3 The water table
The water table is a fundamental reference surface in the study of groundwater. It tends to follow the ground surface, rising under hills and falling at valleys, but the gradient of the water table is usually much less than that of the ground surface (Figure 4). Under hills the water table is usually at greater depths below the surface than it is below valleys. Where the rocks are very permeable, water can flow through them easily, so the water table will be flatter. Where the water table intersects the ground surface (Figure 4c), groundwater will flow out as springs, or directly to streams or rivers.
The water table can be mapped from the elevation (i.e. the height above ordnance datum, which is roughly sea level) of the water in wells. Figure 5a is a map of an outcrop of Triassic sandstones in part of Nottinghamshire, showing both the ground surface and water table contours. Figure 5b is a N-S cross-section across the area in Figure 5a.
Answer the following questions, using Figure 5.
a.Is the water table nearer to the surface of the ground in the northern or southern part of the area?
b.What is the general direction of the slope of the water table?
c.What is the relationship between undulations in the water table and the topography?
d.At what depth below the surface of the ground will the water table be found on the highest ground in the figure (i.e. at about 5 km south of Worksop)?
a.Apart from at the River Poulter, the water table is nearer to the surface of the ground in the northern part (Figure 5b), where it is less than 10 m below the surface in many places. This distance is derived by subtracting the height of the water table above sea level from the height of the land surface.
b.The water table slopes downwards towards the north-east (Figure 5a).
c.Undulations in the water table tend to follow undulations in the topography. The twomajor topographic features are the Ryton and Poulter river valleys, beneath which there are corresponding dips in the water table. A less distinct dip underlies the River Idle.
d.On the highest ground the water table is about 50 m below the surface.
The general slope of the water table in Figure 5 is in the same direction as the slope of the ground surface, and undulations of the water table follow undulations of the ground. The water table does not, however, slope as steeply as the ground surface.
A water table has a seasonal rise and fall. There is a lag between the time of maximum infiltration and the highest water table level. In Britain, for example, the highest rates of infiltration occur in the winter but the water table does not reach its highest level until spring (Figure 6) when infiltration rates are lower, because infiltration is a relatively slow process, and it takes time for water to reach the saturated zone.