Analytical Methods:Watertable Monitoring

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Degradation processes linked to changes in watertable levels
Average watertable depth is an important variable in archaeological heritage preservation. The onset of anaerobic conditions is closely linked with waterlogging, hence organic material permanently below the watertable is less susceptible to decay than that in the permanently or seasonally oxidised soil zones. Watertable depth is a good way of monitoring the dewatering of deposits through drainage, irrigation, climate change, crop changes, development and mineral extraction. Waterlogging may also occur above the groundwater level in perched watertables. Minimum and maximum watertable levels are also highly relevant as this may produce flushing of the intermittently waterlogged zone as well as repeated variations in redox conditions. Fluctuating groundwater levels can also increase oxygen concentrations below the watertable leading to degradation of organic materials in the saturated soil zone (Williams and Oostrom, 2000).

Approaches to monitoring watertable levels
Watertable depth and fluctuations may be inferred from the soil morphology (colour and intensity of mottling). However, such measures are insensitive because it is difficult to compare between soils, and because relict features reflecting previous water table levels may be preserved in the soil.

Dipwells are the commonest means of monitoring watertable fluctuations. These permanent structures need to be installed in areas of low disturbance and must be sufficiently deep to contain free water throughout the monitoring period. The waterlevel in dipwells may be monitored manually or they may be fitted with divers to automatically record the groundwater level. Divers may also record water temperature, conductivity, and dissolved oxygen content.

Sampling strategies
The watertable level varies over time in response to precipitation, evaporation and groundwater abstraction, but it also varies spatially as a result of soil and geology conditions, topography (landform), land use and vegetation type. How many dipwells are needed, therefore, depends on the variability of these factors across the site and on the size of the feature(s) you wish to monitor.

The frequency of measurements required depends on the question being asked. If it is infiltration and seepage of water that is of interest measurements will need to be more frequent than if the long term groundwater regime that is of interest (Smit et al., 2006).

Related techniques

 * Soil moisture
 * Monitoring soil moisture

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