Time series electromagnetic-induction log (TSEMIL) datasets are collected from PVC cased or uncased monitoring wells to evaluate changes in water conductivity over time. TSEMIL datasets consist of a series of individual electromagnetic-induction logs collected at a frequency of months or years that have been compiled into a dataset by eliminating small uniform offsets in bulk conductivity between logs likely caused by minor variations in calibration. These offsets are removed by selecting a depth at which no changes are apparent from year to year, and by adjusting individual logs to the median of all logs at the selected depth. Generally these depths are within the freshwater saturated part of the aquifer, well below the water table. TSEMIL datasets can be used to monitor changes in water conductivity throughout the full thickness of an aquifer, without the need for long open-interval wells which have, in some instances, allowed vertical water flow within the well bore that has biased water conductivity profiles. The TSEMIL dataset compilation process enhances the ability to identify small differences between logs that were otherwise obscured by the offsets. As a result of TSEMIL dataset compilation, for example, the root mean square error of the linear regression between bulk conductivity of the electromagnetic-induction log measurements and the chloride concentration of water samples collected between January 1996 and April 2014, decreased from 17.4 mS/m to 1.66 mS/m in well G-3611. Some of the commonly observed changes in TSEMIL datasets are: (1) variations in bulk conductivity near the water table where water saturation of pore spaces may vary, and water temperature may be more variable, (2) dissipation of conductive water in high porosity rock layers, which may have entered these layers during drilling, and (3) changes associated with saltwater intrusion.
