Measurement of Natural Losses of LNAPL Using CO sub(2) Traps

• Published in: Ground water Vol. 53; no. 4; pp. 658 - 667
• Main Authors: McCoy, Kevin, Zimbron, Julio, Sale, Tom, Lyverse, Mark
• Format: Journal Article
• Language: English
• Published: 01.07.2015
• ISSN: 0017-467X; 1745-6584
• DOI: 10.1111/gwat.12240

Efflux of CO sub(2) above releases of petroleum light nonaqueous phase liquids (LNAPLs) has emerged as a critical parameter for resolving natural losses of LNAPLs and managing LNAPL sites. Current approaches for resolving CO sub(2) efflux include gradient, flux chamber, and mass balance methods. Herein a new method for measuring CO sub(2) efflux above LNAPL bodies, referred to as CO sub(2) traps, is introduced. CO sub(2) traps involve an upper and a lower solid phase sorbent elements that convert CO sub(2) gas into solid phase carbonates. The sorbent is placed in an open vertical section of 10cm ID polyvinyl chloride (PVC) pipe located at grade. The lower sorbent element captures CO sub(2) released from the subsurface via diffusion and advection. The upper sorbent element prevents atmospheric CO sub(2) from reaching the lower sorbent element. CO sub(2) traps provide integral measurement of CO sub(2) efflux based over the period of deployment, typically 2 to 4 weeks. Favorable attributes of CO sub(2) traps include simplicity, generation of integral (time averaged) measurement, and a simple means of capturing CO sub(2) for carbon isotope analysis. Results from open and closed laboratory experiments indicate that CO sub(2) traps quantitatively capture CO sub(2). Results from the deployment of 23 CO sub(2) traps at a former refinery indicate natural loss rates of LNAPL (measured in the fall, likely concurrent with high soil temperatures and consequently high degradation rates) ranging from 13,400 to 130,000 liters per hectare per year (L/Ha/year). A set of field triplicates indicates a coefficient of variation of 18% (resulting from local spatial variations and issues with measurement accuracy).