sum of each transect was calculated and then Md was the determined from the product of the site
estimated Darcy flux and the cross sectional area of each transect.
Md C= Cu.-Ax. (3-2)
The site characterization estimates for hydraulic conductivity (6 m/year) and hydraulic gradient
(0.0025) were used to estimate the site Darcy flux (q) of 0.015 m/day. The mean mass flux (J)
was calculated using Eq. 1-13, the product of C.,um and q. The mass discharge was determined
from the product of the mass flux and the cross sectional area of the transect (Axs) using the
modified version of Guillbeault et al.[2004] and Kubert and Finkel [2006] in Eq. 3-2.
The cross sectional area interrogated by the MLS sampling in the source zone transect
was estimated to be 4 m2, and 14 m2 in the downgradient transect. Source zone MLS sampling
points defined a length and depth of 1.49 m and 1.83 m respectively. The sampling volume was
assumed to extend six inches (0.15 m) on either side and both top and bottom of the MLS
transect, thereby giving a cross sectional area of approximately 4 m2. For the downgradient
transect this same approach yielded a control plane cross sectional area estimate of 14 m2. The
results of mass discharge study are presented in sections 3.5 and 3.6.
3.2 Source Zone Residual Ethanol
In Table 3-1, the subsurface ethanol concentrations in the source zone transect are
reported. Ethanol was monitored for the first nine months, but then was not assessed again until
2002, nearly four years later. By then, under 1% ethanol was detected at all MLS locations. By
2003, very little ethanol remained in the source zone, 1.6 mg/L or less.
The decrease of ethanol over time provided evidence of microbial use of ethanol and the
return of natural gradient groundwater flow. Mravik at al., [2003] reported increases of acetate
as ethanol concentrations decreased due to the microbial oxidation of ethanol. Furthermore,