Water Table Fluctuations

The water table is dynamic in most systems. It changes in response to rain events, flood events, snowmelt, recharge, and pumping. To decipher the interplay of these ongoing processes, periodic water table measurements are essential. Historical data are in many cases available from local water authorities, which conduct routine water table measurements. Measurement of the depth of the water table at the time of each sample collection is essential in order to couple the chemical and isotopic results with the hydrological data.

Repeated water table measurements in a well may be presented on a hydrograph, as a function of time. In the example given in Fig. 4.7, the low water table may be interpreted as reflecting lack of recharge in the winter, and the rise may reflect snowmelt recharge followed by summer rains. Countless combinations of hydrograph shapes and modes of interpretation are possible. Knowledge of local precipitation and climate is needed for proper interpretation. Rapid response to rain or flood events may indicate

Fig. 4.5 Water table transect: (a) data table; (b) well location; (c) the transect, drawn through the numbered wells (x), having a NW-SE direction (o = other wells, not included in the transect).

conduit-dominated intake, whereas response delayed by weeks or months indicates recharge through homogeneous porous media.

Water table fluctuations are occasionally accompanied by measurable variations in water temperature or composition, providing crucial information on mixing of different water types. Water table measurements are an important tool in tracing recharge. Three cases, reported by Winslow et al. (1965) are discussed in the following sections.

Fig. 4.6 Geological cross-section: (a) data abstracted from geological reports based on drill cuttings; (b) geological section (interpretation), along with the topographic (surface) and water table transects of Fig. 4.5.
Fig. 4.7 A hydrograph based on repeated measurements of the water table in an observation well. Possible interpretation: winter months reflect restricted recharge, whereas starting in May snowmelt contributions are noticed, followed by summer rains.
Fig. 4.8 Hydrograph of an observation well at the Saratoga National Historic Park, New York (after Winslow et al., 1965), and local precipitation graph. Interpretation is discussed in the text.
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