This dataset contains summarized historical groundwater salinity observations from wells near the Lost Hills and Belridge oil fields in the southwestern San Joaquin Valley, Kern County, California. Total dissolved solids (TDS) concentration, electrical fluid conductivity (EC), and well construction information were aggregated from public data sources and local water management agencies. Trends in total-dissolved solids (TDS) concentrations and fluid conductivity were assessed on a well-by-well basis to develop a dataset of salinity observations to support the interpretation of resistivity data derived from an airborne electromagnetic (AEM) survey flown in October 2016. Results of the trend analysis were used to identify wells that were stable with respect to three salinity categories between 2004 and 2014: fresh (TDS less than 3,000 mg/L), brackish (TDS between 3,000 and 10,000 mg/L), and saline (TDS greater than 10,000 mg/L). Salinity observations considered to be contemporary with respect to the AEM survey (2015-2018) are also included in this dataset.