Considering Monitoring Well Bias in the Delineation of Benzene Plume Lengths
Publication: Journal of Environmental Engineering
Volume 150, Issue 10
Abstract
Underground storage tanks (USTs) primarily store and dispense fuel products (petroleum hydrocarbons) such as gasoline and diesel as well as some hazardous substances. When a release occurs from an UST, it must be cleaned up and remediated as necessary. As part of the site characterization process, monitoring wells are installed to measure petroleum hydrocarbon concentrations, such as benzene, to help site managers make decisions on remediation strategies and effectiveness, determine site status, and whether sites pose a risk to local receptors—with implications to human health and the environment. Previous studies have used various approaches to estimate benzene plume lengths using these monitoring well data. Examining one state’s large database of monitoring well data from leaking underground storage tank (LUST) cleanup sites, and through statistical and spatial analysis, we find that studies that used existing monitoring well data may have underestimated plume lengths. Our study identified elevated benzene concentrations at the boundaries of monitoring well extents, including at the leading edge of plumes. For example, in 3,495 LUST sites, 83% exceeded a selected threshold concentration of in a monitoring well at the site boundary, and 76% of sites exceeded at the monitoring well furthest from a defined source, the point used to delineate length. Of the total 17,816 monitoring wells that delineated UST site boundaries, 47% had concentrations exceeding . These data analyses and results are done over the lifetime of a monitored UST site, not targeted over time periods when decisions are being rendered. However, results suggest that studies to date using existing monitoring well data may have underestimated plume lengths and extents. Furthermore, results suggest monitoring wells in combination with other site information, such as modeling, should be used to guide site managers in their decisions regarding leaked UST products, the area and extent of contamination, risks to receptors, and remedial approaches.
Practical Applications
USTs, which store fuel at locations such as gas stations, are known to leak fuel products into soil and groundwater. Fuel products contain petroleum hydrocarbons like benzene, a cancer-causing chemical. Previous studies that have attempted to identify the maximum distance benzene plumes travel have been hampered by a bias in available data—specifically, that monitoring wells are often not placed at far enough distances to be able to map a plume to concentrations below concerning levels. This paper identifies this bias and provides insight into how it may be considered in the future to more effectively map plumes and identify potential biases that can lead to inconclusive results related to plume behavior. While traditional physics-based models remain useful, they can be augmented with data-driven models to more accurately estimate plume extents, especially when real-world data are lacking.
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Data Availability Statement
All analysis, statistics, and spatial analyses were completed using R version 4.1.2. All code used in this research, as well as additional exploratory analyses, are publicly available in Murray (2023).
Acknowledgments
All analysis, statistics and spatial analyses were completed using R version 4.1.2. All California LUST data were obtained from the California Water Board’s GeoTracker public portal (SWRCB 2024). The authors would thank Dr. John McKernan for his thoughtful review of the manuscript and assistance in preparing it for publication.
Disclaimer
The views expressed in this paper are those of the authors and do not necessarily represent the views or policies of the US Environmental Protection Agency or the California State Water Resources Control Board. This document, and the research described within, was not supported by EPA funding.
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© 2024 American Society of Civil Engineers.
History
Received: Oct 4, 2023
Accepted: Mar 15, 2024
Published online: Jul 18, 2024
Published in print: Oct 1, 2024
Discussion open until: Dec 18, 2024
ASCE Technical Topics:
- Architectural engineering
- Benzene
- Buildings
- Chemicals
- Chemistry
- Continuum mechanics
- Energy engineering
- Energy sources (by type)
- Engineering fundamentals
- Engineering mechanics
- Environmental engineering
- Equipment and machinery
- Facilities (by type)
- Fluid dynamics
- Fluid mechanics
- Fuels
- Geotechnical engineering
- Geotechnical investigation
- Hydraulic engineering
- Hydrodynamics
- Hydrologic engineering
- Non-renewable energy
- Organic compounds
- Organic compounds
- Petroleum
- Plumes
- Pollutants
- Pressure vessels
- Site investigation
- Storage facilities
- Storage tanks
- Structural engineering
- Structures (by type)
- Tanks (by type)
- Underground storage
- Water and water resources
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