The Need for Complementing Data to Check Deduced Gradients and Flow Directions

Determination of the flow gradient between two wells (Fig. 4.11) is based on the assumption that the two wells are hydraulically interconnected, as shown in Fig. 4.13a. However, wells may be separated by an impermeable rock bed with no hydraulic interconnection (Fig. 4.13b). Thus the gradient measurement between the two wells in the second case is meaningless. The

Fig. 4.11 Determining flow gradient. The drop of water table between wells I and II is Dh — 412.6 — 411.8 = 0.8 m; the distance (read from a map) is 0.76 km. Hence the hydraulic gradient is 0.8m/0.76km — 1.0m/km.

same holds true for deduced directions of water flow—they are meaningful provided the involved wells are hydraulically interconnected. Water flows from I to II in Fig. 4.13a, but it does not flow from III to IV in Fig. 4.13b.

Geological information of the rock sequence and tectonic settings may be instructive but not definitive, as it is hard to translate field data into hydraulic conductivity values: a shale bed may be fractured and let water flow through in one case, and a clay bed may be weathered and act as an aquiclude in another. In addition, a variety of processes lower the local water conductance, occasionally preventing lateral flow. An example of such a process is chemical clogging (Goldenberg et al., 1983).

Other parameters are needed to check for hydraulic interconnections between studied wells. Chemical constituents of the water may indicate

Fig. 4.12 Extracting flow gradients from a water table map (contours in masl). Dh, from B to A, is 300.0m — 298.0m — 2.0m, and the distance (read from the map) is 3.7 km. Hence the gradient between B and A is 0.54m/km. The gradient from B to C is 0 m/km.
Fig. 4.13 Hydraulic interconnection between wells in fissured limestone terrain: (a) wells I and II are interconnected and water flows down-gradient; (b) similar-looking wells, separated by an impermeable rock bed; wells III and IV are not interconnected, in spite of the apparent gradient.

whether studied wells tap the same water type or belong to distinct water groups. In the latter case, hydraulic interconnections can be ruled out (section 6.5). The temperature of groundwater commonly increases down-flow, and hence if colder water is encountered along a suggested flow path, straightforward interconnection between wells is disproved (section 4.8). The isotopic composition of water provides additional clues to hydraulic interconnections (sections 9.10 and 9.11) and so do the radioactive age indicators tritium (section 10.7) and 14C (section 11.9).

0 0

Post a comment