The wellbore skin effect on slug‐test results was analyzed using numerical simulation and field tests for a well at progressive stages of development. The numerical simulation is based on a composite flow model that incorporates a zone of disturbed formation surrounding the wellbore. Field tests were performed on a water‐bearing clayey silt formation at a ground‐water remediation site in Wisconsin. Based on the numerical simulation, the radius of investigation was examined. The results show that the early‐time and late‐time data reflect ground‐water flow in the wellbore skin and undisturbed formation, respectively. Both the numerically simulated and the field slug‐test data define a downward concave curve on a semilog plot of time versus the logarithm of dimensionless head. For the Hvorslev (1951) and Bouwer and Rice (1976) methods, the late‐time segment of the simulated data yields estimates of hydraulic conductivity close to the value defined in the flow model. When a wellbore skin exists, the data curve in a plot of the logarithm of time versus the dimensionless head is shifted horizontally along the time axis. This shift leads to an inaccurate determination of hydraulic conductivity based on the Cooper et al. (1967) method. In the plots of time versus dimensionless head derivatives, the data curve geometry depends on the hydraulic properties of the wellbore skin. Consequently, the wellbore skin effect can be identified and eliminated using derivative‐based type curve methods. For low‐permeability materials, the effect of wellbore skin on estimates of hydraulic conductivity can be minimized through use of the late‐time data. However, proper well installation and development appears to be the most effective and practical solution.
This paper presents analytical solutions for determining non‐steady‐state capture zones produced by a single recovery well and steady‐state capture zones produced by multiple recovery wells. Analysis of non‐steady‐slate capture zones is based on the lime‐dependent location of caplure zone stagnation points and the geometric similarity between steady‐slate and non‐steady‐state capture zones. The analytical solution of steady‐state capture zones is obtained from spatial variations of discharge potential across the capture zone boundary. Both capture zone analyses are based on the assumptions of uniform flow field with a constant hydraulic conductivity, the Dupuit assumption of insignificant vertical flow, a negligible delayed yield, and a fully penetrating well with a constant pumping rate. For a ground water pump‐and‐trcat remediation program, the pumping rate and well location design variables can be adjusted to ensure containment of the ground water contaminant plume.
Concentration ratios of benzene, toluene, ethylbenzene, and xylenes (BTEX) in ground water can be used for ground‐water contaminant plume differentiation and source determination. Computer modeling utilizing BTEX soil‐water partitioning coefficients and biodegradation rates shows that hydraulic dispersion, retardation, and biodegradation do not significantly modify the BTEX concentration ratios in ground water, particularly those of ethylbenzene and xylenes. Therefore, the BTEX concentration ratios are similar in the contaminant plumes that are derived from a common source or sources of similar BTEX compositions. In the vadose zone, the BTEX concentration ratios in downward‐migrating contaminant free‐product remain essentially unchanged because of the dynamic nature of soil adsorption and restricted natural soil ventilation. The only significant change in the BTEX concentration ratios occurs in the partitioning between ground water and contaminant free‐product as the result of differential BTEX solubilities in water. In the partitioning, benzene/toluene and toluene/ethylbenzene concentration ratios of ground water are 3.6 and 3.3 times greater respectively than the ratios at the source, while the ratios of ethylbenzene and xylenes remain unchanged. The geochemical methods were successfully applied and tested at two sites where multiple ground‐water contaminant plumes from different sources were superimposed. The conclusions of geochemical analysis are consistent with the site‐specific hydraulic characteristics and facility operation histories.
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