Groundwater flow lines

The sampling points must be on groundwater flow lines from the source of contamination or above the observation point. The down-gradient observations must accurately reflect the abiotic and biotic processes occurring between the two points. 

Figure 1 Illustration of the development of increasingly complex flow systems as topography becomes more complex. Contours of hydraulic head are indicated by dashed lines and groundwater flow lines by solid arrows. Scale is arbitrary, but might correspond to 100 km in the horizontal direction. In (a), smooth topography produces a regional-scale flow system. In (b) and (c) increasing local topography creates a mixture of intermediate and local-scale flow systems superimposed on the regional one (Freeze and Witherspoon, 1967) (reproduced by permission of American Geophysical Union from Water Resour. Res. 1967, 3, 623-634).

The 3D Configuration of Overall Groundwater Flow Lines Necessary to establish the general pathway for pollutant transport. This requires general delineation of... 

Fig. 17. General diagram of the peat-bog study site. Arrows show groundwater flow lines. Blow up of peat core stratigraphy is attached...

Even where it is not occluded, the mineral surface may not be reactive. In the va-dose zone, the surface may not be fully in contact with water or may contact water only intermittently. In the saturated zone, a mineral may touch virtually immobile water within isolated portions of the sediment s pore structure. Fluid chemistry in such microenvironments may bear little relationship to the bulk chemistry of the pore water. Since groundwater flow tends to be channeled through the most permeable portions of the subsurface, furthermore, fluids may bypass many or most of the mineral grains in a sediment or rock. The latter phenomenon is especially pronounced in fractured rocks, where only the mineral surfaces lining the fracture may be reactive. 

In a typical fixed-bed carbon column, the column is similar to a pressure filter and has an inlet distributor, an underdrain system, and a surface wash. During the adsorption cycle, the influent flow enters through the inlet distributor at the top of the column, and the groundwater flows downward through the bed and exits through the underdrain system. The unit hydraulic flow rate is usually 2 to 5 gpm/ft2. When the head loss becomes excessive due to the accumulated suspended solids, the column is taken off-line and backwashed. 

Application of equation 5 requires caution. In this simplistic form, the equation can be used to find only one component of fluid velocity, namely that defined by the direction over which the gradient is measured, ie, the line between two monitoring wells. In general, however, the direction of groundwater flow at a point is fully characterized by assignment of values in three mutually orthogonal directions. Figure 3 provides an example of such a situation. 

Figure 8.23. Variation of Mg2+, Sr2+ and 813C along the flow lines of groundwater in the chalks of the Berkshire syncline, U.K. (After Edmunds et al., 1989.)...

Fig. 2.19 A more detailed cross-section of the U-shape model area shown in Fig. 2.18 (condensing several figures of Toth, 1963, 1995). Groundwater flow paths deduced by the U-shape flow paths model are marked with arrows denoting flow directions. Three flow zones have been concluded local, intermediate, and regional, with alternating points of discharge (e.g., points A, B, D, F) and points of recharge (points C, E, G, H). The symmetry of the suggested flow lines, centered in the modeled box, reveals that they are a direct outcome of the assumption of the three impermeable flow planes.

Fig. 3.8 A fault has placed a high conducting aquifer against an igneous rock of low permeability. Water ascends along the fault zone, forming a line of springs. Only part of the faults are open and conduct groundwater flow others are clogged by compression and/or mineralization.

Water table maps consist of equipotential lines—contours of equal water table altitudes. The contours are drawn to fit measured water table altitudes, as shown in Fig. 4.4. An immediate outcome is the deduction of the dominant direction of groundwater flow (Fig. 4.4b). 

Fig. 4.4 Drawing equipotential lines (a) a map with well locations and water tables in masl (b) equipotential lines based on the well data the arrows show deduced main directions of groundwater flow.

An often quoted case study (Vogel, 1970) is that of an artesian aquifer in an area near the south coast of South Africa (Fig. 11.8). The decrease in 14C downslope from the aquifer has been taken by the researcher to indicate continuity. One can even calculate the velocity of groundwater flow in the aquifer by selecting two points on the lines of Fig. 11.8, for example, 2 km-4000 years and 18 km-28,000 years. The average flow velocity in the aquifer is... 

Figure 2 Major hydrologic features of hydrologically closed basins (after Eugster and Hardie, 1975) (reproduced with permission of the Geological Society of America from Geol Soc. Am. Bull 1975, 86, 319-334). Flow lines have been added here beneath the playa lake to indicate the possibility of salinity-driven density circulation, and the interaction between fresh meteoric groundwaters and recirculating evolved brines (sources Duffy and...

Figure 1. Location of Devils Hole and salient features in its vicinity. Dashed-dotted line marks approximate boundary of the Ash Meadows groundwater system. Dashed line marks approximate boundary of highly transmissive aquifer. They also coincide with approximate position of the Spotted Range-Mine Mountain structural zone. Arrows indicate the inferred direction of groundwater flow. Shaded areas are approximate recharge areas. Adapted from Winograd et al. (1992).

Figure 4 Groundwater elevation and flow within the Milltown aquifer during Spring 1992. Flow lines are represented by solid arrows and equipotential contours as dashed lines. The flow tube discussed in the text is designated by the heavier two center arrows containing numbered wells used for the discharge and mass loading calculations. The gray arrow represents groundwater flow originating from the Blackfoot River Valley. Contour interval is I m. Data from Titan (1995).

The groundwater flow into a well also can be analyzed by using a flow net. Because the well is symmetrical, lines of constant drawdown (isopotentials) are circles centered at the center of the well (Fig. 3-11). Streamlines are arranged radially around the well if there is no regional flow in the aquifer. 

The hydraulic conductivities and porosities of common rocks are compared in Table 8.1. Clay has the highest porosity, but is among the least permeable of materials. Its low permeability is why clay is used to line the bottom of waste ponds, for example. Basalt and limestone may have low total porosities, but because groundwater flow in basalt and limestone may occur in large fractures and also in cavernous zones in limestone, these rocks often have high permeabilities. 

We selected a few sections containing wells for which analyses can provide some information about the groundwater flow. Fig. 4 is a water-level map of the Madrid aquifer and Fig. 5 shows three sections, along directions which are parallel to the water flow as deduced from the equipotential lines in Fig. 4. 

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