5. Well Testing

In order to effectively and sustainably exploit an aquifer, having determined its hydrodynamic characteristics and behavior (through aquifer tests, as described in Chap. 4) is not sufficient. It is, in fact, necessary to conduct a well test to derive information on the production and efficiency of a water supply system. Operationally, they are step-drawdown tests, consisting in a succession of at least three pumping stages, each with an increased discharge and lasting until a pseudo-steady state drawdown is achieved. The aim of these tests is to correlate each stabilized drawdown value in the well to its corresponding pumping rate, and to discern between head losses attributable to the aquifer system and those deriving from the water supply system (e.g., well design and construction characteristics, formation damage close to the well, non-darcyan flow close to the well and within the screen slots). Step-drawdown test interpretation is based on Rorabaugh’s empirical equation. According to this method, the stabilized drawdown measured in a well as a result of pumping a constant discharge is the sum of a linear term, representing the total head losses resulting from the laminar component of flow, and an exponential term, representing the head losses due to inertial flow in the proximity of the well and to turbulent flow through the screen slots. This chapter provides the means to estimate such terms and how to use them to determine the productivity (quantified by its specific capacity) and efficiency of a water supply well.