API Bull 24:2006 pdf download
API Bull 24:2006 pdf download.Downward Solute Plume Migration:Assessment,Significance, and lmplications for Characterization and Monitoring of “Diving Plumes”.
As such, development of a Site Conceptual Modcl (SCM) is important to assess site conditions and identify the potential for plume dive, and evaluate potential risks to receptors.
If clean water os’erlies a portion of a contaminant plume, this may greatly reduce the potential for volatile emissions from thc plume into soil gas and subsequcnt subsurface vapor intrusion into ovcrlying buildings. Therefore. characterization of the magnitude of plume dive can assist in asscssing 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 ci al. 1999). At one site, comparison of MTBE 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 X,000 jigl occurred at depth in the downgradicnt portions of the aquifer. In addition, the ben ene plume appeared to be shortened in approximately 113 of its actual length, and the averaged concentrations falsely indicated no chromatagraphic separation of the bcnienc and xykne plumes (this is inconsistent with the expected and observed attenuation behavior of these constituents). Ai another site, downward migration of constituents was further induced by nearby supply well pumping from deeper aquikrs.
At a South Carolina site, higher concentrations of MTBE and benzene occurred in the deeper sampling ports of multilevel monitoring wells in an area below a drainage ditch as a result of recharge events that deflected onginally horizontal groundwater flow patterns (l.andnwyer et al. l99t(; Lahvis ci al. 2003). Studies done at a site in Cape Cod, Massachusetts, indicate accretion of precipitation. rather than hydrodynamic dispersion, was the dominating factor controlling vertical migration of a VOC plume (Reynoklselal. 1991).
Stratigraphy can also influence plume behavior. At a site in western Kansas, an MTHI plume in the shallow portion of the waler table aquifer near the source migrated downward along preferential pathways (Ilattan and Blackburn 1999). In California. a natural gradient tracer experiment on an MTBE plume at the U.S. Naval Base Ventura County. Port Hueneme. indicated the plume center of mass deepened with migration dictanec because ot’the dip of the stratigraphy (Anierson and Johnson 2003).
The U.S. EPA Region S 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. Visconsin. and Michigan) indicate higher concentration plume “cores” at progressively deeper intervals with distance from the source as a function of recharge area ancfor 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 evaluatc the potential for and magnitude of diving plumes, and provide suggestions regarding the idetitification and characterization of di ing plumes.
FACTORS CONTROLLING DIVING PLUMES
Diving plumes occur as a result of several hydraulic, geologic, and biogeochernical factors. Although each of these factors contributes individually to the potential for plumes to dive, they may act in combination to influence the overall groundwater flow field and the distribution of contaminants in the subsurface. This section discusses each of these factors and describes how each influence the potential for plumes to dive.
Hydraulic Factors
One of the most important hydraulic factors controlling plume dive is the presence of downward vertical hydraulic gradients, i.e., lower potentiometric head at greater depths. Vertical gradients result from the combined influence of many natural and anthropogenic factors. Natural factors include variations in the magnitude and distribution of surficial recharge, or location within a watershed containing groundwater discharge to or recharge from surface water bodies, or stratigraphic controls on groundwater flow (Figure 2).
