The ground-water component of stream discharge may be determined from the chemical characteristics of the stream water. A chemical mass-balance is used to relate total, direct, and ground-water runoff. To solve the mass-balance equation, it is necessary to estimate the chemical composition of the ground-water and direct-runoff components. The solute concentration of ground water is determined from total runoff during baseflow; the chemical characteristics of direct-runoff are estimated from samples of total runoff collected from selected locations in a basin during peak discharge periods. In three small watersheds in Nova Scotia ground-water runoff constituted from 32 to 42% of peak discharge for the period of analysis.
The attenuation of gamma radiation was utilized to measure changing residual trichloroethylene (TCE) saturation in an otherwise water‐saturated porous medium as clean water was flushed through the medium. A front over which dissolution actively occurred was observed. Once developed, this front varied in length from ≈11 mm to ≈21 mm, lengthening as it moved through the porous medium. Gamma attenuation measurements and analyses of effluent water samples indicate that there was minimal if any transport of TCE as colloidal droplets. Even as trapped TCE ganglia decreased in size due to dissolution, there is no evidence that they became mobile and advected downgradient. An extraction of the porous medium at the completion of one experiment indicated that less than 0.002% of the original TCE mass remained, suggesting that minimal amounts of separate phase TCE remained trapped within the medium after flushing with 290 pore volumes. Mass transfer rate coefficients were computed and are shown to be a function of Darcy flux, TCE volumetric content, and distance into the region of residual TCE.
The movement of the saltwater front in coastal aquifers, including the effect of dispersion, can be determined by using numerical methods. The method of characteristics is used to solve the solute transport equation, and the alternating direction iterative procedure is used to solve the groundwater flow equation for the two-dimensional problem. This approach permits the treatment of transient flow in nonhomogeneous aquifers with irregular geometry. lO
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