pumping water through a test sample (typically in an upward direction) to force NAPL to express from the sample. To be conservative, most test samples are usually collected from specific depths with the most notable NAPL presence. These depths can be identified by slabbing and photographing core samples under white light and ultraviolet (UV) light (Figure 1).

Based on the author’s observations at numerous sites, field hydraulic gradients associated with groundwater flow are typically less than 0.1 (dimensionless). Centrifuge tests and water‑drive tests usually impose hydraulic gradients orders of magnitude greater than those in the field. Therefore, if no NAPL is produced from a given test sample under aggressive laboratory test conditions, the NAPL in the sample is deemed immobile (ASTM Guide E3248 – 20). When the test sample expresses NAPL, that does not necessarily mean that the NAPL is mobile or migrating in the field. However, it indicates that the NAPL in the sample was mobile under the aggressive lab test conditions, and the test data can be used to estimate the NAPL effective hydraulic conductivity (K_n).

Calculating NAPL Hydraulic Conductivity

K_n has units of length over time [L/T] and is the volumetric flow rate [L³/T] of NAPL per unit cross-sectional area [L²] under the influence of a unit hydraulic gradient [dimensionless]. K_n accounts for the key physical factors that influence the ability of NAPL to flow through soil or sediment, including sediment pore sizes; NAPL viscosity, saturation, and relative permeability; and wettability.

NAPL mobility test laboratories typically report the initial and post-test NAPL saturations for each test sample, as well as the sample dimensions, porosity, and other data. The change in NAPL saturation during a given test can be converted to a NAPL volume change within the sample (Figure 2). The volume change and test duration indicate the average NAPL flow rate during the test. The NAPL flow rate, applied hydraulic gradient, and sample geometry are used to calculate K_n (Gefell et al. 2018).

Using the test sample geometry and the applied hydraulic gradient, K_n [cm/s] is calculated using Darcy’s Law as follows:

K_n = Q_n /Ai          (1)

where Q_n [cm³/s] is the average NAPL flow rate from the test sample during the test, A [cm²] is cross-sectional area of the discharge end of the cylindrical test sample, and i [dimensionless] is the hydraulic gradient imposed during the test. For a water-drive test, the hydraulic gradient during the test can be calculated by dividing the pressure difference across the test sample by the test sample length in the direction of flow. For a centrifuge test, the hydraulic gradient can be estimated as the number of gravities of force imposed by the centrifuge (e.g., a 25G centrifuge force equals a hydraulic gradient of 25).

Q_n can be calculated as follows:

Q_n = ΔV_n /t          (2)

where ΔV_n [cm³] is the change in NAPL volume in the sample during the test and t [s] is the test duration (Figure 2). The volume of NAPL (V_n) either before or after the test can be calculated as:

V_n = V_t /nS          (3)

where V_t is test plug total volume [cm], n is porosity [dimensionless], and S is the reported initial or final NAPL saturation [dimensionless]. The laboratory measures V_t and reports S and n based on Dean-Stark extraction and physical analyses. Samples that indicate no change in NAPL saturation have Q_n and K_n values of 0 under the aggressive test conditions, so the NAPL in the sample is deemed immobile and cannot be migrating.

Applying Test Results to Calculate NAPL Flux and Velocity in the Field (if NAPL is Migrating)

Under the hypothetical scenario that NAPL is migrating in the field, K_n values can be used to estimate the effective NAPL Darcy flux (v_d) and pore-scale NAPL velocity (v_n) [both in cm/s] of the migrating NAPL as follows (based on Brooks and Corey 1966):

v_d = K_ni_f          (4)

v_n = K_ni_{f (nS)}          (5)

where i_f is the hydraulic gradient in the field [dimensionless]. To assess the potential velocity of vertical NAPL transport, the field hydraulic gradient should include the “hydraulic gradient due to gravity,” which depends on NAPL and water density values (Cohen and Mercer 1993).

Conclusions

Calculating K_n using NAPL mobility laboratory test results provides additional useful information in NAPL mobility assessments. Some test samples express NAPL but have extremely low K_n and v_n values. It is useful to calculate K_n and v_n to evaluate whether NAPL expressed under aggressive laboratory test conditions is effectively immobile under typical field conditions. If NAPL is immobile based on conservative laboratory testing conditions, it is not mobile or migrating under field conditions.

A Word of Caution

NAPL mobility laboratory test samples should be undisturbed, or minimally disturbed to the extent reasonably practicable, including NAPL retention and distribution within the core. Cores of soft sediment may require particular attention. To be conservative, samples used for testing should include materials that contain the most notable NAPL presence observed in soil or sediment cores. The methods discussed herein are based on average NAPL flow during the test and assume NAPL expresses from the test sample throughout the test period. If the final NAPL saturation in a test sample is inferred to represent the residual (immobile) saturation for the test condition, a conservative factor of 2 multiplier may be applied to the calculated K_n value. Given the inherent heterogeneity of geologic materials and NAPL distribution, and the small scale of laboratory test samples, test results and conclusions based on the calculations discussed herein should be considered in the context of other lines of evidence regarding NAPL movement, if any, at a given site.

References

ASTM E3248 – 20. 2020. “Standard Guide for NAPL Mobility and Migration in Sediment – Conceptual Models for Emplacement and Advection.”

Brooks, R.H., and A.T. Corey, 1966. “Properties of porous media affecting fluid flow.” Journal of the Irrigation and Drainage Division, Proceedings of the American Society of Civil Engineers: pp. 61–89.

Cohen R.M., and J.W. Mercer, 1993. DNAPL Site Evaluation. C.K. Smoley, CRC Press, Boca Raton, Florida.

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

Niemet, M.R., J.L. Gentry, B. Morgan, D.R.V. Berggren, and C.D. Tsiamis, 2015. “Gowanus Canal Superfund Site. I: NAPL Mobility Testing of MGP-Impacted Sediments.” Journal of Hazardous, Toxic, and Radioactive Waste. 19(1): C4014003-1 – C4014003‑12.

Research Corner

Effects of polycyclic aromatic hydrocarbons in urban stormwater on receiving sediment 

Dimitrios Athanasiou
Doctor of Philosophy
Texas Tech University 

Abstract:

Urban stormwater runoff has long been identified as a major influence to the contamination of receiving water bodies and sediment. The episodic nature of storms combined with the imperviousness of urban surfaces, lead to stormwater discharges laden with high levels of solids-associated polycyclic aromatic hydrocarbons (PAHs). These compounds pose a concern due to their toxicity, mutagenicity and carcinogenicity, and many of them have been placed on USEPA Priority Pollutant List. The core objective of this study was to determine the physical, chemical and biological characteristics of stormwater runoff form a mixed use urban watershed and determine the distribution and bioaccumulation potential of its effects on the receiving sediment. Historically, stormwater assessment has been focused on loads rather than impacts on sediments and different sampling approaches were needed to characterize those impacts. The experimental approach involved a 2-year sampling plan in Paleta creek at Naval Base San Diego (NBSD) involving a variety of sampling approaches including intensive sampling of individual storms, water and sediment collection before and after the winter rainy season and settling traps collecting depositing sediments throughout the storm season. Storm runoff samples from 2 storms in January 2016 were collected and size fractionated. Receiving sediments were monitored with water column, sediment and sediment trap measurements. Porewater passive samplers and both in-situ and ex-situ bioaccumulation studies using bent-nose clams (Macoma Nasuta) were conducted in cooperation with US Navy personnel to assess the response of the receiving benthos. Total Organic Carbon (TOC) and Black Carbon (BC) contents were measured to better understand the source of the depositing solids as well as to link PAHs in sediments to their bioaccumulation potential. Sediment and tissue was extracted by pressurized liquid extraction (PLE), storm samples were liquid-liquid extracted (LLE) and final analysis was carried out by HPLC-FLD and GC-TQMS. In preparation for the sediment sampling, a study of PLE was conducted in order to develop an in-house method that would allow us to process large amounts of sediment samples in an efficient and accurate way and, in particular, extract PAHs effectively from weathered and high BC sediment samples. The combination of size fractionated stormwater loads with sediment traps were identified as the most effective monitoring tools to assess sediment recontamination. Analysis of stormwater samples showed most of the PAHs were associated with large particles in runoff and led to rapid near field deposition and sediment recontamination. SEM imaging confirmed the presence of large BC-rich particles in the near field traps. However, bioavailability was limited as indicated by bioaccumulation studies suggesting that sediment recontamination assessment should also be coupled with assessment of bioavailability. Porewater concentrations were also shown to correlate well with the observed bioaccumulation suggesting that either bioassays or porewater assessment could characterize bioavailability for PAHs. Parent and alkylated PAH ratios allowed stormwater from this watershed to be separated from sediments settling in areas away from the stormwater discharges. 

 The primary objective of ANSR is the dissemination of technical information on the science behind the characterization and remediation of Light and Dense Non-Aqueous Phase Liquids (NAPLs). Expanding on this goal, the Research Corner has been established to provide research information on advances in NAPL science from academia and similar research institutions. Each issue will provide a brief synopsis of a research topic and link to the thesis/dissertation/report, wherever available.

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