H₂A is Using LNAPL Transmissivity to Make MDPE More Efficient and Cost Effective

H₂A recently completed an evaluation of LNAPL transmissivity (T_n) at a major international airport in Texas.  The results are being used to further enhance the existing mobile dual-phase extraction (MDPE) remediation strategy for the site by focusing efforts on wells exhibiting the highest LNAPL transmissivity.  By doing so, H₂A has realized increases in product recovery of up to 20 percent over historic efforts of comparable duration.  H₂A also plans to utilize LNAPL transmissivity trends over time to gauge remediation efficacy and to establish active remediation endpoints.  It should be noted however, that although LNAPL transmissivities of between 0.1 to 0.8 square feet per day typically demonstrate the limit of practical LNAPL recovery, potential Texas Risk Reduction Program (TRRP)-32 (Risk-Based NAPL Management) triggers (as established by the Texas Commission on Environmental Quality) must also be considered.

The June 2011 issue of Applied NAPL Science Review (ANSR) was cited in the U.S. Environmental Protection Agency’s July 2011 issue of the TechDirect Newsletter, a publication that identifies new guidance resources related to the assessment and remediation of contaminated media.  ANSR was listed as a web resource that “provides technical insight into the science behind the characterization and remediation of light and dense NAPLs.”  You can sign up to receive the TechDirect newsletter at http://clu-in.org/techdirect, and current and historic issues of ANSR can be viewed/downloaded at http://www.h2altd.com/knowledge-center.

H₂A is currently performing a 12-day, mobile dual-phase extraction (MDPE) event at a former petroleum storage tank site located within TxDOT right-of-way in Angleton, Texas (the Site).  This is the latest of seven MDPE events performed by H₂A at the Site over the past several years.  During a 12-day event performed in March of this year, H₂A performed DPE on three wells located on the east side of the property (six days) and one well located on the west side of the property (six days).  Our system recovered 27.0 gallons of liquid-phase hydrocarbon and 73.9 gallons of vapor-phase hydrocarbon from the east side of the property, and 134.0 gallons of liquid-phase hydrocarbon and 388.1 gallons of vapor-phase hydrocarbon from the west side of the property, resulting in a total event recovery of over 600 gallons of hydrocarbon from the subsurface.

H₂A’s J. Michael Hawthorne, PG, Shannon Walker, PE, and Si Xu, EIT, recently completed an extensive redesign of the LNAPL Conceptual Site Model (LCSM) for a large-scale oil refinery located in southeast Texas. The analysis included a detailed review of all gauging and well construction data, fluid physical properties, aquifer properties, multiple LNAPL recovery and recoverability analyses, and detailed stratigraphic analyses (CPT/ROST and boring log data). The data were captured in LCSM update reports that included various Diagnostic Gauge Plots, Advanced (CPT) Hydrostratigraphs, and Production Hydrostratigraphs, as well as other analyses and cross-references. An initial goal of the LCSM updates was to identify areas of confined or perched LNAPL that result in exaggerated apparent NAPL thicknesses (ANTs) in gauged wells. Once wells exhibiting confined or perched conditions were identified, the mobile LNAPL interval in the formation was determined to remove the exaggeration. Subsequent tasks are to include analytical modeling to evaluate LNAPL specific and recoverable specific volumes, calculation of LNAPL transmissivity for multiple recovery wells based on LNAPL recovery and operational data, and capture analysis modeling of the active site remediation systems.

Abstract of presentation to be given by Mike Hawthorne of H₂A (coauthors Andrew Kirkman of AECOM and Mark Adamski of BP America) at the LNAPL Site Management Strategies Session of the 2011 Battelle Bioremediation and Sustainable Environmental Technologies Conference, Reno, Nevada, June 27-30, 2011.

Background. Existing LNAPL distribution literature is heavily biased towards ideal unconfined conditions representing static water-table elevations.  Mobile LNAPL has been documented at multiple sites to be submerged below the water-table as well as existing perched above the water-table.  Given the substantially exaggerated apparent NAPL thicknesses (ANT) that can occur in wells under these conditions, the identification of confined or perched LNAPL at a site is critical to correctly characterize the mobile LNAPL interval thickness and stratigraphic occurrence in the LNAPL Conceptual Site Model (LCSM).  A more accurate LCSM will lead to better metrics for both LNAPL characterization and remediation.  For example, ANT in wells is a poor metric for remediation because the ANT represents LNAPL confining pressure rather than recoverability or thickness of the mobile LNAPL interval.  The presence of confined or perched LNAPL may also have a profound effect on LNAPL remediation technology selection.

Approach. LNAPL flow vertically is inhibited due to non-wetting phase capillary pressure limitations rather than soil permeability limits.  However, LNAPL confining capillary pressures typically coincide with reduced grain size in unconsolidated sediments.  Therefore, an improved understanding of microstratigraphy results in more precise identification of potential confining or perching layers.  Improved characterization tools (e.g., laser induced fluorescence or LIF, baildown testing, soil core analyses) simplify identification of confined or perched mobile LNAPL intervals.  By combining microstratigraphic characterization tools (e.g., soil core analyses) with mobile LNAPL interval identification tools (e.g., baildown testing, diagnostic gauge plots), stratigraphic intervals containing mobile LNAPL may be identified even under confined, perched or interbedded conditions.

Results. This presentation will discuss methods to identify confined and perched LNAPL, and will include a review of site-specific examples of both simple and complex confined and perched LNAPL sites.  The tools discussed include hydrostratigraphs, diagnostic gauge plots, visual soil boring data, CPT and laser induced fluorescence borings, soil core photography and core analyses, and baildown test data.

One way to look at light non-aqueous phase liquid (LNAPL) drivers for remediation is to break them down into: (1) physical science issues and (2) human science issues.  Physical science drivers include saturation and composition risks.  Saturation risks are primarily related to migration of LNAPL, if the LNAPL mass present is great enough for migration to occur.  Composition risks are the classic human health and ecological risk-based evaluation pathways that typically address exposure to dissolved-phase or vapor-phase hydrocarbon chemicals of concern (COCs).  The primary drivers governing evaluation of remediation need and design of selected remedies should be the physical science drivers, which govern both risk and technical feasibility.

Human science drivers include regulatory requirements, legal issues, business issues, and perception concerns.  The primary regulatory requirement of concern is the requirement to “recover to the maximum extent practicable (MEP)”, which has historically resulted in LNAPL remediation goals ranging from no measurable LNAPL to some arbitrary thickness of LNAPL in wells.  These MEP requirements are not grounded in multi-phase fluid science and are often unattainable with current remediation technologies.  Similarly, the perception that the mere presence of LNAPL requires remediation does not recognize that the LNAPL may pose no risk (if saturations are too low for migration and compositionally the LNAPL contains COCs at concentrations below risk-based thresholds).

Frequently these human science drivers are important, but they may primarily serve as secondary drivers that modify or adjust the selected approach rather than controlling the technology selection or design.  For example, attempting to design to an arbitrary MEP (e.g., 1/8 inch measured LNAPL in wells) may result in endless operation of costly remediation systems that achieve little to no improvement in human health or environmental protection.  By focusing on the physical science drivers that govern risk and technical feasibility, LNAPL remedies can be designed to efficiently achieve meaningful and lasting protection of human health and the environment.

Some states have already begun to implement this approach (e.g., Texas, Minnesota).  For other agencies with arbitrary LNAPL thickness goals, reasonable remedy selection and endpoint metrics may be attainable by demonstrating the “Technical Impracticability” of the arbitrary thickness standard through pilot testing and modeling or analysis of operational remediation data for your site.

Remember – “Have a Plan, Don’t Pump Because You Can.”

Shannon has been a Senior Remediation Engineer at H₂A for almost four years.  She holds both a MS and BS in Chemical Engineering, and a BS in Civil Engineering, from West Virginia University in Morgantown, West Virginia.  She has worked in environmental consulting for over ten years, specializing in site remediation and regulatory compliance.

Q: Shannon, what strengths do you bring to the H₂A team?
A: My dual-engineering background has proven to be a valuable asset.  Two of our key areas of expertise at H₂A, active site remediation and applied NAPL science, require a knowledge of soil science, multi-phase fluid mechanics, and chemical/physical separation processes.

Q: What do you do when you are not saving the planet?
A: I talk shop with my husband, also an engineer, and spend time with my family.  We have two little boys who keep us hopping with their love of dirt and bugs.

Q: What is the last book that you read?
A: Llama, Llama Red Pajama by Anna Dewdney with my two-year-old.

H₂A recently made note of a new visitor frequenting our off-site system maintenance facility.  A common gray fox has taken up residence and is making itself at home in the tall foliage behind the building.

Here are three things you may not know about foxes – they are omnivores, eating both meats and vegetables; they can climb trees and often den in hollow tree trunks; and although more active under the cover of dark, they are not strictly nocturnal.

Texas is currently under drought conditions that may be more substantial than many people realize.  The Texas drought map (republished from the noted source) dated April 5, 2011, demonstrates large areas of Texas that are under severe, extreme, or exceptional drought conditions.  Water conservation requirements have already been triggered in some cities located in harder-hit areas.  We often take for granted the availability of good, clean water – but, our groundwater aquifers are not limitless.  We encourage everyone to get involved in water supply, water quality, and water protection issues.  Let’s be prepared for the future!

H₂A recently obtained closure from the Texas Commission on Environmental Quality (TCEQ) for a waste management unit located at an industrial plant in west Texas.  The unit, which consisted of a grey water pond used to collect discharge water from employee locker room sinks and showers, was regulated under the Resource Conservation and Recovery Act (RCRA) and administered by the TCEQ.  The initial site assessment of the grey water pond revealed detected concentrations of benzo(a)pyrene and total petroleum hydrocarbons in the soil that exceeded applicable action levels.  The soil in the affected area was excavated and subsequent confirmation sampling demonstrated attainment of site-specific Texas Risk Reduction Program critical residential protective concentration levels.