Are there situations where the groundwater table doesn't follow the morphology of the surface?

Question

In most or all geological site reports I read, the groundwater table (of the uppermost groundwater layer) always followed the morphology of the surface. Is this always the case? Are there signs for situation where this might not be the case, something I can see on the surface?

Answer 1

The groundwater table depends on a lot of properties. These include precipitation rates, permeability/transmissivity of the subsurface and regional groundwater flow. There are two different regimes that are distinguished: one with a topography-controlled watertable and the other with a recharge-controlled watertable.

A topography-controlled watertable closely follows local topography, while a recharge-controlled watertable only follows the regional topography, and so 'following the morphology' is partly scale dependent. To give an estimate of which regime is more likely at a location, Haitjema and Mitchell-Bruker (2005) studied a number of models and came to the following equation:

$$ \frac{\overline{R}_r}{K} \cdot \frac{\overline{L}}{\overline{D}} \cdot \frac{\overline{L}}{z_{\text{max}}} \left\{ \begin{array}{ll} > m & \text{topography controlled} \\ < m & \text{recharge controlled} \end{array} \right. $$

Where the terms have the following meaning:

• $R_r$ is the average anual recharge rate
• $K$ is the aquifer's hydraulic conductivity
• $L$ is the average distance between surface waters
• $D$ is the average aquifer thickness
• $z_{\text{max}}$ is the maximum distance between the surface water levels and terrain elevation
• $m$ is either 8 for one dimensional groundwater flow, or 16 for radial-symmetric groundwater flow (in a circular aquifer).

In cases of a relatively low permeability combined with enough recharge by precipitation the groundwater is likely to follow the surface. However, for example in karst areas, where extensive networks of caves and tunnels in soluble rock cause a very effective drainage over long distances, the groundwater will follow the surface on a much more regional scale. Also, areas with high relief groundwater at least isn't parallel to the surface, and for example in sand dunes, the groundwater level is usually relatively flat compared to the dune.