ARCHIVED REPORTS_XR0006230 - Laserfiche WebLink

Home Browse Search

Oc t 10 1996 - 18PH lEG HE ,hPluLuvlt� No J)y d e 43/5d conductivities may be anisotropic, but each horizontal layer may have only one vertical and one horizontal conductivity • The local below-atmospheric pressure field near the wells is neglected. • Density effects are neglected. • The computations assume steady-state conditions. For estimating the capture zone, only convective transport is considered The three-dimensional flow field in the above-defined, limited aquifer region is obtained by superimposition of a horizontal uniform flow field, computed in a vertical cross section and representing the natural groundwater flow, and of radially symmetric, vertical flow fields for each UVB. The superimposition of the different flow fields with their own discre=tion is achieved by interpolating and adding the different flow vectors at the various nodes of a simple rectangular grid with variable grid distances that are independently chosen for each Cartesian coordinate.The rectangular grid can be quickly and simply set up and allows for some refinements near the wells and their screen sections. More details of the numerical computations are given in Herrling and Buermann (1990) Resulting y Flow System ' Before going into more detail, the complex flow field near an individual UVB is clarified for a vertical longitudinal section in the direction of the natural groundwater flow (symmetry plane of the flow problem). In Figure 3, the streamlines of three case studies are illustrated with Darcy velocities (v) of natural groundwater flow of 0.0 m/day, 0.3 m/day, and 1.0 m/day. All other parameters remain constant: the discharge (Q) through the well casing is 20.16 m3/hr, the thickness (H) of the aquifer is 10 m, the anisotropic hydraulic conductivities are Kii = 0.001 msec (horizontal) and Kr, = 0.0001 m/sec (vertical), and the lengths of the screen sections are ag = 1.2 m at the bottom and aT = 2.1 m at the top. Figures 3b and 3c show that the groundwater, flowing from the left, dives downward to the lower screen section and is transported upward within the well casing, ' and that the cleaned water flows out to all sides at the upper screen section. The flow situation can only be calculated and plotted in such a simple way in this longitudinal section, otherwise the complex three-dimensional flow field has to be considered- For a deep aquifer contaminated only m the upper groundwater zone, a UVB installation can be used at a hydraulically imperfect well The resulting flow system is demonstrated in Figure 4, clarified for a vertical longitudinal section in the symmetry ' plane (Fig. 4b). The used parameters are the same as for Figure 3b. The only difference is that the aquifer thickness (H) is 30 m (well length = 10 in, as before), At most of the UVB installation sites, a natural, nonnegligible groundwater flow ' will exist. For a normal withdrawal well, a separating streamline can be determined. all the water within this line is captured by the well, and all water outside of it passes the well. In principle, the situation is the same when using a UVB. In contrast to a normal withdrawal well, where the flow can be considered horizontal, the flow around a UVB must be regarded as three-dimensional. Thus, the water body, flowing toward the UVB from upstream and being captured by the lower screen section, cannot be delimited by a simple separating streamline, but by a curved separating stream surface. This can be calculated as descnbed in Herrbng and Buermann (1940): on the basis of the three- dimensional flow field, a three-dimensional, particle-tracking method is used. The S 1