Mendenhall and others (1916) assessed groundwater resources in California's San Joaquin Valley in 1910 to estimate the availability of groundwater of suitable quality for agricultural, industrial, and drinking water supplies. They inventoried nearly all existing wells, compiled depth-to-water at 4,002 wells, and collected water-quality data at 485 wells. Samples were collected from 114 wells for laboratory analysis of total dissolved solids (TDS), chloride, sulfate, bicarbonate, carbonate, calcium, magnesium, sodium+potassium, and silica (Mendenhall and others. 1916; Dole, 1909). Field assays were used to measure TDS, chloride, sulfate, bicarbonate, carbonate, and total hardness in samples from 371 wells (Mendenhall and others. 1916; Leighton, 1905). Samples from 32 wells were analyzed using both laboratory and field assay methods. These data have been transcribed into a modern database format for use in future groundwater research.