Limitations of Vacuum-Enhanced Extraction: Residual LNAPL in Sandy Soils as a Long-Term Source of Contamination
Tuesday, May 6, 2014: 2:00 p.m.
Platte River Room (Westin Denver Downtown)
Jacob Hartsock
,
Earth Science Department, Emporia State University, Emporia, KS
Tony Dappas, P.G.
,
AMEC Environment & Infrastructure, Inc., Lisle, IL
Marcia Schulmeister, Ph.D., P.G.
,
Earth Science Department, Emporia State University, Emporia, KS
Vacuum-enhanced extraction (VEE) of LNAPL is commonly implemented to prevent LNAPL migration, remove free-phase product, and reduce dissolved-phase constituents from groundwater. However, subsurface heterogeneities, proximity to groundwater recharge boundaries, and incorrect assumptions often reduce the effectiveness of VEE. Additionally, fluctuating water table conditions, which are exacerbated by VEE, create large “smear zones” of residual LNAPL. Residual LNAPL within these smear zones is relatively immobile and difficult to remove, providing a long-term source of dissolved phase constituents. Residual LNAPL is also capable of sufficient remobilization to induce additional migration, which could lead to persistent seeps and sheens if the site is adjacent to a surface-water receptor. Developing accurate conceptual site models to address LNAPL source zones and migration pathways is essential in implementing effective remedial technologies. However, analytical and numerical models used to calculate LNAPL saturation and recoverability are often based on outdated theories or parameter assumptions that are capable of outputs that produce order-of-magnitude variability. In practice, these methods have been known to exaggerate recoverable LNAPL, overestimate VEE capabilities, and set unrealistic remediation timeframes.
A case study will be presented to illustrate common pitfalls of VEE in shallow unconfined sandy aquifers contaminated with diesel fuel and gasoline and located adjacent to engineered waterways. The site has been undergoing VEE for over 15 years, with over one billion gallons of impacted groundwater treated. Less than 60,000 gallons of free-phase LNAPL have been recovered, and daily recovery rates are currently approaching less than one gallon. Benzene concentrations in monitoring wells frequently exceed regulatory limits, and surface-water discharges range from a light sheen to visible product. Contemporary LNAPL theories are used to develop an updated conceptual site model and a more refined evaluation of the effectiveness of VEE in sandy soil conditions.
Jacob Hartsock, Earth Science Department, Emporia State University, Emporia, KS
Jacob Hartsock is a graduate student in the Earth Science Department at Emporia State University with a focus in contaminant hydrogeology. He also works as an Environmental Scientist for AMEC Environment & Infrastructure, conducting subsurface investigations and remediation oversight.
Tony Dappas, P.G., AMEC Environment & Infrastructure, Inc., Lisle, IL
Tony Dappas has more than 25 years of multidisciplinary environmental experience, including due diligence for acquisition and divestiture, CERCLA remedial investigations/feasibility studies, RCRA corrective action and groundwater monitoring programs, Brownfield redevelopment, site assessment and environmental liability evaluation, environmental management, and impact assessment.
Marcia Schulmeister, Ph.D., P.G., Earth Science Department, Emporia State University, Emporia, KS
Marcia Schulmeister is an Associate Professor of Geology and Department Head of the Earth Science Department at Emporia State University. She has also recently served as a Consulting Geochemist for Thailand's Division of Groundwater Resources on one of the first artificial recharge and recovery systems in in Southeast Asia. Her research interests lie in hydrogeology and geochemistry.
