Estimation of Aquifer Transmissivity From Analysis of Long-Term Monitoring With the Thiem Solution

The application of the Thiem equation to support the interpretation of comprehensive long-term monitoring datasets, made possible through modern datalogging technology, is presented as an alternative to constant-rate aquifer testing to obtain representative transmissivity estimates in settings where controlled hydraulic testing may be impractical. Water levels logged at regular intervals can be readily converted to average water levels over time periods corresponding to periods of known pumping rates. By regressing average water levels during multiple time periods of known but variable withdrawal rates, steady-state conditions can be approximated and Thiem's solution applied to estimate transmissivity, without performance of a constant-rate aquifer test. Although the application is limited to settings where changes in aquifer storage are negligible, by regressing long data sets to parse interferences the method may characterize aquifer conditions over a much wider radius than short-term, non-equilibrium tests. As with all aquifer testing, informed interpretation is critical to identifying and resolving aquifer heterogeneities and interferences.