residual ethanol of 2.72 kL in the subsurface [Mravik et al., 2003]. The post flushing

groundwater samples indicate increasing byproducts of reductive dehalogenation of PCE over

time, demonstrating enhanced natural attenuation of residual PCE [Mravik et al., 2003; Sewell et

al., 2006].

1.11 Mass Flux and Mass Discharge

 Although quantifying the source mass or volume is important in determining the size of

the source and the best method for remediation, the amount of mass leaving the source in

dissolved form may be more important [Feenstra et al., 1996]. The risk to a downgradient well

has less to do with the mass of the source and more to do with the hydrogeologic conditions at

the site. A site with little or no groundwater flow maybe of little risk to receptors downgradient,

however an area with high flow through a contaminated site, may create an extensive plume of

dissolved contaminants stretching for miles [Einarson andMcKay, 2001].

 The assessment of contaminant source strength is completed by calculating the mass

leaving the source area in dissolved form [EPA, 2003]. The groundwater flux (q,) is also called

the Darcy flux, and is a measure of the flow per unit area of a region. It is calculated from the

product of the saturated hydraulic conductivity (Ks) and the hydraulic gradient (j) [USEPA, 2003;

Basu et al., 2006].

 q, = -Kj (1-12)

The hydraulic gradient is the change in head (dh) per change in distance (dl). By measuring or

estimating the groundwater flux, the contaminant flux can be calculated when combined with the

concentration in the respective groundwater samples The contaminant mass flux (J,) is the

product of the water flux and concentration of the contaminant in that water[AI /L T] [USEPA,

2003; Falta et al., 2005; Guilbeault et al., 2005; Basu et al., 2006;].