Background: ADWR has three groundwater well data sets for the state of Arizona. The first is the entire database, the 'Wells 55 Registry', which contains all wells registered in the state. The database was created in 1980 to store registration information submitted by well owners and drillers. It contains 200604 well records. The second data set is the Groundwater Well Site Inventory (GWSI), which is a statewide database that contains well locations, construction, and water level measurements for wells that have actually been located and sampled in the field originally by the USGS and since 1990 by ADWR. The GWSI database contains 44239 well records. Of those, 23150 have a 'Wells 55 Registry ID'. The third data set is the 'Wells 35 Registry', which has not been maintained since 1980, when the Wells 55 Registry was created.