Distribution of wells in the central part of the western San Joaquin Valley, California

Information from 5,860 wells in the central part of the western San Joaquin Valley, California, was collected from several sources and compiled into a common data base. Only 2,547 wells had sufficient information for classification into four categories based on the hydrogeology: wells perforated in the semiconfined zone at depths less than or equal to 50 ft, wells perforated in the semiconfined zone at depths greater than 50 ft , wells perforated in the semiconfined and confined zones, and wells perforated only in the confined zone. Additionally, wells perforated in the semiconfined zone at depths greater than 50 ft were classified by the type of deposits in which they were perforated (Coast Range alluvium or Sierran sand). A computerized data base system was developed to manage well information, and to facilitate characterizing the nature and distribution of the wells. Wells perforated in the semiconfined zone at depths less than or equal to 50 ft are evenly distributed over part of the study area underlain by shallow groundwater. These wells generally are used as observation wells. Most wells perforated in the semiconfined zone at depths greater than 50 ft are perforated in the Sierran sand. This concentration of wells indicates a tendency for using the Sierran sand, where it exists, as a source of water. There are 533 wells perforated in both the semiconfined and confined zones and 410 wells perforated only in the confined zone. Most of these wells are upslope of the valley trough in areas where the Sierran sand is not present. Wells perforated only in the confined zone are concentrated near the creeks. (USGS)