New Applications of Field Permeability Tests in Single Observation Well or Borehole

A single observation well or borehole installed in an aquifer is commonly used for monitoring water level and collecting water samples for chemical and microbiological analyses. Field permeability (Lefranc) tests are conducted within the same well or borehole to determine the local hydraulic conductivity K, an essential parameter in engineering geology and environmental projects. The tests were also applied to analyze the anisotropy coefficient of the aquifer and evaluate the groutability of the soil ground. The quality of the well or borehole significantly impacts the reliability of these tests and the accuracy of derived K values. The paper first reviews the two categories of field permeability tests, namely Type Constant-Head and Type Variable-Head tests. It subsequently presents several novel applications of these test to establish a systematic and practical approach for assessing the quality of the well or borehole. The interpretation and comparison of steady-state and transient-state test data of Type Constant-Head tests are utilized to regulate the installation quality of the well. The imperfect condition of the well screen/filter zone is evaluated by analyzing the field permeability tests of opposite flows. The numerical modeling of aquifer-single well systems is conducted using seep/w to provide perfect well condition and quantify the impact of well screen blocking on the apparent K value. The results indicate that even a slight decrease in K value can correspond to a significant levels of screen blockage, which can introduce great errors in contaminant transport predictions displayed by MODFLOW, resulting in inaccurate risk assessment and environment management. Therefore, it is imperative to implement quality control for observation wells and boreholes.