Dakota Home Report Archive FY92 Ann Rep

Kansas Geological Survey, Open-File Rept. 93-1
Annual Report, FY92--Page 9 of 20


Effect of River Valleys..., continued

Determination of Ground-Water Flow Directions

The hydraulic head at a point in an aquifer is a measure of the potential energy per unit weight of ground water at that point and is the algebraic sum of two components, the pressure head and the elevation head (Freeze and Cherry, 1979). In an ideal piezometer, which is in hydraulic connection with the surrounding aquifer only at its base, the hydraulic head is equivalent to the elevation of the water level with respect to a datum. The direction of ground-water flow is determined from the hydraulic head gradient and is downgradient in isotropic porous media.

Potentiometric-surface and water table maps indicate the lateral variation in head and thus are used to determine the lateral directions of ground-water flow. Predevelopment regional water table and potentiometric-surface maps of the major aquifers in southeastern Colorado and Kansas were derived using existing maps of these surfaces. The maps were joined together and carefully edited to make the water-table-surface and potentiometric-surface contours continuous across the Kansas-Colorado border.

Measurements of fluid pressure or the hydraulic head taken at vertically adjacent points below the water table indicate the direction of vertical flow. Where available, fluid pressure vs. depth profiles provide the best indication of the tendency for vertical flow in ground-water systems because they are derived from measurements made at vertically adjacent points. The fluid pressures can be calculated from the difference in elevation between the water level and the midpoint of the screened interval in the piezometer multiplied by 0.433 psi/ft, a factor that assumes a freshwater fluid density of 62.4 lb/ft3. Under conditions of no vertical flow and in a homogeneous and isotropic unconfined aquifer, the fluid pressure versus depth profile approximates a straight line with a slope of 2.309 ft/psi in a freshwater aquifer (Toth, 1979). If the fluid pressure versus depth profile has a slope greater than 2.309 ft/psi, downward flow is indicated; conversely, if the slope is less than 2.309 ft/psi, upward flow is indicated (Figure 20). In a confined aquifer the fluid pressure versus depth profile may be segmented because of the confining layer.

Figure 20--Components of hydraulic head and determination of ground-water flow directions. (a) Components of hydraulic head in a piezometer open only at the bottom for a short distance.

Figure 20b--Determination of vertical flow from measurements of fluid pressure taken at vertically adjacent points in the flow system, assuming a homogeneous aquifer. Profile 1 indicates downward flow because its slope is greater than the slope of the hydrostatic line (2.309 ft/psi) and profile 2 indicates upward flow because its slope is less than the slope of the hydrostatic line.

Other indirect methods were also applied to determine the overall direction of vertical flow in the Dakota aquifer. A plot of the fluid pressure versus the depth to the midpoint of the screened interval can be constructed using a large number of well measurements (Fogg et al., 1983). The slope value of the best-fitting line through the data can be computed by linear regression. If the slope is greater than 2.309 psi/ft, the slope of the hydrostatic line, the head increases with depth and the tendency for flow is upward, which is characteristic of superhydrostatic conditions and discharge zones (Toth, 1972, 1979). Likewise, if the slope of the best-fitting line is less than the slope of the hydrostatic line, the head decreases with depth and the tendency for flow is downward, which is characteristic of subhydrostatic conditions and recharge zones (Toth, 1972, 1979). If the slope of the best-fitting line is not significantly different from the slope of the hydrostatic line, there is no overall tendency for vertical flow.


Previous page--Regional Setting || Next page--Results and Interpretation
Start of this report || Table of Contents
Kansas Geological Survey, Dakota Project
Updated Jan. 1997
Scientific comments to P. Allen Macfarlane
Web comments to webadmin@kgs.ku.edu
URL=http://www.kgs.ku.edu/Dakota/vol3/fy92/rep09.htm