Water Table Measurements

The depth of the water table in a well is measured relative to an agreed-upon mark at the top, for example, the edge of the casing. This reference point should be clearly marked to ensure reproducibility of the measurements. The depth of the water table is obtained in meters below the top of the well or, more meaningfully, in meters below the surface, as shown in Fig. 4.1. AC is the depth of the water table below the reference mark at the edge of the well casing (or any other selected mark). This might differ slightly from BC, the depth of the water table below the surface (Fig. 4.1). In the field notebook the depth AC should be used, as it is exact and properly defined. In a final report reference to the surface may be preferred. In any case, one has to specify how the depth is expressed: "below well head'' or "below surface.''

A variety of simple, accurate, and professional tools are available to measure water table depths to an accuracy of 0.2 cm. These instruments consist of a wire with a metal tip that is lowered and upon hitting the water an electrical circuit closes, indicating the water table (Fig. 4.2). The wire is marked for depth readings. The described mode of water table measurement can be performed only in an open, unequipped well. A pumped well can be specially equipped for water table measurements, and this is of interest as it reveals the local dynamics of the lowering of the water table due to abstraction (section 4.6).

Regional interpretation of water table data obtained in many wells is done in meters above sea level (masl). Each well head has to be surveyed,

Fig. 4.1 Depth of water table: AC is the depth below the reference mark at the edge of the casing; BC is the depth below the surface.

and its altitude has to be determined. The measured water table depth is subtracted from the well head altitude in order to obtain the water table in absolute altitude units (Fig. 4.3).

Fig. 4.2 Depth of water table (WT) measurement, based on the closing of an electrical circuit, by the water: (a) one wire, the casing serving as the second wire (provided it is made of metal and unbroken); (b) two wires lowered into the well.

Fig. 4.3 Expressing water table in altitude units (relative to sea level). The measured depth AB is subtracted from the altitude of point A, determined by direct measurement or read from topographic map. In the example, the reference point A has been determined to be 622 masl and the depth of the water table is 22 m. Hence, the water table in this well is 622 — 22 = 600 masl.

0 0

Post a comment