The impact of urban groundwater upon surface water - eTheses ...

WATER QUALITY INTERACTIONS
glacial sand and gravel deposits (mean 0.4%, n=3). The FOC values obtained from the
analyses of aquifer material (Shepherd, 2002) are substantially lower with a mean value of
0.049%. It can be inferred, therefore, that a considerable difference in contaminant transport
rates exists between the unconsolidated and the bedrock aquifers. Groundwater velocities may
be higher but sorption and retardation will be greater in the permeable alluvial deposits
compared with the sandstone.
The retardation factor may be calculated for PCE, the most sorbing of the chlorinated
solvents, in order to estimate the residence time within the riverbed environment.
R = VL/VS = 1+(Pb/n)*Kd
R = Retardation factor [-]
Pb = dry bulk density, 1.72 gcm -3 (Moylett, 2001) [ML -3 ]
n = porosity, 0.2 [-]
VL = linear groundwater velocity [LT -1 ]
VS = velocity of solute [LT -1 ]
Kd = FOC*Koc [M -1 L 3 ]
FOC = fraction of organic carbon [-]
Koc = organic carbon partition coefficient, [M -1 L 3 ]
The Koc value for linear sorption of PCE was estimated as 588 lkg -1 for aquifer material
(Shepherd, 2002) which is considerably higher than the US Environmental Protection Agency
(EPA) estimate of 265 and is related to the type of organic material present. Using the higher
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