API Bull 24:2006 pdf download
API Bull 24:2006 pdf download.Downward Solute Plume Migration: Assessment, Significance, and
Implications for Characterization and Monitoring of“Diving Plumes’.
If clean water overlies a portion of a contaminant plume, this may greatly reduce the potential for volatile emissions from the plume into soil gas and subsequent subsurface vapor intrusion into overlying buildings. Therefore, characterization of the magnitude of plume dive can assist in assessing the potential for subsurface vapor intrusion.
The phenomenon of plume diving has been observed at several detailed field studies throughout the United States, Plume dive as a function of gradual build-up or accretion of recharge has been noted at several sites located in Long Island, New York (Weaver and Wilson 2000: Weaver et al. 1999). At one site, comparison of’ MTHL analytical results from depth-discrete monitoring wells to vertically averaged results falsely indicated that although averaged concentrations fell below New York State’s threshold value of 10 ig/l, significant concentrations of almost 8,000 tg1 occurred at depth in the downgradient portions of the aquifer. In addition, the beniene plume appeared to he shortened to approximately 1/3 of its actual length. and the averaged concentrations falsely indicated no chromatographic separation of the bcnzene and xylcne plumes (this is inconsistent with the expected and observed attenuation behavior of these constituents). At another site, downward migration of constituents was fl.irther induced by nearby supply well pumping from deeper aquifers.
At a South Carolina site, higher concentrations of MTBE and benzene occurred in the deeper sampling ports of multilevel monitoring wells in arm area below a drainage ditch as a result of recharge events that deflected originally horizontal groundwater flow patterns (Landmeyer et al. 1998: Lahvis et al. 2003). Studies done at a site in Cape Cod, Massachusetis, indicate accretion of precipitation. rather than hydrodynamic dispersion. was the dominating factor controlling vertical migration of a VOC plume (Reynoldset al. 1991).
Stratigraphy can also influence plume behavior. At a site in western Kansas, an MTBE plume in the shallow portion of the water table aquifer near the source migrated downward along preferential pathways (Ilattan and Blackburn 1999). In California. a natural gradient tracer experiment on an MTHE plume at the U.S. Naval Base Ventura County. Port Hucneme. indicated the plume center of mass deepened with migration distance because of the dip of the stratigraphy (Amerson and Johnson 2003).
The U.S. F.PA Region 5 has conducted research on how to best monitor leaking underground storage tank (LUST) sites to characterize diving MTBE plumes. Results at three sites in the Midwest (Illinois.
Wisconsin. and Michigan) indicate higher concentration plume “cores” at progressively deeper intervals with distance from the source as a function of recharge area andor changes in lithology at the water table. (Alvarez, 2003).
The objective of this technical bulletin is to promote a common understanding of the phenomenon of diving plumes. The following sections discuss the factors that can cause plumes to dive, show methods used to evaluate the potential for and magnitude of diving plumes. and provide suggestions regarding the identification and characterization of diving plumes.
Biogeochernical processes can result in spatial variations in contaminant attenuation mechanisms, which may give the impression that a plume is diving or that it is diving at a greater angle than would be observed solely as a result of recharge accumulation. If the recharge water infiltrating into the subsurtce contains greater amounts of dissolved oxygen. aerobic degradation may he enhanced near the water table. The effects of dissolved oxygen with depth on apparent plume dive have been noted by Landmeyer and Bradley (2003). Episodic infiltration of rainwater influenced dissolved oxygen concdntralions in wells screened a few feet below the water table. In the unpaved portion of the site, the plume dove gradually as a result of precipitation accumulation. but the apparent dive was greater than would have occurred solely from hydraulic effects because of enhanced biodegradation near the water table (refer to example in Figure 4).
