Effects of installation of piezometers and wells on groundwater characteristics and measurements

Abstract

The results of a variable-head permeability test can be used to check whether a piezometer or sampling well has been successfully sealed in the soil and to detect hydraulic short circuits and possible cross-contamination between aquifers. An improper seal is a well-known cause of hydraulic short circuit. In some instances, also, although the tubing has been sealed over its full length, the water level in the pipe is not representative of the actual piezometric head. An example shows that the values of hydraulic conductivity determined using several test methods varied in a 1:6 ratio before detection of a piezometric error. The resulting correction reduced the variation in k to ± 12%. The causes of hydraulic short circuits have been investigated using computer simulations. It has been concluded that large errors recorded in the field are due to internal erosion of natural soils around the casing during drilling operations or development. The hydraulic damage to soils is documented. An analytical solution has been written for a simple case of hydraulic short circuit. Its theoretical predictions confirm the validity of the computer simulation. In light of experience, recommendations are proposed to reduce hydraulic damage, therefore improving the reliability of piezometric measurements and representativeness of groundwater samples. Key words: permeability, field test, piezometer, sealing, drilling, water level.